The biology of Phyaces comosus (Araneae: Salticidae), predatory behaviour, antipredator adaptations and silk utilization

Abstract

Recent studies of Portia (Jackson & Blest, 1982; Jackson & Hallas, in press), an unusual salticid genus from Africa, Asia, and Australia, suggest that questions about salticid evolution may not be as intractable as they formerly seemed. Although it moves with apparent ease across open ground and captures prey as a cursorial predator, Portia* also spins prey-capture webs and invades diverse types of alien webs to prey on the host spiders. Lacking acute vision, typical web-building spiders detect and localize prey and predators by interpreting vibratory disturbances of their webs. Using specialized movements of its legs and palps, Portia creates vibrations of the silk that deceive the host spider and assist with predation. Portia also feeds kleptoparasitically on insects ensnared in alien webs and eats the eggs of the host spider. Eggs are an unusual prey for a salticid, since they are non-motile and salticids are generally envisaged as predators of motile arthropods. Morphological specializations give Portia the appearance of detritus in webs, probably affording it protection from visually hunting predators. Normally, locomotion is slow and 'mechanical', rendering Portia difficult to recognize even when moving. When inactive in a web, Portia adopts a specialized posture, the cryptic rest posture, with palps retracted to beside the chelicerae and legs retracted to beside and under the body, thus obscuring the outlines of appendages. Away from webs, P. fimbriata from Queensland has a specialized means of stalking and catching typical cursorial salticids, a prey which, like Portia, has acute vision. This behaviour is unique to this species. When stalking salticids, but not other prey, P. fimbriata exaggerates the slow, mechanical nature of its locomotion, retracts its palps as in the cryptic rest posture and ceases to advance when the salticid faces. Apparently, as a result of these behaviours and the cryptic morphology of P. fimbriata, salticids fail to recognize P. fimbriata as an approaching predator. Although typical cursorial salticids neither spin webs nor use silk in prey capture, they build silken nests in which they moult, oviposit, mate, and generally stay at night and during other periods of inactivity. A salticid in a nest is probably safe from attacks by many of its predators. However, P. fimbriata preys on salticids it locates in nests; it vibrates on the silk, enticing the salticids to come out, or waits patiently until the salticid leaves the nest spontaneously. Although it is a specialized and complex animal, Portia has some morphological characters which are apparently pleisiomorphic (Wanless, 1978, 1984). The occurrence of pleisiomorphic traits in Portia raised the intriguing possibility that some of the behaviours of Portia are also pleisiomorphic. Recognition of this possibility led to a hypothesis, presented in detail elsewhere (Jackson & Blest, 1982), that the Salticidae evolved from web-building spiders with poorly