1992. The Journal of Arachnology 20:217–221

DISPERSAL OF THE SPIDERLINGS OF (ARANEAE, ), A LITTER-DWELLING CRAB

Douglass H. Morse: Graduate Program in Ecology and Evolutionary Biology, Division of Biology and Medicine, Box G-W, Brown University, Providence, Rhod e Island 02912 USA

ABSTRACT. Dispersing spiderlings of Xysticus emertoni (Thomisidae), a litter-dwelling species, placed o n sites similar to their nests on leaves usually descended quickly into the vegetation below . However, those placed on nearby goldenrod (Solidago spp.) flowers remained significantly longer, sometimes hunting, before dropping lower in the vegetation . They seldom ballooned from either site . Xysticus dispersal behavior differs markedly from that of the thomisid vatia, a flower-inhabiting species, which balloons regularly from leave s and remains much longer on goldenrod flowers than Xysticus.

Resource spacing and availability play an im- Misumenops F. Pickard-Cambridge and Misu- portant role in an individual's predisposition t o menoides F. Pickard-Cambridge in these catches , disperse (Southwood 1962 ; Richter 1970 ; Dingle because these are the thomisid genera that dom- 1984). In turn, dispersal patterns influence other inate the aerial fauna—Xysticus is extremely rare aspects of the life styles of the individuals i n in aerial catches (M . H. Greenstone, pers . comm.). question. Combined, these factors should exer t Other than for Richter's (1970, 1971), Green - a dominant impact on patterns of gene flow an d stone's (1982), and Miller's (1984) studies on consequent population structure. It is therefore lycosids, no experimental effort has been mad e instructive to compare the dispersal patterns of to compare the ballooning by different species o f related species with extremely different life styles . a spider family, although Tolbert (1977) pro- Two thomisid crab , Xysticus emerton i vides some comparative information on two ara- Keyserling and (Clerck), pro - neid species. vide such a comparison . Xysticus is primarily an I therefore tested dispersal patterns of Xysticus inhabitant of herbaceous vegetation and litter i n emertoni spiderlings for comparison with recen t fields and pastures of eastern North America results (Morse in press) on the dispersal of Mis- (Comstock 1940 ; Dondale & Redner 1978) . Al- umena vatia spiderlings. In the latter study I es- though it sometimes hunts in flowers at the to p tablished that Misumena readily balloon after of the herbaceous layer, Xysticus occurs much emerging from their egg sacs, although the pro- less frequently, remains for shorter periods, and pensity to do so is strongly influenced by the as an adult experiences considerably lower hunt- substrate they occupy. If the nest sites offer rich ing success in flowers than does Misumena (Morse foraging opportunities for the spiderlings, pri- 1983). The latter species concentrates its activ- marily flowering goldenrods (Solidago spp.) in ities, especially as an adult, at flowers (Morse my study area, they are very likely to remain an d 1984). Nevertheless, Xysticus sometimes places feed in them. Their consequent foraging success its nests in emergent herbaceous plants, as does and subsequent growth at these sites decreas e Misumena. This placement presents Xysticus their probability of ballooning . However, Mis- spiderlings with an excellent opportunity to dis- umena of all ages live above the litter in thei r perse. habitat, concentrating their activities in flowers , Although thomisid spiders regularly occur in a characteristic associated with a nearly contin- samples of aerial "plankton" (Glick 1939 ; Salm- uous quest for insect food. In light of the much on & Homer 1977; Greenstone et al . 1987), re - less frequent occupation of the somewhat ex- ports often do not distinguish between species o r posed sites by Xysticus, and the seemingly more genera. Most frequently, reference is made to homogeneous areas that they occupy low in the 217

