The Life History Of' the Mayfly Isonychia Sicca (Walsh)

A/7

THE LIFE HISTORY OF' THE MAYFLY ISONYCHIA SICCA (WALSH)

(EPHEMEROPTERA: SIPHLONURIDAE) IN AN INTER-

MITTENT STREAM IN NORTH CENTRAL TEXAS

THESIS

Presented to the Graduate Council of the

North Texas State University in Partial

Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

Peter M. Grant, B.S.

Denton, Texas

December, 1978 Grant, Peter M., The Life History of the Mayfly Isonychia sicca (Walsh) (Ephemeroptera: Siphlonuridae) in an Intermittent Stream inNorth Central 'Texas. Master of

Science (Biology), December, 1978, 7 2 pp., 4 tables, bibliography, 69 titles.

The life history of Isonychia sicca (Walsh) was elucidated from samples collected at Clear Creek from Oct.

1976-Jun. 1978, and Elm Fork of the Trinity River from Sept.

1977-Jun. 1978, Denton County, Texas. Adaptations for existence in an intermittent stream were of primary concern.

Eggs are capable of diapausing through hot, dry summers and cold, wet or dry winters. Diapause is broken in the fall after rehydration and/or in the spring. I. sicca is usually bivoltine during a Sept.-Jul. wet period. Observations from

Elm Fork indicate that emergence continues to Oct. if the stream remains permanent. Considerable overlap occurs between overwintering, spring, and summer populations. TABLE OF CONTENTS

Page LIST OF TABLES ...... iv

...... V LIST OF ILLUSTRATIONS . . . . .

Chapter

I. INTRODUCTION . . . .0.0.0 ...... 1

II. METHODS AND MATERIALS . . ." . 6

Stream Sampling Sites Physicochemical Analysis Nymphs Subimagos and Imagos Eggs

III. RESULTS AND DISCUSSION ...... 13

Physicochemica1 Conditions in Clear Creek Eggs Nymphs Subimagos Imagos

IV. SUMMARY ...... 63

BIBLIOGRAPHY ...... 67

iii LIST OF TABLES

Table Page

I. Physicochemical Analysis of Clear Creek,

1977-78 ...... 4..16 ..

II. Results of I. sicca Egg Rearing Experiments, Clear Creek, 1977-78.....*...... 20

III. Summary of Clear Creek Conditions and I. sicca Events During the 1976-78 Study Period...... 36

IV. Thermal Summation for Growth Curves of I. sicca in Clear Creek, 1976-77.*...... 41

iv LIST OF ILLUSTRATIONS

Figure Page

1. Stream Temperature of Clear Creek, Sept. 1976-Jul. 1977...... -.-.-.-.-.15 2. Whole Egg, 400X...... 18

3. Whole Egg, 300X...... 18

4. Interface Between Two Hemispheres, 100oX . . 18

5. Knob-terminated Coiled Threads, 3000X . . . . 18

6. Chorionic Tagenoform Micropyle, 3000X . . . . 18

7. Mature Egg with Eyespots and Yolk Visible, 81X...... 18

8. Eclosion Sequence of I. sicca, 31X ..... 24

9. Eclosion Sequence of I. sicca, 31X . . . . . 24

10. Size Frequency Histogram of Head Capsule Widths for 4469 1. sicca Nymphs, Clear Creek, 1976-77.~*...... 30

11. Size Frequency Histogram of Head Capsule Widths for the 1976-77 Overwintering Nymphs of I. sicca ...... 33

12. Average Final Instar Size of I. sicca During the 1977 Emergence Period at Clear Creek . 35

13. Seasonal Size Distribution of 4469 I. sicca Nymphs, Oct. 1976-Aug. 1977, Clear Creek . 38

14. Light Trap Populations of I. sicca in Clear Creek, 1977 .... 0...... 48

15. Average Hind Femoral Length of I. sicca Subimagos and Imagos During the 1977 Flight Period, Clear Creek ...... 51

V LIST OF ILLUSTRATIONS--Continued

Figure Page

16. Average Subimaginal Fecundity of I. sicca Through the 1977 Emergence Period, Clear Creek...... 53

17. Subimaginal Fecundity vs. Head Width of I. sicca, Clear Creek, 1977 ...... 55 18. Subimaginal Fecundity vs. Abdominal Volume

of I. sicca, Clear Creek, 1977 * . - . . . 57

19. Average Head Capsule Width of I. sicca Subimagos and Imagos During the 1977 Flight Period, Clear Creek ...... 61

vi CHAPTER I

INTRODUCTION

Although a major constituent of most streams, mayflies

(Ephemeroptera) are poorly known from both taxonomic and biological aspects. Of approximately 711 described North

American species (Edmunds et al. 1976), Berner (personal communication) estimates that less than 30% are known in the nymphal stage. The life histories of approximately 35% of the 67 North American mayfly genera have been labeled by

Edmunds et al. (1976) as "unknown." Consequently, knowledge of life histories is lacking for most North American species.

Recently, an increased awareness of the importance of mayflies in aquatic ecosystems has resulted in greater attention to their biology. As primary consumers they occupy an important position in both aquatic and terrestrial food webs, being preyed upon by insects, spiders, fish, birds and mammals (Leonard and Leonard 1962). Gaufin and

Tarzwell (1952, 1956) have emphasized the role of mayflies as water quality indicators. Also, their diverse behavior and morphology provide excellent examples for the study of evolutionary trends, zoogeography, systematics and life history strategies.

1 2

While usually innocuous, in certain situations mayflies create serious problems: (1) mass emergences of Hexagenia create dangerous driving conditions (Fremling 1968), (2) winged stages may evoke allergic responses in sensitive per- sons (Figley 1929, 1940, Parlato 1938), (3) nymphs serve as intermediate hosts of flukes (Hall 1969, Schell 1975,

Knowles and Hall 1976), (4) Povilla nymphs may damage pilings and other submerged wooden structures (Burks 1953), and (5) burrowing nymphs in large numbers may seriously weaken small earthen dikes and dams (Leonard and Leonard 1962).

Isonychia sicca was described by Walsh in 1862, under the name Baetis sicca. Eaton (1871) transferred it to the improper genus Siphlurus Eaton and later to Chirotonetes

Eaton (Eaton 1885), and McDunnough (1923) placed it in

Isonychia Eaton. McDunnough (1931) assigned I. manca Eaton and I. campestris McDunnough as subspecies of I. sicca, primarily on the basis of color andminor genetalic differ- ences. Edmunds et al. (1976) recognized Walsh's I. sicca as the subspecies I. sicca sicca (Walsh). I am calling the species, herein studied, Isonychia sicca (Walsh) and am de- positing a large series of voucher specimens of all life stages in the aquatic entomology museum of Florida A&M Uni- versity. In a stressed, intermittent habitat such as Clear

Creek, Texas, one would not expect 2 subspecies to coexist.

However, reared imagos were variable enough in color and genitalia so that some individuals could be assigned to both sicca sicca (Walsh) and sicca manca Eaton, according to

McDunnough's 1931 descriptions and key (W. L. Peters, personal communication). This suggests further taxonomic analysis of the genus Isonychia is needed.

Isonychia sicca is common in north Texas and central and southwest United States streams (Edmunds et al. 1976).

It is a component in stream drift (Cloud and Stewart 1974); the diet of hellgrammite larvae, Corydalus cornutus L.

