Great Basin Naturalist

Volume 37 | Number 2 Article 12

6-30-1977 The life cycle of Hydroperla crosbyi (: Perlodidae) Reed Y. Oberndorfer North Texas State University, Denton and Brigham Young University

Kenneth W. Stewart North Texas State University, Denton

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Recommended Citation Oberndorfer, Reed Y. and Stewart, Kenneth W. (1977) "The life cycle of Hydroperla crosbyi (Plecoptera: Perlodidae)," Great Basin Naturalist: Vol. 37 : No. 2 , Article 12. Available at: https://scholarsarchive.byu.edu/gbn/vol37/iss2/12

This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. THE LIFE CYCLE OF HYDROPERLA CROSBYI (PLECOPTERA: PERLODIDAE)'

Reed Y. Oberndorfer- and Kenneth W. Stewart-

ABSTRA.CT.— Hydroperla croshiji in Texas exhibited a univoltine, fast hfe cycle over the three-year study period. Adults emerged in February-March when mean daily stream temperature reached ca. 15 C. Reared females in the lab deposited up to three egg masses. Mean fecundity of dissected females was 787 eggs/female. Oviposition in the field was observed and described. Eggs were triangular in cross section, brown, and measured 400 jiim X 535 jum. They underwent an ca. seven- month diapause until mean daily stream temperature decreased to 18 C. Eyespots appeared, and hatching fol- lowed in two to three weeks. First instar nymphs were measured and described. Male and female nymphs under- went 12 and 14 instars, respectively, and could be sexed by the sixth. Fast growth occurred in the coldest season and Simuliidae and Chironomidae larvae were preferred food throughout development. Eggs contained a mean of 6.21 cal/mg. Ash-free mean caloric value of last instar nymphs was 6.0 cal/mg. .\dult males and females lost 33.8 percent and 57.6 percent, respectively, of their caloric pool through their ca.

12 days of life.

The life cycles and ecology of North occurring only in large, permanent rivers. American Plecoptera are relatively un- Life cycles of southwestern should known. The classic works of Needham and differ markedly from those of boreal species Claassen (1925), Claassen (1931), Frison (Harper and Hynes 1972, Harper 1973a, b), (1929, 1935, 1942), and Kicker (1952) were due to post-Pleistocene adaptation to drier, primarily taxonomic, with a few largely de- warmer climates, and the effects of stream scriptive notes on morphology, life cycle permanence and physicochemical condi- events, and ecology. Detailed early studies tions. were made by Wu (1923) for Soyedina val- Hydroperla crosbyi (Needham and Claas-

licularia (Wu) and Holdsworth (1941a, b) sen) is a large perlodid stonefly that is wide- for AUonarcijs proteus (Newman). Recent ly distributed throughout the Mississippi papers by Minshall and Minshall (1966), Valley from Indiana and Illinois to Arkansas Harper and Pilon (1970), Schwarz (1970), and Oklahoma (Ricker 1952). Stewart et al. Tarter and Krumholz (1971), Radford and (1974) first reported the .species from Texas.

Hartland-Rowe (1971a, b). Harper and It is common in streams across the Black- Hynes (1970, 1972), Harper (1973a, b), land Prairie into the Edwards Plateau and Vaught and Stewart (1974), Stanford (1975), Eastern and Western Cross Timbers in

Gather and Gaufin (1975), Hynes and Hynes Texas, and it appears to be a dominant spe- (1975), and Hynes (1976) are indicative of a cies in the macrobenthic community of resurgent interest in Plecoptera life cycles Clear Creek in Denton County, Texas, as supportive knowledge for structural and where this study was made. Presence of functional studies at the population and small nymphs in December, and larger community level. nymphs and adults in March and April, had The only detailed autecological study of a indicated a fast cycle (Hynes 1970). No pre- southwestern Nearctic stonefly was reported vious life cycle studies have been published by Vaught and Stewart (1974) for Neoperia for any Hydroperla species. Stewart and chjtnene (Newman), a widespread species Stark (1977) described the reproductive mor-

Study supported in part bv U.S. Env Asency Traiiieeship T90()115 and North Texas State University Faculty Research Fund Grants .34866 and 34905.

