Dispersal of the Spiderlings of Xysticus emertoni (Araneae, Thomisidae), a Litter-Dwelling Crab Spider

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