Developmental Biology of Spp. (Araneae: ) in the Laboratory and Field Author(s): G. S. Wheeler, J. P. McCaffrey and J. B. Johnson Source: The American Midland Naturalist , Jan., 1990, Vol. 123, No. 1 (Jan., 1990), pp. 124-134 Published by: The University of Notre Dame Stable URL: https://www.jstor.org/stable/2425765

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Developmental Biology of Dictyna Spp. (Araneae: Dictynidae) in the Laboratory and Field

G. S. WHEELER,' J. P. McCAFFREY AND J. B. JOHNSON Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow 83843

ABSTRACT: The developmental biology of two cribellate species, Dictyna coloradensis and D. major, was studied in the laboratory (D. coloradensis) and in the field (both species). Laboratory-reared D. coloradensis matured after 6-10 molts and spent an average of 8-13 days per stadium prior to the sixth stadium. Males required an average of 160 days to reach maturity whereas females required 220 days. Mean carapace widths differed among instars, although their ranges overlapped considerably. Field observations revealed univoltine life cycles for both species, with subadults over- wintering and maturing in late spring. Although size of overwintering individuals was similar at the northern and the one southern site (significantly smaller-sized individuals at the other southern site were attributed to mortality of larger individuals due to pesticide drift), Dictyna coloradensis females were significantly larger at the southern, longer-season sites [3000 growing degree days (GDD)] than at the northern, shorter-season site (1500 GDD). Females were largest when reared in the laboratory compared with all field sites. Spiderling field data suggest that development may be arrested at the southern site, syn- chronizing the population and producing similar-sized overwintering individuals. When reared in the laboratory under the same conditions, however, individuals from both the northern and southern sites developed at the same rate and attained the same adult size.

INTRODUCTION

The cribellate spider, Dictyna major Gertsch (Araneae: Dictynidae) was reported to be the primary mortality factor of the biological control agent Urophora affinis (Frauenfeld) (Diptera: Tephritidae) on spotted knapweed Centaurea maculosa Lamarck (Asteraceae: Cynareae) in Montana (Story and Anderson, 1978; Story and Nowierski, 1984). Since these initial reports, another dictynid spider, D. coloradensis Chamberlin, has also been found in spotted knapweed-infested habitats of northern Idaho (Wheeler, 1985). Dictyna major and D. coloradensis occur sympatrically in northern Idaho and may pose a threat to the biological control of spotted knapweed in that region. Both dictynid species occur on a number of other introduced noxious weeds, some of which also have been targeted for biological control (e.g., yellow starthistle Centaurea solstitialis L.); therefore, the success of other biological control agents may also be threatened by these . With the exception of taxonomic and geographic information (Chamberlin and Gertsch, 1958) and behavioral data (Jackson, 1977a, 1977b, 1978, 1979, 1980; Jackson and Smith, 1978; Starr, 1988), little information is available on the biology and ecology of the dictynids of the western U.S. rangelands. Thus, our objective was to compare life history parameters (e.g., number, size and duration of the different life stages) of Dictyna coloradensis under laboratory and field conditions. Field data for D. major were also collected and compared with D. coloradensis field data.

I Present address: Department of Entomology & Nematology, IFAS 0711, University of Florida, Gainesville 32611-0711