'Department of Biological Sciences, North Te Texas 76203. Oberndorfer's current address: Department of Zoology, Brig- ham Young University, Provo, Utah 84602.

260 June 1977 Oberndorfer, Stewart: Stonefly Biology 261

phology and mating behavior of H. crosbyi, present, and a larger (1 mm) mesh kick net and Prison (1935) reported nymphal micro- was used for later instars. Specimens were distribution and food habits. The purpose of either preserved in 70 percent isopropyl al- this research was to make a thorough aute- cohol or transported live to the laboratory cological investigation of H. crosbyi, with for rearing studies. Six quantitative samples emphasis on its hfe cycle. were taken each month from November 1975 to March 1976 by kicking up an area of 25 cm- in front of a fine jum) mesh Study stream (363 net; care was taken to sample all typically All sampling and observations were done available habitat types, including leaf debris at several sites along Clear Creek in Denton and rocky substrate. These were preserved County, Texas, from Highway 455, ca. 0.5 in 70 percent isopropyl alcohol and taken km west of Bolivar, Texas, to the bridge on to the laboratory for sorting and identi- Highway 428, ca. 4 km northeast of Den- fication of ail organisms. Last instar exuviae ton. Clear Creek originates in Montague were collected in March 1976, to aid in a County, Texas, and flows through Cooke more precise estimate of the adult sex ratio and Denton counties, merging with Elm since adults were difficult to obtain in the Fork of the Trinity River ca. 5 km above field. In an attempt to find eggs or smaller

Lake Lewisville. Stream discharge is highly instars, 5 liters of damp substrate from areas variable, depending upon season and rain- with no surface flow and from pools were fall, with the highest levels usually occur- collected monthly during the period of ring in spring and lowest levels occurring March through September 1974, preserved during the summer. In dry years, the stream in 99 percent isopropyl alcohol, and becomes intermittent during July-Septem- brought to the laboratory for elutriation ber. The substrate consists mainly of sand, (Stewart 1975). with gravel and rock rubble riffles. Packs of Aspects of adult emergence, mating, and leaves and other debris collect on large oviposition were determined from field and rocks and fallen tree limbs in riffle areas. laboratory observations. The longevity of