(Stewart et al. 1973); and is vertically distributed in the substratum (Poole and Stewart 1976). Very little is known of its biology, except for generic traits such as its probable use of the foreleg hairs in filter feeding and its good swimming capability.

Comparatively little is known of the life cycles and biology of other Isonychia species. Morgan (1911, 1913),

Needham (1905) and Harper and Magnin (1971) provided brief biological notes on Isonychia bicolor (Walker), and Ide

(1935) illustrated and described its eggs and first 2 instars. Cooke (1942) published brief notes on matingbehavior and the effect of temperature on the emergence of Isonychia christina Traver, and Brodsky (1973) discussed swarming behavior of the genus. The classic mayfly works of Needham et al. (1935), Berner (1950, 1959), Burks (1953), Leonard and

Leonard (1962) and Edmunds et al. (1976) mainly provided generic-level information on biology. The studies of Clemens

(1917) and Sweeney (1976) on I. bicolor represent the only 4 major biological studies directed to species of Iso- nychia.

Other papers have dealt with toxicity of chemicals to

Isonychia nymphs (Surber and Thatcher 1963, Gregg and

Benfield 1972, Dolan et al. 1974, Kellogg and Bulkey 1976), the range of chemical conditions in Isonychia-inhabited streams (Roback 1974), substrate preference (Crisp and Crisp

1974), egg morphology (Koss and Edmunds 1974), diurnal variation in nymphal respiration (Ulanoski and McDiffet 1972), drift (Zimmer 1976), Isonychia as carriers of fecal coliform bacteria (Sarai 1976) and as dispersal agents for small aquatic organisms (Stewart et al. 1970).

The abundance and apparent success of Isonychia sicca in an intermittent stream suggested to me that this species had possibly undergone major and potentially very interesting life cycle adaptations, probably involving some form of diapause for survival during hot, dry summer conditions. This and the paucity of knowledge about I. sicca's biology were the major stimuli for this study. My major objective, therefore, was to determine the life cycle of this local mayfly, giving particular attention to its adaptive survival strategies in a stressed, intermittent stream of north central

Texas.

I originally intended to compare its life cycle in the intermittent Clear Creek, Denton County, Texas, with that in a permanent, modified stream, the Brazos River, Palo Pinto 5

County, Texas. Stewart (unpublished data) found I. sicca occurring in the Brazos River at an average density of 571 nymphs/m2, Aug. 1971, and specimens from the North Texas

State University Aquatic Entomology Museum had been collected during Oct. 1972. However, intensive sampling in Aug. 1976 yielded no I. sicca nymphs in the Brazos River; continued sampling until Dec. revealed only 3 nymphs. Sampling was thereafter concentrated in Clear Creek, and later (Sept. 1977) in a more permanent stretch of Elm Fork of the Trinity River, of which Clear Creek is a tributary. CHAPTER II

METHODS AND MATERIALS

Stream Sampling Sites

The primary Clear Creek'sampling site was 1 km south of

Bolivar, Texas, near the F.M. 2450 bridge. Occasional sampling was also done ca. 18 km downstream, near the F.M. 2164 bridge, 5 km north of Denton. Substrate in riffle areas consisted of shallow limestone rubble, underlain with packed sand. The stream flowed at low to moderate levels Sept.

1976-Jul. 1977, and Jan.-Jun. 1978. A short period of flow in Aug. 1977, produced by a 6.02 cm rainfall, occurred during the 1977 dry period. Riverine vegetation consisted mostly of American sycamore (Platanus occidentalis), ash (Fraxinus sp.), eastern cottonwood (Populus deltoides), and hackberry

(Celtis sp.).

The Elm Fork of the Trinity River site was 1 km east of

Burger Road on McKiriney Bridge Road, Denton County, Texas, ca. 13 km above its confluence with Clear Creek. Substrate and vegetation were similar to those on Clear Creek and there was continuous flow during the Oct. 1976-Jun. 1978 study period.

6 7

Physicochemical Analysis

A 1.0 1 H20 sample was collected monthly, May 1977-Apr.

1978, at the Bolivar site. It was immediately analyzed upon return to the laboratory, or stored at 1 C until analysis within 24 h. Tests for total alkalinity (mixed brom cresol green-methyl red indicator method) and hardness (EDTA titri- metric method) were performed according to Standard Methods

(American Public Health Association 1976). Either a Corning

Digital 109 general purpose pH meter or a Fisher Accumet

Model 210 pH meter was used to measure pH. Conductivity was measured with YSI model 31 conductivity bridge; results were temperature corrected to 25 C. Dissolved oxygen was measured in the field with a YSI Model 54 oxygen meter.

A Tempscribe remote bulb temperature recorder was originally installed in the stream at the Bolivar site. Due to vandalism in Dec. 1976, it was replaced the following month with a submersible, continuously recording Ryan Model

G thermograph, encased in a vandal-resistant, stainless steel housing. Water temperature was also measured on each sampling date with a Kahlsico 297WA105 porcelain scale thermometer in a Kahlsico 297WA080 shock-protected water thermometer frame.

Relative humidity, wind velocity and light intensity were measured, respectively, with a Bacharach Sling Psychrom- eter, Bacharach Florite Air Velocity Meter Model MRF and 8

Kahlsico 268WA620 light meter, during each subimago-imago

sampling date.

Nymphs Nymphal sampling began in Clear Creek during the recruitment from presumed diapausing eggs, after the summer 1976

dry period. Samples were taken monthly, Oct. 1976-Jul. 1977.

On each date 4 quantitative samples were collected with a

363 p mesh opening Wildco drift net, having a 45 x 30 cm

opening with an attached 363 p mesh opening Wildco plankton bucket. An area of 40 x 45 cm was scoured in front of the net with a hand rake.

In addition, cumulative qualitative samples were taken

each month in Clear Creek with a 2-stage net having 1 mm

and 153 p mesh openings in the 1st and 2nd stages, respec.-

tively. A 363 p Wildco plankton bucket was attached to the

2nd stage. This net increased chances of collecting early

instars.

Two-stage samples were also taken monthly on Elm Fork,

Sept. 1977-Apr. 1978, and biweekly the following May and

Jun. I. sicca recruitment in Clear Creek, after the long,

dry period of 1977, was monitored by biweekly 2-stage net

samples, Jan.-Jun. 1978.

During dry periods in the stream, substrate was removed

with a posthole digger to determine water table level and

collect vertical layers of substrate for subsequent elutri-

ation (Stewart 1975) and sucrose flotation (Anderson 1959). 9

Existing pools were sampled with a 1 mm mesh opening kick

net, and the substrate stereoscopically examined for the presence of small nymphs or eggs.

Live nymphs were transported in styrofoam coolers to

the laboratory and reared in glass aquaria containing natural

substrate and aerated stream H20. Nymphal feeding and movement behavior were observed in these aquaria and in the

stream. Newly hatched nymphs from incubation experiments were reared in individual 20 ml vials of stream H20, in a simulated temperature-light environmental chamber. Behavior was observed periodically under a stereomicroscope.

Live nymphs for gut examination were either transported to the lab on ice and frozen, or killed in 80% isopropanol at the site and later preserved in 10% formalin. Fore- and midguts were extracted by dissection, placed in distilled

H 20 on a slide and scanned under 10OX. Head capsule widths of all nymphs were measured at their greatest width, including the eyes, with an ocular micrometer. Temperature sum- mations for nymphal growth periods were calculated by adding

H2 0 temperature taken from thermographs at 1800 h each day, from proposed 1st day of hatch to proposed 1st day of emergence.