124

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METHODS AND MATERIALS

Laboratory studies. -Dictyna coloradensis egg sacs and maternal webs containing recently emerged spiderlings were collected during the spring and early summer of 1982 and 1983 from two southern sites: the Pond site, 8.3 km NW of Culdesac, and the Central Grade site, 6.4 km NE of Hatwai, Nez Perce County, Idaho. The sites were located ca. 19 km apart, on S-facing slopes, at ca. 658 and 750 m elevation, respectively, and were dominated by yellow starthistle. Additionally, one northern site was studied at Farragut State Park, 1.6 km E of Athol, Kootenai Co., Idaho. This site was located ca. 240 km N of the southern sites, on level ground, ca. 720 m elevation and was dominated by spotted knapweed. Postembryonic development of Dictyna coloradensis in the laboratory was observed twice daily in petri dishes (60 x 15 mm) lined with moistened black filter paper to facilitate observation of egg development. Spiderlings and adults were reared individually and main- tained in 0.185-liter, plastic, snap-top vials and 0.95 liter jars, respectively. All rearing occurred at 27 + 2 C, 45-55% RH and a 14:10 (L:D) photoperiod. The first instar spiderlings emerge from the egg sac as the first free-living and feeding stadium. Preliminary field studies revealed that primarily the first, and to a lesser extent second, instar Dictyna coloradensis spiderlings (recognized by a distinctly darker dorsal coloration of abdominal setae compared with the first instar), remained aggregated in the maternal web. Thus, the first instar spiderlings were allowed to remain in the maternal web and the second instar spiderlings were transferred to individual vials. The diet of the spiderlings consisted of four prey individuals from a mixture of small, acalypterate Diptera and Hymenoptera adults collected from alfalfa and supplied every other day. As winter approached and field-collected prey became scarce, the natural diet was replaced with two wild and two vestigial winged Drosophila melanogaster (Meigen) (Diptera: Drosophilidae) adults every other day. Spiders were observed daily to record all molts, and size was determined by measuring the carapace width of recently ecdysed individuals (?0.02 mm at 50x). Fifty first and second instar spiderlings were sacrificed and measured in 70% ethanol. Later instar spi- derlings and adults were measured live while restrained within a clear plastic sandwich bag. A subset of the entire reared population of Dictyna coloradensis (144 individuals) was measured during the first four instars; all fifth instar through adult individuals were mea- sured. Field studies.-Dictyna spp. development was studied during 1982-1983 at the three locations previously described. During the field season (April-October) all sites were sampled according to a systematic stratified sampling method ca. weekly by hand collecting 20 Dictyna spp. webs along with the resident spider. Five spiders and their webs were collected (one each at 10-m intervals) along each of four randomly selected 50-m transects. Transect directions were selected by randomly determined compass bearings. The spiders were char- acterized by sex, species (when possible), and carapace width using the methods described above. Sex determination was possible only for adults and penultimate males. Only the adult Dictyna individuals or spiderlings in a web inhabited by an adult could be positively identified to species (W. J. Gertsch, pers. comm.); thus, data regarding third through penultimate instars included both species. Additionally, duff and litter samples were collected during the late autumn-winter of 1982-1983 on 31 October and 21 December 1982 and 26 February 1983 (under 0.15-0.5 m of snow for the later two dates) and processed with Berlese funnels. All statistical analyses were conducted using the Statistical Analysis System (SAS Institute Inc., 1987). Experimental designs and analyses [ANOVA, ANCOVA, Student's t test, x2,

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Tukey's HSD (P = 0.05), simple linear regression and homogeneity of slopes] followed Sokal and Rohlf (1981). Degrees of freedom for the Student's t test were determined by the Satterthwaite's procedure for unequal sample sizes. When least square mean (Searle et al., 1980) multiple comparisons were conducted, the Bonferroni inequality was used to maintain the 5% experiment-wise significance level. All data are presented as means with the standard error of the mean in parentheses. Growing degree days (GDD) were calculated for the field data by the modified sign-wave method (Allen, 1976) and for the laboratory data by summing the daily degrees between the assumed developmental thresholds of 10 and 35 C.

RESULTS

Laboratory studies. -All growth parameters were analyzed by site (origin of collection). Only duration of the first instar differed, with spiderlings developing from egg masses collected from the northern site requiring significantly more days (13.1 days, +0.6; n = 117) than those from the southern sites (8.4 days, ?0.8; n = 30) t = 4.8; df = 65; P < 0.0001). Therefore, site data were combined for the remaining analyses. Field-collected adult Dictyna coloradensis spiders maintained in the laboratory deposited an average of 3.0 (?0.2; n = 28) egg sacs per female with each egg mass containing an average of 14.6 (+0.7; n = 66) eggs. Egg clutches were separated by an average of 3.5 days (?0.3; n = 17). No significant differences were found in the number of eggs per sac (one- way ANOVA; P > 0.5) or in the number of days between clutches with regard to the position of the clutch in the sequence (one-way ANOVA; P > 0.5). Eggs oviposited in the laboratory from field-collected females measured 0.62 mm (?0.01; n = 40) in length. The size of the two postembryonic stages that occur within the egg sac was not recorded. The mean duration of the first and second postembryos was 1.4 (?0.1; n = 61) and 2.4 days (?0.1; n = 248), respectively. Mean carapace width of the laboratory-reared Dictyna coloradensis increased significantly (least square means; P < 0.05) at each molt until maturity (Table 1). Females were statistically larger than males of the same instar. The carapace width of adults regardless of instar at which maturity occurred averaged 1.10 mm (?0.01; n = 38) for males and 1.26 mm (+0.02; n = 24) for females. Six spiderlings completed 10 instars but only two of these matured. Although the average carapace width of each instar was distinct, considerable overlap in their ranges occurred among instars. Thus, it was not possible to accurately estimate the instar of field-collected individuals. The number of instars required to reach maturity varied for each sex. Females most commonly required nine instars (range: 7-10, n = 24) whereas males required either seven or eight instars (range: 6-10, n = 38). Male and female stadia duration remained relatively similar, both between the sexes and among stadia, until the spiderlings approached maturity. Males and females spent signif- icantly more time in the stadia prior to maturation (instars 6-7 for males; 7-9 for females) compared with earlier stadia (Table 1). Male Dictyna coloradensis matured approximately 60 days earlier than females (t = 4.2; df = 36; P < 0.001). The mean total development time for males was 159.7 (?15.6; n = 37) and for females 220.1 (?27.5; n = 23) days. Males and females required 2714.9 and 3741.7 growing degree days (GDD), respectively. Longevity of laboratory-reared adults differed between the sexes (t = 2.12; df = 28; P < 0.05) with males surviving 60.6 (?5.6; n = 37) days and females surviving an average of 93.4 (?14.4; n = 23) days. Analysis of covariance revealed that adult spider size (carapace width) did not influence (P > 0.5) adult longevity for either sex. Field results. -Data for Dictyna major and D. coloradensis from the three field populations