The streamside forest is dominated by Cot- laboratory-reared adults was determined by tonwood {Populus deltoides), sycamore {Pla- holding them in 8-dram, cotton-stoppered, tanus occidentalis), black willow {Salix glass shell vials at room temperature, ca. 24 nigra), and several species of elm {Ulmus C. spp.). As leaves become conditioned by mi- Caloric values of last instar male and fe- crobes, a successive invasion by macro- male nymphs and their exuviae, newly benthos, including shredders, collectors, emerged virgin males, spent (postulated) scrapers, and predators, ensues (Cummins males, newly emerged virgin females with 1974). The role of H. crosbyi in this se- ova and after ova removal by dissection, quence was suspected to be that of a domi- spent females (after oviposition), and depos- nant macropredator. ited egg masses were determined and com- pared. Energy allocated to reproduction (Pr) was equated to the caloric contents of the Materials and methods total amount of eggs deposited by a female. Seasonal growth, instar analysis, food An instantaneous measure of reproductive habits, sex ratios, and numbers/m- were de- eff^ort (IRE) was expressed as a percentage termined from quantitative and qualitative of energy going to production of eggs (Pr) samples taken from January 1974 through and to growth (Pg). After obtaining live March 1976. Biweekly qualitative samples weights, individuals were dried in a vacuum were taken from October through March in oven at 80 C (-15 psi) for 24 hours, each year except in October and November weighed, ground with a mortar and pestle, 1974, when heavy fall rains prevented ac- and pressed into pellets of known weights cess to the stream. A fine (363 jum) mesh (10-20 mg).The pellets were combusted in a net was used when earlier life stages were Gentry-Weigert modification of a Phillipson 262 Great Basin Naturalist Vol. 37, No. 2 microbomb calorimeter (Phillipson 1964). electivity (E) (Ivlev 1961) were calculated The drv weight of material not combusted for determination of food preferences. was considered to be ash weight; therefore, Stream temperature was taken on each caloric data were expressed as calories/mg sampling date with a total immersion ther- ash-free dry weight. Organisms were not mometer. A seven-day continuous recording bombed individually, but pooled to produce thermograph (Tempscribe remote bulb re- the pellets. corder) was placed in the stream in June Groups of eggs deposited in March by 1975 and left for the remainder of the reared, mated females were incubated in study. Stream flow was taken on each sam- 1974 and 1976. They were held in loosely pling date from August 1975 to March covered 8-dram vials at 22 ±2 C until Octo- 1976, using a Kahl Pygmy Flow Meter cali- ber (1974) and September (1976), then brated at 0.4714 meters per revolution. Sea- transferred into a Percival E-50 environ- sonal water samples were taken and ana- mental chamber, lowered 1 C per day down lyzed for pH, conductivity, and alkalinity to 18 C. according to standard methods (American Public Health Association Eggs "in uteri" and deposited eggs were 1971). counted, photographed, measured, and de- scribed, with special attention given to any Results and Discussion gelatinous layers and/or other attachment During the study, pH varied from 7.6 structures and chorionic sculpturing (Stew- (summer 1975) to 8.1 (winter 1976), con- art et al. 1969). Scanning electron micro- ductivity 530 iumhos (fall 1975) to 1,180 graphs were made using an IS! Mini-SEM. jumhos (summer 1975), and total alkalinity First instar nymphs, hatched from eggs 212 ppm (fall 1975) to 408 ppm (winter obtained in March 1974, were preserved in 1976). Stream flow was highly variable, 70 percent isopropyl alcohol for drawings with the highest level observed in October- and descriptions. The interocular distance of November 1974 and no flow in June-August these and field-collected nymphs was meas- 1974. From August 1975 to March 1976, ured using a calibrated ocular micrometer, stream flow remained relatively constant at and sex was determined when possible. Live ca. 1.4 m/sec. Temperature ranged from 30 nymphs were placed in styrofoam cups with C in June 1974 to 2 C in February 1976. creek water and kept in the environmental Such highly variable and intermittent condi- chamber at simulated creek temperature tions of Clear Creek are typical of moder- and photoperiod to determine size changes ate-sized streams on the western edge of the and to rear them to adults. After one sam- temperate deciduous forest. Life cycle ad- pling date in February 1976, 40 last instar justments of stoneflies such as H. crosbyi nymphs were divided into three groups and and Pcrlesta pkicida (Hagen) through natu- kept at three constant temperatures, 7 C, ral selection, which have enabled success in 15 C, and 25 C, respectively, to determine such harsh environments, were of prime in- the effect of temperature on the time re- terest in this study. quired for development and emergence. Other dominant associated with H. Size-frequency histograms, constructed crosbyi in Clear Creek included Isomjchia from interocular distances of field-collected sicca manca (Eaton), Choroterpes (Neo- nymphs, and the of methods Cassie (1954) choroterpes) mexicanus (Allen), P. pkicida, and Janetschek (1967) were used to inter- Corijdalus cornutus (Linnaeus), Hijdropsijche pret growth and instar development. To simulans (Ross), Cheumatopsyche campyla supplement these analyses aid in and inter- (Ross) and C. lasia (Ross). pretation, field-collected nymphs of differ- Adult.— H. crosbyi displayed a relatively ent sizes were reared to successive instars. spontaneous emergence, with adults occur- Food habits were determined by foregut ring from mid-February through March dur- analysis (Richardson and Gaufin 1971). For- ing the three-year study period. A similar age ratio (FR) (Hess and Swartz 1941) and recurrent emergence pattern has been re- 1977 Oberndorfer, Stewart: Stonefly Biology 263 ported for several stonefly species (Nebeker tion. Similar emergence has been reported 1971b, Miiller 1973, Kerst and Anderson for Paragneiina media (Walker) (Tarter and 1974). Average daily stream temperatures at Krumholz 1971) and transmarina the onset of emergence were ca. 15 C with (Newman) (Harper 1973a). Adults then ei- 19 C maxima and 13 C minima. Emergence ther fly or walk to streamside vegetation or was essentially over and nymphs gone when debris near the water. Preemergent nymph- average daily stream temperatures reached al sex ratios were ca. 1:1, but collections of 22 C. As emergence progressed, there was a exuviae in the middle of the emergence pe- corresponding decline in nymphal popu- riod in March 1976 favored females,