Subimagos and Imagos

Emergence of subimagos and flight activity of imagos were monitored during the spring-summer of 1977, using 2 10

91 cm, 30 watt bulbs: a cool white and a Black Raymaster, powered by an 0.85 hp Honda ER400 portable generator. The

black light rested in a 24 cm diam funnel emptying into a

2 1 collecting jar. The white light rested on a white 30 x

30 x 6 cm box. Mayflies attracted to the white light were

dumped into the funnel every 0.5 h. This light trap was

operated weekly from 0.5 h before sunset to 2 h after,

during the entire emergence cycle. Streamside vegetation was swept during the day on these same dates. During peak

emergence, 6 Jun. 1977, the lights were operated continuously

throughout the night until sunrise the following morning.

These stages were preserved in 80% isopropanol for later

enumeration and body measurements.

Nymph-subimago molts were observed both in the field

and laboratory, and subimago box cages (Edmunds et al. 1976) were used to observe subimago-imago molts. Reared individ-

uals and their exuviae were carefully correlated and labeled

for identification and study. Longevity estimates were

based on nymphal molt-imago death intervals in the box cages.

Fecundity was determined from field-collected sub-

imagos. Light-attracted imagos were usually devoid of eggs.

To examine potential changes of fecundity with decreasing

subimaginal sizes throughout emergence, relative volumes of

? abdomens were estimated, assuming them to be simple cylin-

ders, and calculated as V='r21, where r=0.5 x width of ab53 and l=length of ab2-ab8, inclusive. 11

Eggs

Egg sacs from light-attracted imagos were either immediately placed in a covered dish of distilled H20 for attempted rearing, or 80% isopropanol for later photography.

Distilled H20 was used to minimize fungal contamination and growth. Additional eggs were obtained by dissecting reared and light-attracted subimagos and imagos. All eggs were incubated in the laboratory at room temperature (23 C), or in a Sherer Controlled Environment Laboratory Model CEL 4-4 under simulated stream temperature and photoperiod. Dis- tilled H20 was changed every 1-2 wk.

Eggs in distilled H20 were measured with an AO micrometer (2 mm/.01 mm divisions). Eggs dissected from preserved subimagos and imagos were distorted and could not be accu- rately measured.

Methods described by Towns and Peters (1978) were used to prepare eggs for scanning electron microscopy. A Film-

Vac Inc. Mini-coater and Omar SPC-900/EX critical point dryer were used, and eggs were photographed with a Polaroid

CU-5 land camera with a 7.6 cm lens, universally mounted on an International Scientific Instruments Mini-sem electron microscope.

Artifical fertilization was attempted by mixing dissected eggs from a reared imago in distilled H20 on a depression slide with the crushed abdomen of a reared 2 imago. This mixture was left standing for 1-5 min and eggs 12

were then placed in fresh distilled H2 0 .and reared at room temperature or simulated stream conditions. The forced

copulation method of Huff and McCafferty (1974) was also attempted.

Parthenogenesis was investigated by rearing mature 9

nymphs to imagos, dissecting out and incubating the eggs in

distilled H20 at both room temperature and stream conditions,

Some egg sacs from light-attracted imagos were collected

during the summer 1977 flight period, dried, and stored in

an air-tight vial. They were rehydrated with distilled H20 in Apr. 1978 and incubated at both room temperature and stream conditions. CHAPTER III

RESULTS AND DISCUSSION

Physicochemical Conditions in Clear Creek

Mean weekly temperatures in Clear Creek ranged 5-30 C during its flowing phase, Sept. 1976-Jul. 1977 (excluding

Dec. and Jan.) (Fig. 1). Daytime recordings during Dec. and

Jan. ranged 1-12 C. A similar seasonal range was indicated by daytime recordings on H20 sample dates over the study period (Table I). During the 1977 flowing phase, DO, specific conductance and total alkalinity (as CaCO3 ) ranged

6.9-8.2 ppm, 960-4770 mhos and 131-192 ppm, respectively.

Ranges of these parameters during the 1978 flowing phase were 8.2-8.4 ppm, 680-1008 pmhos and 204-270 ppm, respectively. The pH remained stable at 8.0-8.3 during both periods.

Water hardness of 264-344 ppm as CaCO3, is considered hard to very hard (Sawyer and McCarty 1967).

Eggs

I. sicca eggs (Figs. 2, 3) are spherical and average

242 p in diam. The knob-terminated coiled threads (KCTs)

(Koss and Edmunds 1974) are densely. packed on one hemisphere and scattered on the other (Figs. 2, 3, 4, 5). Chorionic tagenoform micropyles (Fig. 6) were found only on the hemisphere with few KCTs. These eggs fit the generic

13 Fig. 1. Stream temperature of Clear Creek, Sept. 1976- Jul. 1977. Solid line = x weekly temperature from recording thermograph; dots = daytime readings on sample dates. 15

0 0 xs

ro 0 -~0N Oo 38fliV83dV4J3i d3IVM 16

TABLE I

PHYSICOCHEMICAL ANALYSIS OF CLEAR CREEK, 1977-78

Specific Total Hard- Water Dissolved Conductance Alkalinity ness Temp. Oxygen (pmhos) (ppm as (ppm as Date (C) (ppm) 25 C pH CaCO) CaCO) 3_3_ V-1977 25 8.2 4770 8.3 136

VI 29 7.3 1140 8.0 131 ** VII 28 6.9 960 8.1 192 294

VIII-XII * * * * * *

1-1978 2 ** 1008 8.1 270 332 II 4 * 680 8.2 214 264 III 18 8.4 920 8.1 204 344 IV 23 8.2 765 8.2 209 278 *Stream dry **Not measured 17

Figs. 2-6. Scanning electron micrographs of I. sicca eggs Fig. 2. Whole egg, 40OX

Fig. 3. Whole egg, 300X

Fig. 4. Interface between two hemispheres, 100OX

Fig. 5. Knob-terminated coiled threads, 3000X

Fig. 6. Chorionic tagenoform micropyle, 3000X

Fig. 7. Mature egg with eyespots and yolk visible, 81X 18

!jw't

41A

- -g-

6 1"l ip

414

ff 110111 19

description of Koss and Edmunds (1974). Eggs collected over the period 1 Jun.-l Jul. 1977, from light-attracted imagos, exhibited no size or structural change.