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TABLE 1.-Mean carapace widths and stadium duration of laboratory-reared Dictyna coloradensis individuals

Mean carapace width mm (?sE) Mean stadium duration days (?sE)

Instar Juveniles Males Females Males Females

I 0.36 (?0.01) i* 11.6 (?0.6) de 12.5 (?1.1) de II 0.41 (?0.01) h 10.8 (?2.2) de 12.0 (?1.0) de III 0.56 (?0.01) g 9.1 (?0.6) de 9.2 (?0.6) de IV 0.69 (?0.01) f 7.5 (?0.4) e 8.0 (?0.6) de V 0.82 (?0.01) e 12.9 (?1.6) de 13.3 (?1.2) de VI 0.92 (?0.01) d 1.02 (?0.07) c 44.8 (?4.5) c 25.8 (?2.5) d VII 1.02 (?0.01) c 1.09 (?0.02) b 1.21 (?0.03) a 59.5 (?6.9) b 56.5 (?7.2) bc VIII 1.11 (?0.02) b 1.11 (?0.03) b 1.25 (?0.04) a 38.4 (?6.9) cd 78.8 (?8.3) a IX 1.13 (?0.03) b 1.13 (?0.03) b 1.27 (?0.02) a 36.4 (?9.3) cd 76.9 (?10.3) ab X 1.15 (?0.06) b 1.04 ? 1.40 ? 8.0 ? 17.0 ?

* Mean carapace widths or stadium durations followed by the same letter are not significantly different at the 5% level (least square means test) ? Indicates only one individual matured at this stadium revealed univoltine life cycles with subadult stages overwintering. During the winter months (late October to mid-March), no spiders were found in webs. However, small spiderlings (0.61 ? 0.2 mm; n = 27) (instars 1-3) were recovered from duff and litter at the northern site. Medium (instars 4 through 5) and large (instars 6 through penultimate) spiderlings were found in webs in the early spring at the sites where sampling began early (Pond, Farragut) (Fig. 1). Maturation began during late spring and was followed by the production of offspring in early July. Sampling began at the Central Grade site after most of the spiders had matured. Adults.-Mean adult carapace widths differed among spider species, sex and site of collection (northern vs. southern, but not between the two sites within the southern region) (least square means, P < 0.05). Regardless of site or sex, adult Dictyna coloradensis were larger (Table 2) than D. major adults (P < 0.0001). The sexes of either species did not differ within a site, in contrast to the laboratory data (Table 1), in which females were larger than males. The southern sites produced significantly larger female D. coloradensis and male D. major individuals relative to the northern site. Dictyna coloradensis females grew to a significantly larger size in the laboratory (1.26 mm) than in the field [1.16 mm (t = 3.0; df = 54; P < 0.01) and 1.08 mm (t = 5.0; df = 58; P < 0.001) at the southern and northern sites, respectively]. The laboratory-reared D. coloradensis males were not significantly larger than their counterparts collected from the northern site (t = 2.0; df = 26; 0.1 > P > 0.05), nor from the southern sites (t = 0.30; df = 4; P > 0.05) (note that few males were collected). These data suggest that the final size attained by these dictynid females varies according to rearing conditions. Under laboratory conditions where the temperature was more constant and resources were plentiful, D. coloradensis females grew larger than field individuals. The two dictynid species differed in their abundance at the three sites. Dictyna major was significantly (X2 = 32.1; df = 2; P < 0.0001) more frequently encountered in webs at the three sites than D. coloradensis over the entire season (Fig. 2). Additionally, the cumulative frequency of D. coloradensis adults leveled off, approaching a plateau much earlier at all sites than D. major, suggesting that peak adult density of the former species occurs earlier than that of the latter.