lations. These observations and results of 1.0 cf :1.5 ? , indicating at least a slightly laboratory holding of mature nymphs in en- protandrous emergence. Schwarz (1970), vironmental chambers suggest that the Heiman and Knight (1970), Sheldon (1972), mean daily stream temperature cue for and Harper (1973a, b) have reported similar emergence in H. croshiji is ca. 15 C. Field- findings for other species. collected, mature nymphs held at 7, 19, and A seasonal size variation was exhibited in 25 C showed a highly significant difference adult H. croshiji. The interocular distance (P<0.01) in the mean number of days to decreased as emergence continued from a emergence, with most time(X = 44.8 days) mean of 2.0 mm to 1.6 mm for females, and at 7 C, shortest successful emergence (X = from 1.7 mm to 1.4 mm for males. Khoo 25.6 days) at 19 C, and 100 percent mortal- (1968a), Schwarz (1970), Sheldon (1972), ity at 25 C. and Cather and Gaufin (1975) have shown

The emergence of some stonefly species is similar variation, indicating that the emer- apparently dependent on the number of de- gence cue hurries smaller specimens so that gree days or temperature accumulation dur- full size is not attained (Khoo 1968a). ing nvmphal development (Cummins 1974, Adults did not feed, but they were ob- Stanford 1975). Several investigators have served to drink water; Hynes (1976) indicated that events such as emergence and suggested this is probably true of all adult hatching result when a certain temperature Systellognatha. Dissection indicated that the cue is attained during periods of increasing digestive tract was much reduced. or decreasing temperatures (Sheldon and A period of two to five days is required Jewett 1967, Heiman and Knight 1970, Har- for maturation of eggs in adult females. Iso- per and Pilon 1970, Nebeker 1971a, b, Rad- perla clio (Newman) (Harper 1973a), Iso- ford and Hartland-Rowe 1971, Sheldon perla goertzi lilies, and Diiira hicaudata 1972, Harper 197.3a, b). The emergence re- (Linnaeus) (Schwarz 1970) require a similar sponse range (tolerance) to such cues is nar- egg maturation period. Males find females row in nymphs such as H. croshiji and oth- by a tactile or possibly by a pheromone re- ers, effectively synchronizing emergence sponse, and mating takes place on stream- into a short time period (Macan 1958, side vegetation or debris. Reproductive Nebeker and Gaufin 1967, Nebeker 1971c, morphology, mating, and sperm transfer Hynes and Hynes 1975). Since accurate have been described bv Stewart and Stark temperature data were not available over (1977). Sperm transfer in this species is ex- successive seasons, and a major egg-in- ternal, and the aedeagus does not function cubation experiment was lost due to envi- as an intromittent organ. The species exhib- ronmental chamber malfunction, it could its polygamous mating habits. Mated fe- not be determined whether temperature ac- males flew to tree branches from 5 to 10 m cumulation (Baskerville and Emin 1969) in- above the stream on ca. the fifth day after fluenced the development and emergence of emergence. An egg mass was extruded from H. croshiji. the genital opening located at the rear of Collections of cast exuviae indicated that the abdominal sternum 8. Between ca. one mature nymphs crawled out of the water to one-half hour before sunset, the female onto rocks or debris above the surface of began her oviposition flight, approaching the water or along the bank for transforma- the site from upstream. She glided down 264 Great Basin