Developing eggs gradually changed from transluscent white to opaque brown. Compound eyes, ocelli and yolk were the only nvmDhal structures identifiable through the chorion

of the mature egg (Fig. 7). Results of egg experiments are shown in Table II. The total number of eggs utilized in each test was not determined, but hatching success was poor, probably less than 5%. Eggs, from Jun. 1977 light-attracted imagos, began hatching after 11 days at simulated stream and room conditions. This incubation time compares favorably with those of Clemens (1917), where eggs of I. bicolor hatched in 14 days at 22.5- 25 C, and 25 days at 13 C. Hatching continued for 11 and 17 days at stream and room conditions, respectively. No further hatch occurred until after 70 days of incubation, when 2 eggs hatched at the 23 C room temperature. This confirmed a differential hatching rate. The live-appearance and sur-

vival of many of these eggs f o 11 mo strongly suggested they were in diapause; those that deteriorated were possibly infertile. Most of these eggs came from ?s whose oviducts were empty, and were carrying small egg sacs. Egg sacs that had been shelf-dried in vials for 282 days slowly fell apart when placed in distilled H20, as the eggs swelled to normal size and appearance. No apparent 20

TABLE II

RESULTS OF I. SICCA EGG REARING EXPERIMENTS, CLEAR CREEK 1977-78

Duration Duration First Day of Hatch of Expt. Experiment Incubation of Hatch (Days) (Months)

Egg Sacs Simulated From Light- Stream 11 11 11.0 Attracted Imagos, Temp. 11 59 11.0 ca. 23 C

Simulated No Rehydrated Stream Hatch - 2.5 Egg Sacs 1977 Room Temp. No ca. 23 C Hatch - 2.5

Artificial Fertiliza- Simulated tion, 1977 Stream 14 178 11.0

Forced Copulation Simulated (1 pair) Stream 31 1 9.0 1977

Partheno- Room genesis Temp. 41 1 9.0 1977 ca. 23 C 21

development or hatch occurred for the 2.5 mo of incubation.

Artificially fertilized eggs began hatching after 14 days

and continued for 14 days (Table II). The reasons for their

longer minimum incubation time than those from field col-

lected imagos is not apparent, since temperatures during

the 2 experiments averaged 23 C. One egg hatched at 154

days, and another at 192 days, further substantiating the

capability of differential hatching rates and possible egg

diapause ability. Low hatching success with this method

could have been due to the short, 5 min contact time of eggs

with sperm. Clemens (1917) and Pescador and Peters (1974)

successfully used this fertilization method with I. bicolor

and Baetisca rogersi Berner, respectively.

Less than 1% of the eggs from the forced copulation

experiment hatched after a period of 31 days incubation at

simulated stream conditions. However, since 2 partheno-

genetic eggs hatched at 41 days, it is not known whether the

forced copulation method was successful, as the c never

grasped the T for longer than 1 sec. Huff and McCafferty

(1974) achieved good results using this method with Stenonema

femoratum (Say). The hatching of 2 unfertilizedeggs of I.

sicca was similar to results reported by Sweeney (1976), who observed 1 of ca. 30,000 unfertilized 1. bicolor eggs to hatch. Many additionalinfertile I. sicca eggs developed eye

spots, but failed to hatch over a 9 mo period. Partheno- genesis has been demonstrated in Ameletus ludens Needham 22

(Clemens 1922), Ephoron album (Say) (Britt 1962), Baetisca

rogersi (Pescador and Peters 1974), Stenonema femoratum, S.

interpunctatum (Say), S. pulchellum (Walsh), and S. vicarium

(Walker) (Huff and McCafferty 1974); Cloeon triangulifer

McDunnough (Gibbs 1977); and Baetis macdunnoughi Ide, B.

hageni Eaton, B. frondalis McDunnough, B. spinosus McDunnough,

(Bergman and Hilsenhoff 1978).

These qualitative experiments established that I. sicca

eggs (1) display a differential hatching rate of 11-192

days at simulated stream and constant room conditions, (2)

have potential diapause ability, and (3) are capable of

parthenogenetic development. Additional quantitative egg

experiments, run for longer periods of time at several tem-

perature regimes and simulated conditions, are needed to

determine (1) which conditions induce and break diapause, and

influence differential hatching rates; (2) if the observed

differential hatching rate and apparent low parthenogenetic

success is a genetic phenomenon; and (3) maximum diapause

duration capability. Also, methods of obtaining adequate

numbers of fertile eggs for such experiments must be sought.

Nymphs

Eclosion was preceeded by nodding movements of the

nymphal head, followed by its sudden burst through a split

in the egg (Fig. 8). The nymph remained motionless in this position for ca.4 min, then began squirming from the egg, 23

Figs. 8, 9. Eclosion sequence of I. sicca, 31X 24 25

with its legs held close to the body (Fig. 9). When nearly

free, the legs were extended for support. The nymph walked

away from the egg, pulling free the abdominal tip and cerci.

The entire eclosion process lasted ca. 5-6 min.

Laboratory-hatched nymphs were successfully reared to

the 3rd instar. The 1st stadium of all nymphs lasted 2 days

at 23 C, and the 2nd averaged 4.2 days (ranged 3-5). Clemens

(1917) reported that the 1st stadium of I. bicolor lasted

6 days. Third instars died 1-3 days after the 2nd molt.

Attempts to rear small and mid-size, field-collected nymphs

to maturity in the laboratory were unsuccessful. First and

2nd instars closely fit the description for those of I. bicolor (Ide 1935), having no evident gills and 3 differen- tiated antennal-and caudal filament segments.

Gill lamellae (as tiny finger-like extensions), fibril-

lar gills and wing pads first became evident in field-col-

lected nymphs at 0.17, 0.40 and 0.73 mm head capsule width

(hcw), respectively. Gills of mature I. sicca nymphs resembled those of I. bicolor (Needham et al. 1935:Fig. 65).

Forceps first appeared on S'sof 0.90 mm hcw, and their eyes were consistently larger than S s in later instars. The number of hairs on the front legs increased after molts to the 2nd and 3rd instars; this trend continued until nymphs had acquired the full complement of hairs necessary for efficient filter feeding. 26

Nymphal sex ratio of pooled samples was 3:5 (713:

1139). Females may undergo a longer developmental time, and this ratio may thus compensate for a higher expected mortality rate.

Second instar nymphs fed by removing detritus, which had accumulated on the tarsi from their movements in the rearing dish. The leg was bent at the tibio-femoral joint, bringing the tarsus close to the mouthparts for detritus removal and consumption, and only 1 tarsus was cleaned at a time. These nymphs also fed by actively lowering their heads to graze on detritus in the rearing dish.

The front legs of middle and late instars are equipped with 2 long hair rows on the mesal margin of the femur, tibia, and tarsus (Edmunds et al. 1976:Fig. 45). These hairs are used as a seine to filter out small particulate matter from the water. Field-collected nymphs, observed in aerated aquaria and in the field, faced the current with outspread front legs. The current separated the hair rows, forming a v-shaped seine. Filtered particles were transferred to the hairy mouthparts by rhythmically bending each leg at the tibio-femoral joint. This action allowed the current to

collapse the hair fringe, forcing filtered particles into the mouthparts, where they were entrapped and then collected by the palpi. Legs were always flexed alternately. One nymph, with a missing front leg, was observed filter feeding with the other in the normal manner. The tarsal cleaning and 27

grazing behavior, reminiscent of the 2nd instar, was ob-

served for 1 late instar nymph crawling on a dense mat of

algae in an aquarium. Mouthparts of late instar nymphs resembled those of I. bicolor, illustrated by Needham (1905: Plate 6).

These observations indicate that I. sicca may shift

from predominantly grazing in early instars to filter feeding

in later instars. However, once filter feeding behavior is

established, the nymph does not lose it ability to graze, as

shown by the large nymph grazing on algae. This flexibility would be advantageous for I. sicca in an intermittent stream, permitting filter feeding during normal flows and grazing in pools during dry periods. This possibility was not established in field situations during the study, since timing of stream flow termination in both years occurred just after the major emergence, when large nymphs were naturally decreasing. Sur- vival of larger nymphs in pools in spring and early summer during drought years would further substantiate this hypoth- esis,

Inspection of 10 nymphal fore- and midguts collected in

Elm Fork during late fall and early winter, 1977, indicated that I. sicca was predominantly a detritivore, occasionally ingesting algal cells or diatoms. Two arthropod legs and a head capsule were found in 1 gut, and a microcrustacean exoskeleton in another. Needham (1905), Morgan (1913), and

Clemens (1917) reported I. bicolor occasionally ingested 28 simulids, chironomids and mayflies. Shapas and Hilsenhoff

(1976) reported that guts of unidentified Isonychia nymphs contained 97% detritus and 3% diatoms. Coffman et al.