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A C 20 20

16

Clo1 10

C0 01

April May June JWly Aug Sept Oct o B =ADULTS MSMALL EJMEDIUM 3ILARGE Month ~0

E 16 z 10,

6

0 April May June MJut Aug Sept Oct

FIG. 1.-Developmental biology of Dictyna spp. from three field sites in northern Idaho, 1983. Arrows indicate pesticide applications. Sites: A = Farragut, B = Pond and C = Central Grade. Small spiderlings = instars 1-3, medium = instars 4-5 and large = instars 6-penultimate

Eggs.-Eggs were present from early June through mid-July at the northern site and from early June through mid-August at the southern sites. These final dates correspond to 43 and 66% of the entire season's cumulative GDD, or 832 and 984 GDD still available for growth and development at the northern and southern sites, respectively. The number of egg sacs per web did not differ among the sites; however, Dictyna coloradensis webs contained significantly more egg sacs (2.2 ? 0.2; n = 12) than webs inhabited by D. major (1.2 ? 0.2; n = 16) (t = 3.4; df = 25; P < 0.01).

TABLE 2.-Mean adult D. major and D. coloradensis carapace widths (mm). Samples were collected from northern (Farragut) and southern sites (Central Grade and Pond) during 1983

Species Sex Site Mean carapace width mm (?sE, n)

D. coloradensis F South 1.16 (?0.02, 33) a* D. coloradensis M South 1.09 (?0.02, 5) ab D. coloradensis F North 1.08 (?0.01, 41) b D. coloradensis M North 1.03 (?0.01, 9) b D. major M South 0.89 (?0.02, 26) c D. major F South 0.85 (?0.01, 233) cd D. major F North 0.84 (?0.01, 9) cd D. major M North 0.83 (?0.02, 13) d

* Means followed by the same letter are not significantly different at the 5% level according to a least square means test

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A c 20 Apr20 C 16

160

100 -100 0

10 76 10 - - - g_

V 0)

< D~ 00~ ~ ~~~~~~~~5 0. 50 E April May June July Aug __ MonthM= E~2 12 coioradensm Month Sol20iB l 0ar Ea Cum r 7D e E 16 -1250

Z -100

10 - 75 06ilililiiL ~~~~~~~~-50

April May June July Aug Month FIG. 2.-Adult web inhabitants from three sites in northern Idaho, 1983. Data include only the adults from a total of 20 spiders sampled. Vertical bars represent frequency of adults of each species. Solid lines and dashed lines represent cumulative numbers of adult D. maj'or and D. coloradensis, respectively. Sites: A = Farragut, B = Pond and C = Central Grade

Juveniles. -First and second instar Dictyna coloradensis spiderlings were frequently ob- served inhabiting the maternal web with an adult female spider. The maternal spider was observed capturing and subduing prey while the spiderlings remained in the web periphery. As prey ceased to struggle, the maternal spider and the spiderlings fed together on the prey. Web cohabitation by mature males and females was frequently observed for both species. However, males and females of either species were never seen sharing prey. Two adult females were observed briefly in the same web only once. In late July and again in early August spider development at the Pond site was disrupted by pesticide overspray (dimethoate + methyl parathion) from a neighboring field (Fig. 1); this may have led to the distortion of the population structure at that site. At the time of the disturbance, the number of small-sized spiderlings (instars 1 through 3) was decreasing as medium spiderlings (instars 4 through 6) were developing. During this same time at the other southern site (Central Grade) small spiderlings made up less than half the population. However, after the pesticide sprayings small spiderlings at the Pond site comprised the majority of the population until the final sampling date (18 October). Possibly, larger individuals occupied the web sites most exposed to airborne pesticide, accounting for the apparent selective kill of larger individuals. Smaller spiders from less exposed canopy positions may have replaced the larger individuals, such that the spiderlings sampled were significantly smaller than the spiderlings at the other southern site (Central Grade) on similar sampling dates. Temperature-dependent developmental rates. -Growing degree day accumulation (GDD) at the southern sites amounted to twice (3000) that of the northern site (1500), and yet all