Naturalist Vol. 37, No. 2 and rested briefly on smooth water from 2 cantly different from females in all stages to 10 m from the head of a riffle. Contact (Student's t; P<0.05). Few calories (16.34) with water and a slight movement of the appear to be lost in transformation, through abdomen resulted in separation of the mass, loss of the exuvium (mostly epicuticle). which then sank. The female was carried by Males lose 33.8 percent (27.13) of their ca- the current ca. 1 m during this process, then loric pool as metabolic heat through their rose off the water, flying back into adjacent ca. 12 days of life (Table 1). trees. Laboratory-held females extruded up A mean of 58.99 calories (deposited egg to two more egg masses over the next three masses) constituted the energy allocated to to five days. Mean life span of mated adult reproduction (Pr) by females. However, a males and females held in the laboratory at difference of 110.45 calories between newly simulated light and temperature conditions emerged virgin females with ova and newly was 11.4 and 11.6 days, respectively. emerged virgin females with ova removed Mean numbers of eggs in successive mas- by dissection indicated that 51.46 calories ses for reared females were 330, 235, and might be resorbed by the female from un- 172, respectively. Fecundity of dissected, deposited eggs to be used for maintenance gravid females was from 442 to 1,418 and metabolic costs involved in mating and eggs/ $ , with a mean of 787/ ? . Smaller oviposition. Energy allocated to growth (Pg) females generally contained fewer eggs. was estimated as the mean biocontent of a Deposited eggs had the highest caloric newly emerged virgin female with ova re- value (6.21 cal/mg) of any stage in the life moved (106.95 calories), since the first instar cycle (Table 1). Mean caloric value of ca. was very small and its calories negligible. 6.0 cal/mg ash-free for late instar H. An instantaneous reproductive effort (IRE) crosbiji nymphs generally agree with those was thus calculated as: Pr (58.99)/[Pr of Cummins and Wuycheck (1971) for im- (58.99) -I- Pg (106.95)] = 35.5 percent. An- matures of various aquatic insects, McDif- other measure of IRE is the ratio of Pr to fett (1970) for Pteronarcys scotti, Lawton female carcass calories. Expressed this way, (1970, 1971) for Pyrrhosoma mjmphula and IRE of H. crosbyi was: Pr (58.99) /female its prey, Hofsvang (1973) for Tipula excisa, carcass calories (106.95) = 55.1 percent. and Brown (1974) for Corydalus cornutus. These values compare favorably with RE's Mean ash-free cal/mg of males were signifi- determined by Brown (1974) for C. com-

Table 1. Caloric values of major stages in the life cycle of H. Crosbyi.

Material June 1977 Oberndorfer, Stewart: Stonefly Biology 265

titiis. A later discussion of food habits of H. 1949, Hynes 1970). A gelatinous mushroom- crosbyi will show that its general food re- shaped attachment structure, emerging from sources are similar to those of C. cornutus. the crown area of the egg, was prominent Females lose 57.6 percent (125.21) of their for at least the first 48 hours after depos- caloric pool as metabolic heat and Pr dur- ition. Its "carapace" portion appeared as a ing their adult life. shriveled mass in older eggs in water and

Egg.—Eggs are oval shaped, triangular in those preserved in alcohol (Fig. 1). Both the cross section, brown in color, and an aver- envelope and the attachment mushroom age 400 jum wide by 535 jum long (Fig. were very effective in gluing eggs to glass