(1971) found ca. 60% of the caloric ingestion of I. bicolor to be animal, 25% algae and 15% detritus. Clear Creek nymphs, frozen for gut analysis, were so fragile after thawing that guts could not be successfully extracted for analysis.

One of 2 final instar guts from Elm Fork, Oct. 1977, was completely empty; only the posterior half of the other was full. Final instars were never observed feeding in the laboratory or the field. This probably allows purging of the alimentary canal prior to emergence, thereby lightening the flight load.

I. sicca is probably a nonspecific feeder, ingesting whatever is collected in the hair seine or during grazing.

Some reports of carnivory (including this one) may have resulted from ingestion of drifting exuviae, collected during filter feeding.

A frequency histogram was constructed from the 4469 nymphs collected during the study (Fig. 10) in an attempt to arrive at an estimate of instar numbers. No well-defined, instar separations were evident, and there was a considerable amount of overlap. Measurement of the reared first 3 instars substantiated this. Similar overlap was reported for

Baetisca rogersi (Pescador and Peters 1974) and Choroterpes mexicanus Allen (McClure and Stewart 1976). A frequency 29

Fig. 10. Size frequency histogram of head capsule widths for 446 9 I. sicca nymphs, Clear Creek, 1976-77. (1 micrometer unit = 0.0244 mm). 30

I-

-0 (X

z 0

0 fl0 t 0 Lo 0 to N~ 0 0It) M

SHdIAN A10"0N 31 histogram, constructed for only the overwintering nymphs

(Fig. 11), in an attempt to avoid overlap between generations or broods caused by a temporal decrease in nymphal size, as

H 0 temperature 2 increased during the emergence cycle, also produced no reasonable separation.

The size of final instar nymphs gradually decreased throughout the emergence period (Fig. 12), with 9 always larger than 5. This decrease has also been described in

Baetisca rogersi (Pescador and Peters 1974), Epeorus pleuralis (Banks) (Minshall 1967), Leptophlebia cupida (Say)

(Clifford 1970), and Baetis scambus Eaton (Elliot 1967).

Table III summarizes Clear Creek conditions and I. sicca events during the 1976-78 study period. Seasonal growth and size distribution of nymphs are shown in Fig. 13.

A small population of nymphs appeared in Oct. 1976, within

1 mo of rehydration; they grew through the winter and began emergingin late Apr. (Fig. 13, solid line). These nymphs hatched from short-diapausing eggs, deposited during the previous summer, 1976, since intensive sampling of pools during that dry period yielded no nymphs.

Recruitment of small numbers of early instars continued

Oct.-late Nov. from those same summer 1976 eggs, that continued to break diapause differentially. All of these nymphs will hereafter be called Generation I, overwintering brood

(GI-ow). The Dec. population exhibited the greatest size range, including 3 black wing pad nymphs (lS,2) which 32

Fig. 11. Size frequency histogram of head capsule widths for the 1976-77 overwintering nymphs of I. sicca. (1 micrometer unit = 0.0244 mm). 33

0 0

0 C/

O F- z

0 w 0 CC)

-mo- *0 I- U, 0

2 0

4 No r*0 ),.

am 0 i Go -U

U 0 1 In N (D ro 0 SHdNAN JO0ON 314

Fig. 12. Average final instar size of I. sicca during the 1977 emergence period at Clear Creek. 35

- 2.3 2. = I.-- w 2.1

- 1.9

1.6

IV V VI Vil 36

U) H 0) Uo 0 00 S H H 0 q H) U) 0 Cd H 0 z 0

co Cd

S cc _ > Cd Cd 2H

HI Cd 4-)~ Cd H0

E10 H- Cd F70, ~ OCo H 00 LA cc H H r H 4 I H Z a o t ~ o~ Cd Co D H) H0 0 ro

H H) Cd C-) U 4-)p O H 0

01\r._ C Hr13

0D 00100 U) t-- Z Z

rli% 0 Cd 0 'H H O C4 0 Cd 0 H 37

Fig. 13. Seasonal size distribution of 4469 I. sicca nymphs, Oct. 1976-Aug. 1977, Clear Creek. Vertical line above 0.90 mm size class separates J's (left) and _'s (right). Solid curved line = proposed Generation I, overwintering brood (GI-ow) from fall-broken diapause; dashed line = proposed Generation I, spring brood (GI-s) from spring-broken diapause; dotted line = proposed Generation II (GII). 38

0 y>-

LO *c'j (.0 '

C\J0

HZZ

z ui 1Li

(0 o OD

z tb 6 Lb 6 o ( Wj.Cj - H (NN 1Oi'x) HACIMv GV3H 1lVHdL/'4N 39

could have been included in early-emerging adults and would

have contributed a 2nd generation. Dec. water temperatures

in Clear Creek ranged 8-12 C (air temperature -9-22 C) after

the appearance of these black wing pad nymphs, and emergence

probably did not occur until Apr., when they became synchro-

nized with slow developers. No nymphs were found in Jan.,

indicating the possibility of their elimination. Sweeney

(1976) found that only 5% of the black wing pad I. bicolor

nymphs exposed to a 10-12 C thermal regime successfully

emerged; none emerged at the 8-10 C regime. He also found

that mature nymphs collected in Dec., and reared at ambient

stream temperatures experienced a high winter mortality (>70%), with death occurring during ecdysis.

Diapause termination of the remaining 1976 eggs led to

a large recruitment in mid-Mar. 1977, with nymphs growing

quickly and emerging late May-early Jun. (Fig. 13, dashed

line). This hatch preceeded any possible significant

emergence (source of non-diapausing eggs), and therefore

resulted from long term 1976 summer-winter diapausing eggs.

These shall hereafter be called Generation I, spring brood

(GI-s).

Clemens (1922) suggested that the diapausing eggs of

Ameletus ludens enabled it to survive summer dry periods,

and Williams and Hynes (1976) reported that Paraleptophlebia

ontagrio (McDunnough) probably had a similar strategy. Britt's

(1962) experiments proved Ephoron album eggs were capable of 40 withstanding a limited amount of dessication. Heptagenia

(naculipennis?) and Ameletus lineatus Traver were found after a summer dry period, during 2 consecutive spring seasons

(Clifford 1966). Summer diapausing eggs have also been reported for 2 stonefly species in Clear Creek: Hydroperla crosbyi (Needham and Claassen) (Oberndorfer and Stewart 1977) and Perlesta placida (Hagen) (Snellen 1978).

The dotted line in Fig. 13 represents the short summer

Generation II (GII). Non-diapausing eggs from GI-ow and/or

GI-s adults began hatching in early May and a substantial recruitment occurred late Jun.-early Jul. Some of these could also have been recruited from long diapausing 1976 eggs.

However, previously mentioned short-term incubation time

(ca. 2wk)of Jun. 1977 eggs from imagos, substantiate the probable source of this 2nd generation recruitment. Thus, this population of I. sicca, along with I. bicolor (Sweeney

1976) and I. japonica (Ulmer) (Gose 1973), appears to be bivoltine.

