The influence of web monitoring tactics on the tracheal systems of spiders in the family Uloboridae (Arachnida, Araneida)

Summary

Uloborids that spin reduced webs more actively monitor them than those that construct orb webs. Hyptiotes use both their first and fourth legs to tense their triangle-webs, whereas Miagrammopes rely principally on their first legs to monitor and jerk the threads of their irregular webs. The respiratory systems of these spiders include tracheae that extend into the prosoma, bifurcate, and enter the legs. To determine if the legs responsible for active web-monitoring tactics have more extensive tracheal supplies, the total cross sectional area has been computed of the tracheae entering the legs of mature female orb web and reduced web uloborids. Each leg's value has been divided by the cross sectional area of the tracheal trunks that enter the prosoma. These indexes reveal no significant differences between the relative tracheal supplies of the orb weavers investigated (Waitkera waitkerensis, Tangaroa beattyi, Uloborus glomosus). But the first, third, and fourth legs of H. cavatus and the first legs of M. animotus and M. pinopus have greater relative tracheal supplies than those of the three orb weaving species. Relative to leg volume, the first and fourth legs of H. cavatus have the greatest and the first legs of Miagrammopes species the next greatest tracheal supplies. When tracheal lengths are considered, these differences in potential oxygen supplies remain, showing that area differences do not simply compensate for differences in the distances over which oxygen must diffuse. These differences are leg-specific and not species-specific, and uloborids with the most extensive tracheal supplies are found in moist habitats. Thus the observed differences are best explained as adaptations to meet the greater oxygen demands of legs responsible for active web-monitoring tactics and not as adaptations to reduce respiratory water loss.