This content downloaded from 86.59.13.237 on Thu, 01 Jul 2021 09:54:54 UTC All use subject to https://about.jstor.org/terms 130 THE AMERICAN MIDLAND NATURALIST 123(1) three populations were univoltine. Spiders located at the different sites must either develop at different rates, or overwinter in different stages of development. The average carapace width of the spiderlings collected on the last sampling date (12 October) at the northern site was 0.84 mm (?0.03; n = 20) while at the southern sites (19 October) it was 0.76 (+0.03; n = 20) (Central Grade), and 0.66 mm (?0.03; n = 20) (Pond). Only the Pond site value differed significantly from the northern site value (P = 0.05; Tukey's HSD). The smaller spiderling size at the Pond site may be related to the pesticide application as discussed previously. Thus, the similarity in overwintering size and stage (instars 4-5) between the northern site and southern site undisturbed by pesticides suggests that developmental dif- ferences occurred in the spiderlings from the different sites. Regression analysis of the frequency of replacement of small spiderlings by medium and large spiderlings as a function of cumulative GDD suggests developmental differences at the different sites. The slope for the northern site [0.07 (?0.004)] was significantly greater than that for the southern site (Central Grade) [0.03 (?0.002); P < 0.0001] (Fig. 3).

DISCUSSION

Dictynid life cycle. -Members of the Dictynidae are generally considered to have an annual life cycle with adults present during the spring, summer and/or autumn (Chamberlin and Gertsch, 1958). Several workers have demonstrated that other spider species exhibit a biennial life cycle in more northern latitudes whereas annual life cycles occur in the more southern regions (Almquist, 1969; Edgar, 1972). Cool summers (Dondale, 1961) or high elevation (Albert, 1982, as cited by Schaefer, 1987) may also cause a similar change in spider life cycles. Additionally, low levels of food (Miyashita, 1968; Deevey, 1949) and lower relative humidity (Browning, 1941) may cause extended life cycles. Our field data for Dictyna coloradensis and D. major suggest the occurrence of annual life cycles at all sites, with adults being present only during the spring and summer. This annual life cycle was maintained despite the large differences in GDD and the possible, but unquantified, nu- tritional and relative humidity differences between the two sites. Under laboratory conditions spiderlings matured after a variable number of instars (6- 10). Increased size would be expected with additional molts but our data indicate that maturation beyond the sixth instar for males and the seventh instar for females was of only minimal advantage in terms of greater body size. However, it is conceivable that delayed maturation may have allowed the accumulation of more energy reserves (e.g., lipids) not detected by these carapace width measurements. Larger size may be beneficial in terms of increased fecundity (Kessler, 1971), and we would expect larger spiders to live longer due to increased energy and nutrient storage. However, the laboratory data show no correlation between greater adult size and increased adult longevity. Since we did not measure the size of the field-captured females from which we obtained eggs, and since we did not measure egg production by our laboratory-reared females, we cannot address the effect of body size on fecundity. Even though female Dictyna coloradensis grew larger in the laboratory compared with the field, and there was some evidence that sites with longer growing seasons produced larger individuals, increased size may have been of negligible benefit as the smaller species (D. major) was more commonly collected at all sites. Our results for D. coloradensis males suggest that size is not significantly influenced by the rearing conditions studied. Indeed, there may be only minimal benefit for larger males as medium-sized male spiders may have increased chances of survival compared with larger males in search of mates (Vollrath, 1980).

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80

-+- Southern 70- _ 09 * Northern 60 +- ? 45 50

40-Cn3L =

1:0 30

E'o20- oE 10 - I-

0 500 1000 1500 2000 2500 3000 CUMULATIVE GROWING DEGREE DAYS FIG. 3.-Cumulative frequency of medium (instars 4-5) and large (instars 6-penultimate) spiderlings of Dictyna spp. at northern (Farragut) and southern (Central Grade) sites in Idaho, 1983. (Pond data omitted because of pesticide disturbance; see text.) Farragut: Y = 42.9 + (GDD*0.07), R2 = 0.97, P < 0.0001. Central Grade: Y = -28.0 + (GDD*0.03), R2 = 0.95, P < 0.0001