Temperature summation has been suggested as a possible explanation of growth curves (Cummins 1974, Stanford 1975).

I. sicca nymphs growing through the winter emerged at a larger size than those in the spring and summer (Fig. 12).

However, if their development was geared to temperature summation, the required amount of heat units (degree days) to mature physiologically should be ca. the same for all nymphs, regardless of brood or generation. Table IV shows 41

TABLE IV

THERMAL SUMMATION FOR GROWTH CURVES OF I. SICCA IN CLEAR CREEK, 1976-77

Time Period Degree Growth Curve (Hatch to Duration Days Description Emergence) (Days) (C) Generation I, Over- wintering Brood 1 Oct.- 205 2658 (GI-ow) (Summer 23 Apr. diapause)

Generation I, Spring :Brood (GI-s) (Summer- 14 Mar.- 72 1570 winter diapause) 25 May

Generation II (GII) 1 May- 68 1801 7 Jul. 42

that the I. sicca GI-ow brood required more degree days than

the GI-s and GII broods. The unknown exact length of egg

diapause in these mayflies may, in part, account for this

lack of conformance with the temperature summation theory.

It is possible that temperature summation operates in some

aquatic species and not in others.

Vertical substrate samples taken in mid-Jul. 1976 and

1977 at Clear Creek, after the stream dried, produced no

nymphs. The water table at this time was ca. 33 cm below

the surface and movement of any nymphs would be highly re-

stricted by the sub-surface packed sand layer. A 6.02 cm

rainfall 1 Aug. 1977 rehydrated the stream. First instars

were found the following day, along with a few larger nymphs,

that were too large to have developed in 1 day. They had

possibly survived the 2 wk dry period in moist areas of sur-

face rubble, or in pools. First instars found at this time

presumably came from mature, diapausing eggs located in the moist areas of surface rubble or in pools. The stream dried

again within 1 wk, and did not run again until Jan. 1978,

due to semidrought conditions. Usual rehydration would have

occurred after fall rains in Sept. Sampling of surface sub-

strate, vertical substrate levels and existing pools yielded no I. sicca nymphs during that dry period. Water table level

in mid-Oct. had dropped to ca. 76 cm below the surface.

Removal of surface rubble did reveal damp areas which may have supported diapausing eggs, but were probably not of 43

sufficient moisture content to support nymphs. Hyporheic

burrowing was dismissed because of the hard-packed sand

beneath the surface rubble, lack of fossorial appendages on

nymphs and absence of nymphs in vertical samples.

Clear Creek rehydrated 11 Jan. 1978, after a 3.15 cm

rainfall, but nymphs were not found until late Mar. Recruit-

ment occurred continually from then through mid-Jun. Emer-

gence began in late May, when 1 final instar exuvium was

found, and continued through mid-Jun., as indicated by the

presence of black wing pad nymphs. This growth pattern

closely resembled that of the 1977 GI-s brood, and Clear

Creek was dry again by 1 Jul. 1978.

Elm Fork flowed throughout the 1976-78 study period

and contained a large population of I. sicca in Sept. 1977.

A light trap was run post-sunset in mid-Sept., and 10 &Is

(6 subimagos, 4 imagos) and 6 * subimagos were taken.

Emergence continued through Oct., as indicated by black wing pad nymphs, and recruitment occurred from Sept.-Oct. How-

ever, populations appeared to synchronize near the 2.20 mm hcw in Nov. (only a few larger nymphs found) and Dec. (no

larger nymphs found). Recruitment did not occur in Dec.

The heavy rains that rehydrated Clear Creek prevented sampling in Jan. 1978, and only 3 nymphs were collected in Feb.

at the synchronized size. No nymphs were found in Mar., even though samples from Mar. 1977 had produced nymphs larger than the synchronized size, indicating a GI-oow brood 44

similar to that of 1976-77 at Clear Creek (Fig. 13, solid

line). Nymphs first appeared in mid-Apr. 1978, and black wing pad nymphs were first found in mid-Jun. This Elm Fork brood was 2- wk later than those of the 1977, 1978 GI-s broods at Clear Creek, Continual recruitment followed from

mid-Apr. through mid-Jun. Emergence occurred from mid-May

through mid-Jun., indicated by black wing pad nymphs.

The unusually cold winter of 1978 may have caused the

synchronization of nymphs in Nov. and Dec. in Elm Fork.

Synchronization also occurred in I. bicolor during the win-

ter (Sweeney 1976). Cold temperatures probably prevented

diapause termination or killed early instar nymphs. The 2.5 mo delay in hatching after Clear Creek rehydrated in 1978 may also have been due to the severe winter.

These observations and data show an amazing plasticity in I. sicca development, needed for survival in the harsh, unstable environment of the intermittent Clear Creek. Its eggs are capable of diapausing through both hot, dry summers and cool, wet or dry winters. A small portion of the eggs break diapause in the fall after rehydration, forming the GI-ow brood. A few eggs break diapause throughout the winter, but mostly in the spring when optimum growth conditions are more predictable for nymphs. If the stream remains flowing, a 2nd generation can develop by Jul. A permanent stream, such as Elm Fork, may continue to produce additional generations until growth is supressed by cold winter temperatures. 45

Further research involving laboratory incubation of eggs

collected from 1's throughout the entire emergence period would be necessary to establish the source of diapausing

eggs for the following generations.

Subimagos

Black wing pad nymphs appeared near the surface on emergent rocks and objects near sunset. Emergence began ca.

20 min after sunset, and continued for ca. 2 h, with peak emergence during the 1st h. Nymphs crawled to the air-water interface with only their heads projecting out of the water.

Abdominal undulations produced a trembling motion of the body, until a longitudinal split appeared on the mesothorax.

The thorax and head were then slowly pushed out by continued undulations, with the head bent ventrad. The wings and legs were held close to the body during the molt, until both were free. Undulations then stopped, the legs took a foothold, and the wings were quickly expanded and held over the body.

They then walked away from the exuvium, pulling the last few abdominal segments and the cerci free. Subimagos took flight immediately after molting and were strongly attracted to light.

The entire molting process was completed in ca. 20 sec.

Occasionally anymph would molt with its body partly or wholly out of the water, but most observed molts and final instar exuviae occurred below the water surface. Clemens (1917) 46

reported that I. bicolor nymphs molted after crawling com-

pletely out of the water.

The subimaginal cerci become twice as long as the

nymphal cerci through elongation of the subimaginal cercal

segments as they are pulled from the nymphal exuvium. This

elongation process was observed under astereoscope several

times. The terminal filament degenerates in the final in-

star. The ks eyes expanded greatly during molting, becoming very bulbous and nearly contiguous dorsally.

Emergence occurred 23 Apr.-ll Jul. 1977, with the peak

on 6 Jun. (Fig. 14), when 2023 subimagos were attracted to the light trap. The 13,619 total subimagos and imagos attracted at. this 1 site over the 15 wk light trapping period suggested an astounding production of I. sicca in this intermittent stream. Both S's and V s were highly attracted to the light trap. Relative numbers of both sexes attracted, throughout the period, followed a very similar pattern, with

V's always outnumbering i's. The same was true for imagos, except that 's always outnumbered &ts.