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vegetation, one may predict a considerably lower end of two hours I censused them twice or mor e probability of ballooning than for Misumena , daily to determine the approximate time at which even though Xysticus nests are often placed in they dispersed from the substrate on which they positions that make this type of dispersal pos- were released . I recorded movements of these sible. spiderlings, the time they remained on the sub- In the present study, I tested the propensity of strates to which they were introduced, and the second instar Xysticus spiderlings, newly emerged methods by which they left these sites (balloon- from their egg sacs, to balloon in experimenta l ing, dropping on lines, etc.). I then compared situations similar to those under which I ha d these results with those for Misumenaspiderlings tested Misumena spiderlings. I then compared that had been exposed to similar experiment s the two species, paying particular attention t o (Morse in press). the implications of these differences for dispersal, potential gene flow and population structure in RESULTS general. Location of Xysticus nests in the field.—I found nine Xysticus emertoni nests during 1989-1991 , METHODS of which a majority (five) were on milkweed, tw o I found several Xysticus emertoni nests on on aster (Aster sp.), one on chokecherry (Prunus broad-leaved vegetation in the study area while virginiana), and one on raspberry (Rubus sp.). pursuing other work . I measured key parameters The nests were constructed at the ends of leaves, of their locations as I encountered them, but mad e and on all but the aster the distal tip was turne d no special attempt to hunt for them in the litter under the rest of the leaf and secured, the eggs layer. Thus, I do not claim that those nests are laid between the two resulting thicknesses of leaf, typical of all X. emertoni . Young were taken from and the sides drawn tight by silk into a compact these nests for release experiments (discussed be - nest. These nests resembled those of Misumena low), and others were taken from broods laid i n (figure 1, Morse 1985), except that Xysticus the laboratory, for a total of 22 broods . mothers ensconced themselves inside their nests , I released over 200 newly-emerged, second in- rather than guarding the nests from the outside . star Xysticus spiderlings from locations that re- In contrast, Xysticus folded the narrow aster sembled one of their frequent nest sites, leaves leaves twice, permitting a nest to be fashioned of the common milkweed Asclepias syriaca ; and by essentially providing a third side from plant inflorescences of nearby goldenrod clones (Soli- material . In a sample now exceeding 1500, I have dago juncea and S. canadensis) that attracte d never seen a Misumena nest built by folding a large numbers of tiny insects . Five young from leaf in the latter way. a single brood were placed on a substrate (milk- Nests were all located in low vegetation at a weed leaf or goldenrod inflorescence) at a time , mean height of 54 .9 (± 12.1 SD) cm, in vege- the maximum number that an observer could tation of 71 .3 ± 17.8 cm. The nests on milkweed carefully watch and record under these condi- most frequently occupied the third pair of leave s tions. These densities frequently occur when they from the top, ranging from the second to the fifth emerge from a nest . All statistics were run on the from the top. responses of groups of spiderlings, with only one Movement of Xysticus spiderlings on various group used from a brood in any experiment . Pri- substrates .—Xysticus spiderlings have a strong or to release the spiderlings were lightly dusted tendency to descend into the litter when place d with powdered red micronite dye, which in- on substrates similar to the ones on which their creased their visibility to the observer . Earlier parents normally build their nests. This behavior experiments with Misumena spiderlings had occurred both in young placed on the normal demonstrated that this manipulation did not af- sites (milkweed leaves) and on nearby goldenro d fect their subsequent behavior (Morse in press) . flowers at the peak of bloom (Table 1), a sourc e The studies were carried out in a field in Bremen, at which large numbers of tiny insects, potential Lincoln Co ., Maine, an area that I have describe d prey of these spiderlings, often congregate. How- in detail elsewhere (Morse 1979, 1981) . ever, they responded quantitatively differently to I observed these spiderlings continuously dur- these two substrates, remaining significantly ion - ing the first two hours following release, or until ._ger on goldenrod than on milkweed (Table 1) (P they had all dispersed . If any remained at the < 0.05 in a two-tailed Mann-Whitney U-test) .

MORSE—DISPERSAL OF XYSTICUS SPIDERLINGS 219

Table 1.—Time (min) that newly emerged Xysticus spiderlings remained on different substrates, with similar results on Misumena for comparison (Morse in press) . a = Misumena remained 176.1 ± 251 .5 min on milkweed, 3820.0 ± 3435 .5 min on goldenrod (Morse in press) . b = 19 of 50 Misumena released on milkweed were observed to balloon, but none of 50 released on goldenrod were observed to balloon (Morse in press) .

Time remained Method of dispersal Substrate Year N (z ± SD) Drop Line Balloon Not Know n Milkweed, 1987 92 8.1 ± 11 .9 64 17 8b 3 1990 65 18.0 ± 21 .0 53 4 0 8 Goldenrod' 1990 38 114 .8 ± 169 .1 11 0 2b 25 1991 10 144.4 ± 203.8 5 0 0 5

