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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. Instituto Smithsoniano de Investigaciones Tropicales (STRI) Universidad de Costa Rica Smithsonian Tropical Research Institute, Panama, USA UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Biología

When disturbed, Uloborus glomosus either remain in position at the hub of their orb webs, jump from the web, move to the edge of the web, or shake the web. Juveniles more frequently exhibited moving and jumping responses, whereas the majority of adults jumped from the web or remained in position. Adults with linear stabilimenta tended to shake their webs in the morning and to jump from their webs in the evening. Juveniles with linear stabilimenta shook their webs in the afternoon and evening, whereas those with circular stabilimenta tended to move to the edge of the web or shake the web during all times of day. Adult females with egg-sac chains were usually aligned with the egg sacs and tended to remain in position when disturbed. Both adults and juveniles lacking stabilimenta or egg-sac chains tended to jump from their webs or move to the edge of their webs. Spiders filmed in a natural setting responded to contact by insects. Predatory wasps repeatedly flew near the spiders without eliciting a response. A tethered predatory wasp held above both juveniles and adults in the laboratory also failed to elicit responses from the spiders. The data agree with the hypothesis that moving to the edge of, shaking, and jumping from the web may be predator avoidance strategies elicited by direct contact.

Differences in web structure and cephalothorax features suggest that Hyptiotes cavatus should exert more force while monitoring its vertical triangle-web than Uloborus glomosus exerts while hanging beneath the hub of its horizontal orb-web. When this hypothesis was tested by measuring the force that instars of each species exerted on a horizontal thread, Hyptiotes cavatus was found to exert significantly more force throughout development than did Uloborus glomosus. This relationship holds when either first femur length or body weight is used as an index of spider size.

Members of the genera Uloborus, Hyptiotes, and Miagrammopes have similar web-monitoring postures, but very different webs and tactics for monitoring them. Orb weavers of the genus Uloborus construct horizontal webs and hang from their hubs, whereas reduced-web uloborids construct vertical webs and monitor them from a single thread. To determine if changes in spider strength accompanied web reduction, resting and maximum force measurements were taken of a developmental series of Hyptiotes cavatus, Uloborus glomosus, Miagrammopes animotus, Miagrammopes pinopus, and an undescribed Costa Rican Miagrammopes using a glass needle strain gauge. Both carapace length and spider weight were used as indexes of spider size. Regression analyses of forces show that H. cavatus exerts the greatest relative force and Miagrammopes species the least. This is consistent with requirements for the operation of each web type: Hyptiotes cavatus tenses its entire triangular web and suddenly releases this tension when a prey strikes its web, whereas a Miagrammopes species jerks a single thread that has captured a prey. Within the genus Miagrammopes, the species with the most highly modified carapace expressed the greatest resting force.