Only 1 subimago was captured by sweeping low-lying riverine vegetation during the day. It escaped, flew straight up to tree top level (ca. 12 m), crossed the stream, and disappeared into the trees. Subimagos probably seek resting sites in high tree foliage. Subimagos were observed resting in a head-up, vertical position in the laboratory, with the front legs raised and held close together. They would 47

Fig. 14. Light trap populations of I. sicca in Clear Creek, 1977. 48

0 0 00

0000

0: L:o

0 n0 o Co 00 0 0 0 0

G3~iflidV3 O

o 0 0- 0 0

N 0 o (.0 N 49 occasionally take flight when provoked, always flying to a light source, regardless of time of day, or walk a short distance using all 6 legs, and then resume the resting position.

The average size of subimagos decreased through the emergence period, as indicated by hind femoral length (Fig.

15). Females were consistently larger than Jtis. This was a reflection of the same trend in size of mature nymphs

(Fig. 12).

Sex ratio of subimagos was 106:7 (5550:3909), almost a reversal of the 3&:5. sex ratio of nymphs. If both sexes are equally attracted to lights, this suggests that g nymphs must experience a high mortality.

Average subimaginal fecundity decreased markedly through the flight period, from 4440-621 eggs/i, 30 Apr. and

11 Jul. 1977, respectively (Fig. 16). Paraleptophlebia mollis (Eaton) (Ide 1940), Baetis rhodani (Pictet) (Benech

1972), Baetisca rogersi (Pescador and Peters 1974), and

Choroterpes mexicanus (McClure and Stewart 1976) also exhibited this phenomenon. Fecundity was correlated with both head width (Fig. 17, r=0.97) and estimated abdominal volume

(Fig. 18, r=0.97). Estimated volume (assumed cylinder) of dissected subimagos decreased by 85%, and fecundity by 89% over the emergence period, yet those emerging early in the season were completely filled with eggs, while those emerging later in the season had a considerable amount of 50

Fig. 15. Average hind femoral length of I. sicca subimagos and imagos during the 1977 flight period, Clear Creek. 51

2.5

2.4

2 2.3

2.2 wZ 2.1\

a:2.0 w 1.9 \- w I- \ z

. 1.'? (' SUBIMAGOS > IMAGOS S1.6 1 SUBIMAGOS 1.5 IMAGES

1.4 30 17 30 13 27 11 Iv V VI Vil 52

Fig. 16. Average subimaginal fecundity of I. sicca through the 1977 emergence period, Clear Creek. 53

5000

4000 z w 3000 z

4 2000

Cf)

1000

040L 11 30 17 30 13 27 IV V vi Vil 54

Fig. 17. Subimaginal fecundity vs. head width of I. sicca, Clear Creek, 1977 (r=0.97). 55

5000

00 4000

I- 5 0 z wC) 3000 LL rn-i 4 z 2000 * * 0 4

00 C,) 1000-

I I --I 3 0 1, 1 1 1 I I U 1.5 1.7 1.9 2.1 2.3 2.5 HEAD WIDTH (MM) 56

Fig. 18. Subimaginal fecundity vs. abdominal volume of I. sicca, Clear Creek, 1977 (r=0.97). 57

5000

0 4000

0z w 3000 LL I. "I z .. 0 / 2000 *0 .0

Cl) I,0 1000o jr0

O - L I 4 1 I 0 ao 40 60 80 ABDOMINAL VOLUME (MM 3 ) 58

abdominal space. Average and range of subimaginal life span in the laboratory for both sexes were 25 and 23-29 h, respectively.

Imagos

Subimagos became restless just prior to the proximate

molt. They alternately walked and rested, then anchored all

tarsal claws to the substrate. A median, mesothoracic ecdysial split then appeared as the abdomen began to undulate,

and the wings were gradually lowered parallel to the substrate.

This split widened as the wings were pulled along side the

abdomen. The head was bent ventrad while it and the thorax

were pushed out by abdominal undulations. The wings and

legs were held close to the body during this time. The

emerging imago then bent backward, until the wings and legs

were completely free and only the tip of the abdomen remained

in the exuvium. The imago was completely upside down on a

vertical surface at this time. It remained motionless in

this position for a few seconds, then quickly jerked the

body to an upright position, and extended and anchored the

legs on the substrate. The wings at this point were extended

vertically over the body. The tip of the abdomen and cerci were then freed from the exuvium by raising the posterior portion of the abdomen, after which they became quiescent.

The molting time averaged 9.5 min. 59

Female imagos were the 1st arrivals at the light trap

88% of the time, beginning ca. 21 min after sunset. Male

and imagos came to the light trap before subimagos, sug-

gesting that mating probably occurs near sunset. Oviposition probably closely followed, since most light-attracted .ts were devoid of eggs. Peak imaginal attraction occurred ca.

30 min after sunset, 23 Apr.-25 Jul. 1977 (Fig. 14). The relatively fewer e imagos at the light trap could have been

due to high post-mating mortality or differential attraction.

The late Apr.-early May emergence probably came from the

GI-ow brood, the early Jun., peak from the fast growing GI-s brood, and the secondary peak late Jun.-early Jul.from GII.

Overlap undoubtedly occurred between these 3 groups. It would seem most advantageous if imagos of all these groups were capable of producing diapausing eggs. That would insure a continual supply of eggs for diapausing, should the stream dry up at any time during the flight period.

The how and hind femoral length of imagos decreased markedly over the flight period. Mean 2 hcw and femoral length decreased 2.46-2.07 mm (Fig. 19) and 2.05-1.49 mm

(Fig. 15), respectively. Females exhibited a similar trend; their body size and femoral lengths were always larger than 2's, but due to larger eyes, &'s had wider hcw than iT s.

Figs. 15 and 19 show subimagos and imagos on given dates to be about the same size, whereas c imagos were larger than subimagos. TheS hcw increase was due to further expansion 60

Fig. 19. Average head capsule width of I. sicca subimagos and imagos during the 1977 flight period, Clear Creek. 61

2.5

2.4* *~-0 AO UU\ 24

-2.3

~2.I

2.0 4* w 1. 9

1.8 dlSUBIMAGOS....-

I IMAGOS -- - . 1.7 - SUBIMAGOS --

1.6 IMAGOS ..---

1.5 30 17 30 13 27 11 IV V VI Vil 62

of the eyes, at the subimago-imago molt. The eyes are not contiguous dorsally on the subimago, but are on the imago.

Sex ratio of light-attracted imagos was not calculated, since low ; numbers attracted did not seem representative of the population, as indicated by nymphal ratios.

Two S's and 1 - imagos were found during the day resting underneath sycamore leaves. This was the only imaginal resting site found. Males lived an average of 3.4 days in the laboratory (range 2-5), and 2s 4.4 days (range 1-9).

Mating and ovipostion were not observed, even though a

24 h monitoring station was set up near the light trap during peak emergence. A few %-imagos had small egg sacs that were hemispherical in shape, and ca. 1.0 mm diam. A few larger, cylindrical egg sacs measured ca. 3.5 long x 1.0 mm diam.

Fresh, moist egg sacs were light yellow, and immediately fell apart when placed in H20. Older, dried egg sacs were darker yellow, and were slower to fall apart in H20. The oviducts of 2 light-attracted imagos with small egg sacs were full of eggs, which may indicate multiple extrusion of egg sacs. Nematodes were frequently found coiled in the oviducts of dissected ts, and when placed in distilled water, they uncoiled and moved about. Male and ?imaginal descriptions, and &Sgenitalia illustrations are found in Needham et al. (1935) and McDunnough (1931). CHAPTER IV

SUMMARY

1. The life history of Isonychia sicca (Walsh) was determined from observations made and samples taken Oct.

1976-Jun. 1978, in Clear Creek, Denton County, Texas.