Further, numbers of spiderlings observed quick- the wind moving the line-laying spiders from a ly dropping on lines directly to the litter fro m vertical to horizontal position while they wer e milkweed far exceeded those dropping from leaving the original sites. Although the result s goldenrod (P < 0.002 in a two-tailed Mann- clearly indicated that Xysticus does balloon, it Whitney U-test) . This difference, combined with was the least common of the activities recorde d observations of two Xysticus spiderlings captur- in these releases (Table 1). ing tiny midges on goldenrod during these pe- Movement of Xysticus and Misumena spider- riods, suggested that they used these flowers as lings.—Although Xysticus spiderlings resembled hunting sites . This result is consistent with oc- Misumena spiderlings in remaining significantly casional observations of early-instar Xysticus on longer on goldenrod (foraging substrate) than on these sites under unmanipulated conditions (Ta- milkweed (nest substrate), they stayed on bot h ble 2). of these sites only a small fraction of the tim e The most frequent movement from the release that Misumena did (Table 1), differences that ar e sites was directly into the litter, either on vertica l highly significant (P < 0.002 for goldenrod, P < lines or by crawling down the vegetation (Table 0.001 for milkweed in two-tailed Mann-Whitne y 1). These movements occurred significantly more U-tests) . These results are consistent with thei r frequently than dispersal via horizontal lines t o parents' habits and with the relative scarcity of adjacent grass stems (P < 0 .01 for milkweed, Xysticus spiderlings on goldenrods and othe r P = 0.05 for goldenrod in Wilcoxon matche d flowers. In censuses of spiderlings on goldenrods pairs signed ranks tests) . The outcome of mos t only two Xysticus spiderlings were found on over moves on horizontal lines probably did not diffe r 900 inflorescences, in comparison to over 60 0 in function from the vertical movements, how - Misumena spiderlings (Table 2) . ever, since all such individuals that I could follow Also striking is the difference in dispersal modes on horizontal lines used their new locations a s of Xysticus and Misumena spiderlings from the staging sites from which to move into the litter, release sites . Although a majority of Xysticus spi- rather than for aerial dispersal . Some of the latter derlings left these sites for the litter, either di- movements could have been a consequence of rectly by lines or by crawling down the vegeta -

Table 2.—Numbers of Xysticus and Misumena spiderlings on flowering goldenrod. a = Each clump is distinct from others and probably a clone . b = Involves daily counts over 2½ to 3½-week period following release of a total of 404 Misumena spiderlings .

Number of Number of Number of Number flowering Xysticu s Misumena Sample of goldenrod of clumps, stems spiderlings spiderlings Randomly chosen 25 277 1 248 > 5 m from Misumena nest 10 151 0 27 <1 m from Misumena nest 12 439 0 359 Release of 4 Misumena broodsb 4 42 1

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tion, Misumena spiderlings never descended t o patchy flower resources, but Xysticus does not the litter (Table 1) . This difference is highly sig- depend primarily on this resource . Even when nificant (P < 0.002 for both milkweed and gold- Xysticus does hunt on flowers, it remains there enrod in two-tailed Mann-Whitney U-tests), as for much shorter periods than Misumena, with is the difference in frequency with which the two a periodicity suggesting that its poor hunting suc- species balloon from milkweed (P < 0 .002 in a cess may account for the short tenure (Mors e two-tailed Mann-Whitney U-test) (Table 1) . Al- 1983). Because Xysticus obtains a major part of though a few Xysticus were observed to balloon its prey away from the flowers, within the veg- off goldenrod as well as milkweed in the trials , etation and litter layers, its resources are not like - Misumena were observed to balloon only off ly to be as patchy as those of Misumena . Xysticus milkweed. Nevertheless, the frequency of bal- should therefore balloon less frequently than looning from goldenrod by Xysticus was so low Misumena, as observed . Thus the two specie s that the two species did not differ significantly appear to respond to the same habitat in dis- (P > 0.05 in a two-tailed Mann-Whitney U-test) . tinctly different ways, notwithstanding their sim- Since many Xysticus left goldenrod within the ilar size and close phylogenetic relationship. two-hour period of continuous observation (Ta- These results, plus the infrequent natural pres- ble 1), the probability of observing them bal- ence of Xysticus spiderlings on goldenrod, sug- looning from goldenrod was far higher than fo r gest that the latter spider seldom moves above Misumena . Misumena remained for long peri- the litter layer . Consequently, its dispersal dis- ods, often several days, on goldenrod (Table 1) , tances as juveniles are likely to be low . Gene flow and they were only censused a few times a da y should thus be much lower in Xysticus than in after the original observation period . Misumena, which should in turn generate dif- ferences of population structure in the two spe - DISCUSSIO N cies. Nevertheless, since Xysticus sometimes bal- The behavior of Xysticus spiderlings resem- looned, it clearly retains the ability to initiat e bled that of Misumena spiderlings in that dis- long-distance movement. persal time was related to substrate in both spe- cies. This difference strongly suggests that they ACKNOWLEDGMENTS discriminate between sites . However, actual times I thank M . H. Greenstone, J. D. Parrish, G. required to disperse were considerably shorte r Stratton, and R. B. Suter for reading a draft of for Xysticus than for Misumena . This brief ten- this manuscript. My research on Misumena i s ure is consistent with Xysticus's distribution at supported by the National Science Foundation similar heights as adults (Morse 1983) . (BSR85–16279 and BSR90–07722) . I thank H. Richter (1970) and Greenstone (1982) have Heller, J. Kotanchik, N . McKay, and J. Rollen- provided the only previous experimental studies hagen for assistance with the field work, E. B. on between-species differences in ballooning pat - Noyce for kindly permitting use of the study site , terns, although they have all been performed i n and M. H. Greenstone for information on tho- the laboratory using artificial wind, heat, and light misid genera in aerial samples. sources. Young wolf spiders of different Pardosa species vary in their ballooning tendencies, which LITERATURE CITED Richter attributed to the abundance and stabilit y Comstock, J . H. 1940. The spider book, revised and of their habitats, and Greenstone to the predict- edited by W . J. Gertsch. Cornell University Press, ability of the habitats . Propensity to balloon dif- Ithaca, New York. fered inversely with each of these traits . Abun- Dondale, C . D. & J. H. Redner. 1978. The insects dant habitats often have a low level of patchiness . and of Canada, Part 5. The crab spiders This study thus suggests that spatial patchines s of Canada and Alaska . Canada Dept. of Agriculture may be added to the variable of temporal patch - Publ. 1663:1-255. iness as a factor affecting ballooning. Both thorn- Glick, P. A. 1939. The distribution of insects, spiders, and mites in the air . United States Dept. of Agri- isids live in similar habitats, but as a result o f culture Tech. Bull., 671:1-150. their markedly different demands on these hab- Greenstone, M . H. 1982. Ballooning frequency and itats, they probably view patchiness at strikingly habitat predictability in two wolf spider species (Ly- different scales. For most or all of its stages, Mis- cosidae: Pardosa) . Florida Entomol., 65:83-89. umena depends on insects drawn to extremely Greenstone, M. H., C. E. Morgan, A .-L. Hultsch, R.