1). They are similar to descriptions of other or other substrate. perlodid eggs such as Hydroperla fugitans Eggs of H. crosbyi underwent a long, ca. (Frison 1935), frontalis and seven-month diapause from February- Isogenoides zionensis (Knight et al. 1965), March, when deposited, to October-Novem- microcephala and hicaudata ber, when fall rains occurred and mean (Schwarz 1970), and Pictetiella expansa and daily water temperature declined to ca. 18 Isogenoides zionensis (Baumann 1973). The C. Elutriation (Stewart 1975) of sand sub- chorion surface generally lacks the sculptur- strate from an area where there was no sur- ing so evident in perlids (Stark and Gaufin face flow in July 1974 yielded one H. 1976a, b), the , and other stone- crosbyi egg and several Perlesta placida eggs flies. The crown ridge is reduced and from that appeared turgid and alive. The dia- two to four micropyles occur near the pause and development of eggs were similar middle on each of the three sides of the to those of Diura bicaudata and Brachyp- egg- tera risi (Khoo 1968b, c) and Arnphinemura Each egg was enclosed by a gelatinous delosa, A. Undo, A. nigritta, and Prostoia envelope, whose possible functions include: completa (Harper 1973b), in that eyespots

(1) prevention of desiccation after extrusion appeared only two to three weeks before and prior to deposition, (2) action as a ce- hatching. menting substance to retain eggs in masses, One group of March 1974 eggs held at and (3) attachment of individual eggs to 25 ±2 C until September, when the temper- substrate near the site of oviposition (Brinck ature was reduced 1 C every three to five days, hatched spontaneously at 19 C. This and early November appearance of first in- star nymphs in Clear Creek suggest that rejuvenation of streams by fall rains and de- clining temperatures cue the termination of diapause. Another group of 2,000 March 1976 eggs, held in the laboratory at a con-

stant 23 ± 1 C, hatched over an extended time period of five to eight months.

The diapause is a great advantage since it allows survival through hot summer, when water temperatures might be lethal to yoimg nymphs and many of the intermittent streams inhabited by this species become dry. Several species such as Zealeuctra ar- noldi, Z. claasseni, and Z. hitei and Perlesta placida that are endemic to, or extend their ranges into, the ecotone between the tem- perate deciduous forest and grassland are apparently capable of similar fast cycles.

This adaptation is similar to several species

Fig. 1. Hydroperla crosbyi egg; SEM, 400x. reported by Hynes and Hynes (1976) in 266 Great Basin Naturalist Vol. 37, No. 2

semiarid regions of Australia. Further work Although both H. crosbyi and Perlesta on their life cycles should provide some in- placida occupy similar habitats, have similar teresting comparisons with fauna of similar diets, undergo an over-summer diapause, regions of the two continents. Many stone- and have a relatively synchronous egg hatch

flies undergo an egg or nymphal diapause to (October-November), differential growth avoid adverse effects of extended cold or rates appear to separate them temporally. hot temperatures (Hartland-Rowe 1964, Nymphs of H. crosbyi grow faster through Khoo 1968a, b, c, Harper and Hynes 1970, winter, leading to an emergence in Febru- 1972, Harper 1973a). ary-March at the time when early instars of Eclosion (Fig. 2) follows the same general P. placida are just beginning their growth pattern reported by Heiman and Knight spurt, leading to an April-May emergence. (1970) and Vaught and Stewart (1974); the A thorough comparison can be made in chorion is torn, and the nymph pushes the southwestern streams such as Clear Creek, hinged cap away and crawls out. Hatching when another current study of the life cycle in the field is apparently synchronous, since of P. placida is completed (Rosalyn Snellen, first instar nymphs in November 1975 were personal correspondence). found only over a four-week period. Macan Nymphs.— After hatching in October, (1958) postulated that synchronous hatching nymphs grow rapidly through the winter is a disadvantage since competition for food until just prior to February-March emer- and space would be very intense. This would only apply, however, in situations t t t t where numbers approach or exceed the car- rying capacity. This theoretical asymptote (K) has not been defined for stream systems,

where it would be expected to vary geo- graphically and from year to year. Also, drift might play a large role in dispersing synchronously hatched nymphs.