Samples were also collected in Elm Fork of the Trinity

River, Denton County, Texas, Sept. 1977-Jun. 1978.

2. Water temperatures during running phases of Clear

Creek ranged 1-30 C. Dissolved oxygen, specific conductance and total alkalinity ranged 6..9-8.4 ppm, 680-4770 pmhos, and 131-270 ppm as CaCO3 , respectively. The pH was relatively stable at 8.0-8.3 and the water was hard at

264-344 ppm as CaCO 3 ' 3. Eggs were spherical and averaged 242 p in diam.

Knob-terminated coiled threads (KCTs) were densely packed on 1 hemisphere. Chorionic tagenoform micropyles were found only on the hemisphere with sparse KCTs. Eggs changed color from transluscent white to opaque brown during development.

4. Fertilized eggs from various sources displayed a differential hatching rate of 11-192 days at ca. 23 C. They remained apparently alive and possibly in diapause for up to 11 mo at simulated stream temperatures. Eggs, shelf-

63 64

dried for 282 days, did not hatch after 2.5 mo incubation. Two parthenogenetic eggs hatched in 41 days at 23 C.

5. Eclosion took ca. 5-6 min, and laboratory-hatched nymphs were reared to the 3rd instar. The 1st and 2nd stadia lasted 2 and 3-5 days respectively at 23 C. Third instars died 1-3 days after the 2nd molt. Gill lamellae, fibrillar gills, and wing pads appeared on field-collected nymphs at 0.17, 0.40 and 0.73 mm how, respectively. Second instars primarily fed in the laboratory by grazing; later instars fed primarily by filter feeding. Guts contained mostly detritus, with occasional algae, diatoms .and animal remnants.

6. A frequency histogram from 4469 nymphs produced no distinct instar separations. Considerable overlap, from probable instar size variations, precluded estimation of instar number. Size of final instar nymphs decreased markedly during the Apr.-Jul. emergence period: c"s from

2.08-1.74 mm, and ?'s 2.24-1.83 mm.

7. A Generation I, overwintering brood (GI-ow) was produced in Oct. 1976 from summer 1976 eggs, after an inter- vening Jul.-Sept. dry period. Egg diapause through that period was the only plausible explanation for survival, since neither adults nor nymphs were present in the few pools or substratum. Differential diapause termination contributed small recruitment from Oct.-Nov. that year.

Synchronization of these overwintering slow and fast de- 65

velopers contributed to the early Apr. emergence.

8. A Generation I, spring brood (GI-s) was produced in mid-Mar. 1977 from diapause termination of the remaining summer 1976 eggs, that had diapaused through summer-winter

1976-77. They grew rapidly and emerged late May-early Jun.

9. Generation II (GII), of bivoltine I. sicca, repre- sented a short summer generation produced from non-diapausing eggs from the GI-ow and GI-s broods. Nymphs grew rapidly, emerging in late Jun.-early Jul.

10. An extended drought, Jul. 1977-Jan. 1978, occurred on Clear Creek after the 1977 GII emerged. Eggs undoubtedly survived that period in diapause, and recruitment of 1st instars did not occur until Mar. 1978, a delay of 2.5 mo after rehydration. The streamdried again by 1 Jul. 1978 at termination of the study.

11. Temperature summation did not seem to be a feasi- ble explanation for development of these diapausing mayflies, since degree days calculated for the GI-ow brood

(2658) were considerably more than those for the GI-s brood

(1570) and GII (1801).

12. Mature nymphs crawled to emergent objects and molted just below the water surface, with only the head projecting out of the water, during the 1st h after sunset.

The nymph-subimago molt lasted ca. 20 sec. Emergence, as indicated by light trap samples, occurred 23 Apr.-11 Jul.

1977. Male subimagos always outnumbered Is; 13,619 66

subimagos and imagos were captured at the light trap over the 15 wk period. Subimagos apparently flew into the streamside forest canopy to await the final molt.

13. Size of subimagos decreased markedly throughout the emergence period, corresponding with a similar decrease in nymphal size. Sex ratio was 103: 7 , almost a complete reversal of the 36:5 nymphal ratio. Fecundity of subimagos decreased from 4440-621 eggs/?, 30 Apr. and 11 Jul.

1977, respectively. This decrease correlated statistically

(r=0.97) with decrease in head size and estimated abdominal volume.

14. The subimago-imago molt required ca. 9-10 min.

Peak imaginal attraction to light occurred ca. 30 min after sunset. Head capsule width and femoral length of imagos decreased markedly over the flight period. Rating and oviposition were not observed, though spent condition of most light-attracted imagos suggested that mating and oviposition should have occurred near sunset.

15. I. sicca exhibited a highly flexible bivoltine life cycle, with an egg diapause, enabling it to survive normal summer intermittency and probably extended periods of drought. BIBLIOGRAPHY

American Public Health Association. 1976. Standard methods for the examination of water and wastewater. American Public Health Association, Washington, D.C. 1193 p. Anderson, R. 0. 1959. A modified flotation technique for sorting bottom fauna samples. Limnol. Oceanogr. 4(2): 223-225.

Benech, V. 1972. La fecondit6 de Baetis rhodani Pictet. Freshwater Biol,. 2(4):337-354.

Berner, L. 1950. The mayflies of Florida. Univ. Fla. Stud., Biol. Sci. Ser. 4(4):1-267.

Berner, L. 1959. A tabular summary of the biology of North American mayfly nymphs (Ephemeroptera). Bull. Fla. State Mus., Biol. Sci. 4(1):1-58.

Bergman, E. A. and W. L. Hilsenhoff. 1978. Parthenogenesis in the mayfly genus Baetis (Ephemeroptera:Baetidae). Ann. Entomol. Soc. Am. 71(2):167-168. Britt, N. W. 1962. Biology of two species of Lake Erie mayflies, Ephoron album (Say) and Ephemera simulans Walker. Bull. Ohio Biol. Surv., N. S. 2(5):1-70.

Brodsky, A. K. 1973. Swarming behavior of may-flies (Ephem- eroptera). Entomol. Obozr. 52(l):51-62. Burks, B. D. 1953. The mayflies, or Ephemeroptera, of Illinois. Ill. Nat. Hist. Surv., Bull. 26:1-216.

Clemens, W. A. 1917. An ecological study of the may-fly Chirotenetes. Univ. Toronto Stud., Biol. Ser. 17:1-43. Clemens, W. A. 1922. A parthenogenetic mayfly (Ameletus ludens Needham). Can. Entomol. 54(4):77-78.

Clifford, H. F. 1966. The ecology of invertebrates in an intermittent stream. Invest. Indiana Lakes Streams. 7(2):57-98.

Clifford, H. F. 1970. Variability of linear measurements throughout the life cycle of the mayfly Leptophlebia cupida (Say) (Ephemeroptera:Leptophlebiidae). Pan-Pac. Entomol. 46(2):98-106.

67 68

Cloud, T. J., Jr. and K. W. Stewart. 1974. The drift of mayflies (Ephemeroptera) in the Brazos River, Texas. Kansas Entomol. Soc. 47(3):379-396.

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