To initiate maturation, spiderlings may require some environmental cue (e.g., shortened photoperiod, cumulative chilling units, or changes in dietary quality or quantity) not present under the laboratory conditions. The generally greater thermal units required by the lab- oratory-reared spiderlings (2715-3742 GDD) compared with either field population, sug- gests that the proper cues are important for the timing of maturation. After attaining a certain size, further development of the laboratory subadults may have been arrested due to a lack of such cues and additional instars were required before maturation was finally achieved. Developmental differences. -Sexual dichronism has been reported to occur frequently in web-building spiders (Vollrath, 1980). If receptive females are present, early maturing males may mate earlier and possibly more frequently than later maturing males. Early maturation may enable males to locate and more safely mate as the females molt to the adult stage (Robinson and Robinson, 1973; Lubin, 1986). If the developmental time difference observed here is an indication of that found in the field, males would not be expected to mate with females from the same clutch, though sibling matings between members of different clutches produced by the same female are possible. As the average adult male longevity is 60 days, equal to the average difference in male- female developmental time, only the latest maturing, longest lived males would be expected to occur at the same time as early maturing females. Such a strategy to avoid sibling mating may have evolved because it increases the genetic heterogeneity of the offspring (Levy, 1970). Alternatively, additional female developmental time may be required simply to grow larger or to mature eggs.

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Primitive social behavior.-The web sharing and brood care exhibited by Dictyna colora- densis may be an initial stage in the evolution of permanent social spider species (Kullmann, 1972). This 'maternal-social' mode of communal living breaks down as the food demands of a developing brood become too much for an aging female (Krafft et al., 1986). Communal feeding is commonly recognized among all stages of the social dictynid, Mallosgregalis Simon (Burgess, 1979). In addition to our results, females of other solitary dictynids (e.g., D. volucripes Keyserling) remain in the web with offspring (Scheffer, 1905; Jackson, 1978; Gertsch, 1979). However, there is a limited period of brood care as spiderlings are seldom found in maternal webs after the first instar. Egg sacs of Dictyna coloradensis from the northern site, when reared in the laboratory, took longer to complete the first instar than eggs collected from the southern sites. Since the first instars remain in the maternal web, this suggests the northern population is programmed to extend maternal care longer (1.6-fold longer) than the southern population, perhaps in response to variable and unpredictable weather conditions prevalent during this period. Based on this hypothesis, we would not expect a difference in first instar duration between the northern and southern populations of Dictyna major, since web cohabitation between D. major females and spiderlings was never observed. Unfortunately, our field collections for the laboratory rearing resulted in too few D. major individuals to provide reliable data on instar duration. Developmental plasticity.-Despite large differences in season length of the sites studied (1500 GDD at the northern and 3000 GDD at the southern site), spiderlings attained the same overwintering stage (instars 4-5) at both sites. However, no differences were found in developmental rates of laboratory-reared spiders from females or egg masses collected at the different sites. From our laboratory data we calculate that 714-944 GDD were required to develop to the overwintering stages and our field data suggest that more or less sufficient thermal units (832 and 984 GDD) were available to attain this stage even for the last egg sacs collected at either site (mid-July and mid-August). Thus, we suggest that earlier emerging spiderlings (early June at both sites) arrested their development toward the end of the growing season (as seen in the laboratory study) such that their size at overwintering was similar to that of individuals emerging later in the season. Because twice the GDD accumulated at the southern site compared to the northern site, sufficient thermal units were probably available for a second spider generation. We suggest, however, that the risk of prey scarcity to reproductive adults and juvenile spiderlings toward the end of the growing season would preclude the evolution of a second generation at the southern site. Yellow starthistle flowers attract abundant pollinators during spring and early summer but under the arid conditions of the rangeland habitat, flowering ceases during mid-late summer, decreasing the abundance of pollinators. Thus, despite abundant thermal units the entire population is synchronized with a univoltine life cycle.

Acknowledgments. -We gratefully acknowledge the identification of these dictynid spiders by Dr. W. J. Gertsch. Assistance in statistical analyses was provided by Jim Norris. Frank Merickel, Mark Cole and Jude Sirota assisted in the field studies. We appreciate the contributions of critical reviews of an earlier draft of this manuscript by F. Slansky, Jr., M. Quinn and M. Rice. This research was supported, in part, by a grant awarded to R. H. Callihan and J. B. Johnson of the Department of Plant, Soil & Entomological Sciences, University of Idaho, Moscow, and Idaho Agricultural Exper- iment Station Project 061 -R838, a contribution project to Western Regional Project, W-84. University of Idaho Agricultural Experiment Station Publication number 8971.

LITERATURE CITED

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SUBMITTED 3 JANUARY 1989 ACCEPTED 6 JULY 1989

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