Fig. 3. Seasonal cycle and growth of Hydroperla crosbyi, March 1974' to March 1976, Clear Creek, Denton County, Texas: Vertical line = range, number at top of line = niunber measured, solid circle = mean, solid square and triangle within bars = mean female and male interocular distance, respectively, ver-

Fig. 2. Eclosion sequence; Htjdroperla croshiji. tical bar =-!-&- ISD, arrow = emergence. June 1977 Oberndorfer, Stewart: Stonefly Biology 267

gence (Fig. 3), when water temperatures are fulva, I. patricia, Sweltsa coloradensis, Su- coldest (Fig. 4). The species thus fits Hyne's wallia pallidula, and Zapada columbiana (1961) classification as an F^ (fast) univoltine (Stanford 1975) exhibit slow cycles, with cycle. Similar cycles have been reported for most growth occurring at moderate to Isoperia clio and decisiis (Minshall warmest seasonal water temperatures. Tem- and Minshall 1966); Atnphinemura delosa, perature compensation (Gummins 1974) is Prostoia completa, and Shipsa rotunda (Har- thus manifested in this species through rap- per 1973b); and confusa, id winter growth, early spring emergence, trava, U. columbiana, Doddsia occidentalism and diapause for protection against drying and Taenionema pacifica (Stanford 1975). or lethal temperatures in summer. This This emphasizes the wide range of adapta- growth pattern is a selective advantage in tion in Plecoptera nymphal growth charac- allowing avoidance of competition for food teristics since many species such as Zapada and space with such species as Corydalus columbiana, (Hartland-Rowe 1964), Tae- cornutus (Brown 1974) and Perlesta placida. niopteryx neuhulosa, Nemura meijeri, First instar nymphs were obtained from A. borealis, N. cinerea, Leuctra hippopus, egg-hatching experiments and Glear Greek. Capnia atra, Diura nanseni, Isoperia dif- They were unpigmented, had 9 antennal formis, I. grammatica (Svensson 1966), and 4 cereal segments, and the mean inter- Isoperia goertzi, Diura bicaudata (Schwarz ocular distance was 0.18 mm (Fig. 5). Hynes 1970), Paragnetina tyiedia (Heiman and (1976) indicated that many first-instar Ple- Knight 1970), Pteronarcys dorsata, Isoperia coptera exhibit these characters. frontalis (Nebeker 1971b), Brachyptera risi, Distinctive color patterns became evident Protonemoura meyeri, Amphinemura sulci- through the middle and later instars, and collis, A. borealis, A. standfussi, Capnia numbers of cereal and antennal segments in- atra, Siphonoperla burrneisteri (Benedetto creased to 30 and 50, respectively. Mouth- 1973), Neoperla clymene (Vaught and Stew- parts of first-instar nymphs are shown in art 1974), signata (Gather and Fig. 6. Life cycle studies of stoneflies should Gaufin 1975), and Pteronarcella badia, give attention to good descriptions of first parallela, Diura knowltoni, Isoperia instars to facilitate accurate identifications

30 u 25 ^ 20 EMERGENCE

S 15

10

- J J A S O N D J F M « 1975 ^ 1'^^^^^

Fig. 4. Mean weekly stream temperature, June 1975 to March 1976. 268 Great Basin Naturalist Vol. 37, No. 2 for future ecological studies. Comparison of Brittain (1973), and Vaught and Stewart Figs. 6 and 7 show the major changes that (1974). take place in presence and pattern of setal Estimation of nymphal instar number was arrangement and shape of mouthparts be- determined from 971 field-collected nymphs tween first and last instars; number of pal- by using a size-frequency histogram (Fig. 8) pal segments remain unchanged. and the methods of Cassie (1954) and Ja- Nymphs can first be sexed at the sixth in- netschek (1967) (Figs. 9 and 10, respective- star, by differences in the posterior setal ar- ly). An expanded discussion of application rangement of the eighth abdominal sternum. of these methods is given by Harper (1973b) The fringe is interrupted in females and and McClure and Stewart (1976). These continuous in males. Dimorphism in setal methods indicated 12 and up to 14 instars arrangement, nymphal size, and even even- for males and females, respectively. Similar tual presence of the male epiproct, becomes dimorphism in instar number has been re- more distinct in postsixth instars. The sex ported by Holdsworth (1941b), McDiffett ratio of 870 postfifth instar nymphs sexed (1970), and Vaught and Stewart (1974). The j* throughout the study was 1.0 :1.1 9 . size change between most instars was con- The larger size of female nymphs appears firmed and these estimates substantiated by typical of all stoneflies and has been report- rearing field-collected nymphs of various ed by Holdsworth (1941a, b), Heiman and sizes through at least one molt. Of course, Knight (1970), Tarter and Krumholz (1971),

