Spider Body Size Research Articles

Negative effects of ant foraging on spiders in Douglas-fir canopies

Spiders and ants are potential competitors and mutual predators. Indirect evidence from previous research has suggested that ant foraging may significantly lower the abundance of arboreal spiders in young Douglas-fir plantations in western Oregon. This study tested the effect of foraging by ants, dominated by Camponotus spp., on spider assemblages in Douglas-fir canopies in a 5-month ant-exclusion experiment. The biomass of potential prey organisms on foliage, dominated by Psocoptera, increased significantly by 1.9- to 2.4-fold following ant exclusion. The removal of ants did not affect the abundance of flying arthropods in the vicinity of tree canopies as indicated by sticky trap catches. The abundance of hunting spiders, the majority being Salticidae, increased significantly by 1.5- to 1.8-fold in trees without ants in the late summer; neither the abundance of web-building spiders nor the average body size of hunting and web-building spiders were significantly affected by ant removal. Spider diversity and community structure did not differ significantly between control and ant-removal trees. The majority of prey captured by ants were Aphidoidea (48.1%) and Psocoptera (12.5%); spiders represented only 1.4% of the ants' diet. About 40% of observed ants were tending Cinara spp. aphids. Our observations suggest that the lower abundance of hunting spiders in control canopies with ants may be due to interference competition with ants resulting from ant foraging and aphid-tending activities. Direct predation of spiders by ants appeared to be of minor importance in this study system. This study did not provide sufficient evidence for exploitative competition for prey between ants and spiders.

アリを専食するミジングモ属（Ｄｉｐｏｅｎａ）３種の餌構成

We examined species and size compositions of prey in the three Dipoena spiders specializing on ants. Dipoena punctisparsa fed exclusively on small ants, Lasius. In this spider, the number of prey fed at a time increased with the spider body size. Dipoena castrata mainly captured Componotus and Lasius, and most spiders fed on a single prey. The body size of prey fed upon by this species increased with the spider size. Dipoena mustelina captured a variety of ant species, ranging from very small to large ones.

Size-dependent survivorship in the web-building spiderAgelena limbata.

Stage-specific mortality rates and mortality factors for the web-building spiderAgelena limbata, which is suggested to be food-limited, were studied, and the relationship between body size of spiders and survivorship for instar 3 to adults was examined. The mortality rate of the egg sac stage including eggs, deutova (prenymphal stage), and overwintering instar 1 nymphs was low. The low mortality of this stage was partly due to maternal care that reduced the mortality caused by predation and/or abiotic factors. From emergence of instar 1 nymphs from egg sacs to reproduction, the stagespecific mortality rates were almost constant, 32-47%, and the time-specific mortality rates were also constant. These results suggest a Deevey (1947) type II survivorship curve inA. limbata, in contrast to other reports on the wandering or burrowing spiders which suggested type III curves. Important mortality factors for nymphs and adults were parasitism by an ichneumonid wasp and predation by spiders. There were great variations in body size (carapace width) ofA. limbata in the field. Smaller individuals survived at a lower rate to the next stage than larger individuals. This tendency was clearer for the population living under poorer prey availability.A. limbata was unlikely to starve to death in the field because every stage ofA. limbata could survive starvation for a long time in the laboratory, 22-65 days on average. I suggest that the size-dependent survivorship of this spider is associated with vulnerability of smaller individuals to parasitism and predation.

Life History Consequences of Bioenergetic and Biomechanical Constraints on Migration

In this paper I test the hypothesis that bioenergetic and biomechanical constraints to migration play a pivotal role in shaping the life history characteristics of migrants. Firstly, I examine bioenergetic constraints on the ability to migrate by active transport and how they shape the life histories of insects and fish, and, secondly, the consequences of biomechanical constraints to the migration by passive transport in insects and spiders. In both insects and fish the mass-specific energetic costs of active transport (flight and swimming, respectively) decrease with body size, and hence selection should favor large size in migrant species. Because their habitats are ephemeral, migrant insects must grow rapidly. In fish, migrant species are able to exploit resources unavailable to more sedentary species and hence should also show an enhanced rate of growth. These predictions are supported by comparisons within populations, between populations, and among species in both groups. In contrast to the above, biomechanical factors limit the upper size at which insects and spiders can migrate by passive transport. Theory predicts that ballooning will be most likely in spiders considerably less than 6 mm in length. Therefore, species that migrate as adults are predicted to be smaller than those that do not. This prediction is supported by a comparison of migratory and non-migratory spider species from the United Kingdom. The average length of species that migrate as adults, and of migrating young of spiders too large to balloon as adults, is about 2 mm. Further, within this geographic species assemblage, the size distribution of adult spiders is markedly peaked in the 2 mm region, suggesting that biomechanical constraints on ballooning may have a major influence on the evolution of body size in spiders.

Epigeic spiders, their potential prey and competitors: Relationship between size and frequency.

In an analysis of 67 pitfall trap studies in different environments a positive correlation is found between the abundance of spiders and their potential prey, individual main prey groups and individual predator groups. The body-size of spiders and potential prey is significantly correlated both during the day in one locality and between five different localities. Spiders match the size spectrum of their potential prey by an almost equally broad spectrum, whilst the size spectrum of other predator groups is narrower. Therefore, in all size classes spiders exercise optimal predator pressure upon their potential prey. It is suggested that there may be a significant role for spiders as a multi-predator complex in reducing a multi-prey complex.

Scaling of subunit structures in book lungs of spiders (Araneae).

The relationships between dimensions of book lung subunits were measured and analyzed as a function of body size in diverse spiders over a body mass range of 3.4 to 3,190 mg. Book lungs are the characteristic respiratory gas exchange organs in these arachnids. Actual gas exchange occurs across numerous air-filled cuticular plates, which invaginate hemolymph sinuses within the abdomens of these animals. Characteristic linear dimensions of these air-filled compartments reflecting diffusion paths scaled to the 0.2 power of body mass and showed only a fourfold increase over the size range in the sample. This deviation from isometric scaling in the direction obtained and its numerical similarity to scaling of alveolar dimensions to body size in vertebrates was interpreted as an adaptation to reduce diffusion distances between these compartments and vascular fluids. Conversely, lengths and widths of these plates scaled to the one-third power of body mass, isometric scaling, and increased between six-and eightfold over the size range. This result is consistent with the hypothesis that respiratory gas distribution within spider lungs is achieved by convective mixing as has been recently hypothesized.