Feeding by Philoponella vicina (Araneae, Uloboridae) and how uloborid spiders lost their venom glands

Abstract

Feeding by uloborid spiders is unusual in several respects: cheliceral venom glands are absent; prey wrapping is extensive (up to several hundred metres of silk line) and severely compresses the prey; the spider’s mouthparts usually never touch the prey; and the entire surface of the prey is covered with digestive fluid. This paper presents observations on Philoponella vicina O. Pickard-Cambridge, 1899, which provide possible causal links between these traits. The spider begins ingesting soon after it wets the prey, gaining access to the prey’s interior through a broken cuticle that was broken during wrapping and by digestion of the prey’s membranes. The more abundant of the two types of wrapping lines is also digested, but the remaining shroud of wrapping silk is dense and filters digested prey particles. Robust setae on the palpal tarsus and the spread position of the anterior legs during feeding probably protect the spider from contact with the digestive fluid. Spiders extracted about 65% of the wet contents of the prey, but feeding was slow and involved substantial water evaporation. We propose that selection in uloborid ancestors to recover wrapping silk led to increased wetting of the prey’s surface and that compressive wrapping facilitated this wetting. These traits could have led to loss of the now superfluous cheliceral poison glands.