0.1 mnn

0.5 mm Fig. 6. Hydroperla crosbyi first instar nymphal mouthparts: A = labrum, B = mandible, C = max-

Fig. 5. Hydroperla crosbyi first instar nymph. illa, D = labium. June 1977 Oberndorfer, Stewart: Stonefly Biology absolute determination of instars can only be made through rearing individuals from egg to adult, under various physical condi- tions (Harper 1973b). Stanford (1975) sug- gested that temperature may influence the number of instars in hemimetabolous in- sects. Seasonal food habits of nymphs are shown in Table 2. All calculations are based on numbers of dietary items in the foregut and environment. Gut contents were examined using methods of Richardson and Gaufin (1971), and food availability was calculated from quantitative samples. Larvae of Sim- uliidae and Chironomidae always made up over 85 percent of the diet (Table 2), and electivity (Ivlev 1961) and forage ratio (Hess and Swartz 1941) consistently in- dicated that these organisms were preferred 1.0 mm or more available. Nymphs of Isonychia sic- Fig. 7. Hydroperla crosbyi last instar female nymph- ca manca were the third most common al mouthparts: A = labrum, B = mandible, C = dietary item, but preference for them was maxilla, D = labium. always negative (or they were less available

^^'9

y 50

o mLU

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

INTEROCULAR DISTANCE (mm)

Fig. 8. Interocular distance—frequency of 971 Hydroperla crosbyi nymphs, illustrating instars. 270 Great Basin Naturalist Vol. 37, No. 2

as June 1977 Oberndorfer, Stewart: Stonefly Biology 271 uliids was greater in both these months even Krumholz (1971), and Gather and Gaufin though in February their numbers were de- (1975). Fat stores are probably relied on for creasing because of emergence. Proportion necessary energy from one to five days prior of empty stomachs was highest just prior to to emergence and for transformation. Finni emergence (Table 2). Molting nymphs al- (1975) reported preemergent fat reserves for ways had empty stomachs. Similar findings AUocapnia granulata. were reported by Hanson (1960), Tarter and Mean nymphal standing crops were 8.0,

U

CO o

UJ DQ

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

INTEROCULAR DISTANCE (mm)

Fig. 10. Instar analysis of 971 Hijdropcrla crosbiji nymphs by Janetschek (1967) method: A = running mean of five of the frequencies, B = modal periodicity of instars (difference between Fig. 8 histogram and A). 272 Great Basin Naturalist Vol. 37, No. 2

25.6, 40.0, and 21.3/m- each month from growth of setipalpian Plecoptera in southern On- December 1975 to March 1976. The lower tario. Oikos 24:94-107. 1973b. Life histories of Nemouridae and number in December was probably due to in southern Ontario (Plecoptera). Hy- sampling error, peculiarities in micro- drobiologia 41:309-356. distribution, or difficulty in detection of tiny Harper, P. P., and H. B. N. Hynes. 1970. Diapause in first-instar nymphs. Decreases from Febru- the nymphs of Canadian winter stoneflies. Ecol- ary-March were likely an indication of sur- ogy 5i:925-927. vivorship and emergence. 1972. Life histories of Capniidae and Taeniop- terygidae (Plecoptera) in southern Ontario. Arch. Hydrobiol., Suppl. 40:274-314.

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