MORSE—DISPERSAL OF XYSTICUS SPIDERLINGS 221

A. Farrow, & J. E. Dowse . 1987. Ballooning spi- Morse, D . H. in press . The relationship between dis- ders in Missouri, USA, and New South Wales, Aus- persal by spiderlings from their nests and earlie r tralia: family and mass distributions. J. Arachnol., foraging patch decisions made by their mothers. 15:163—170 . Ecology . Miller, G. L. 1984. Ballooning in Geolycosa turricola Richter, C. J. J. 1970. Aerial dispersal in relation to (Treat) and Geolycosa patellonigra Wallace: high habitat in eight wolf spider species (Pardosa, Ara- dispersal frequencies in stable habitats . Canadian J . neae, Lycosidae) . Oecologia, 5 :200—214 . Zool., 62:2110—2111. Richter, C. J. J. 1971. Some aspects of aerial dispersal Morse, D . H. 1979. Prey capture by the crab spide r in different populations of wolf spiders, with par- Misumena calycina (Araneae: Thomisidae). Oec- ticular reference to Pardosa amentata (Araneae, Ly- ologia, 39:309-319 . cosidae). Misc. Pap., Landouwhogeschool, Wagen- Morse, D . H. 1981 . Prey capture by the crab spider ingen, The Netherlands, 8 :77—88 . Misumena vatia (L.) (Thomisidae) on three com- Salmon, J. T. & N. V. Homer. 1977. Aerial disper- mon native flowers . American Midl . Natur., 105: sion of spiders in North Central Texas . J. Arachnol., 358—367. 5:153—157 . Morse, D . H. 1983. Foraging patterns and time bud- Southwood, T. R. E. 1962. Migration of terrestrial gets of the crab spiders Xysticus emertoni Keyserling in relation to habitat . Biol. Rev., 37: and Misumena vatia (Clerck) (Araneae : Thomisi- 171—214 . dae) on flowers. J. Arachnol., 11 :87—94 . Tolbert, W. W. 1977. Aerial dispersal behavior of Morse, D . H. 1984 . How crab spiders (Araneae: two orb-weaving spiders . Psyche, 84 :13—27 . Thomisidae) hunt at flowers . J. Arachnol., 12:307— 316. Manuscript received 2 January 1992, revised 3 Septem- Morse, D. H. 1985. Nests and nest-site selection o f ber 1992. the crab spider Misumena vatia (Araneae, Thomis- idae) on milkweed. J. Arachnol., 13:383—390 .