Notes on the biology of anelosimus pacificus levi, 1963 (Theridiidae, araneae)—Evidence for an evolutionary reversal to a less social state

All hitherto studied Anelosimus species are social, either temporarily (subsocial) or permanently (quasisocial); the genus represents the largest known clade of social spiders. We provide behavioural data on Anelosimus pacificus Levi, 1956 (Theridiidae, Araneae) a species that hitherto was known from only a few specimens collected half a century ago. Anelosimus pacificus, unlike its better known congeners, is solitary for the majority of its lifespan with the juveniles passively receiving maternal care before dispersal at an early instar. Given the phylogenetic position of A. pacificus, this is an example of a behavioural reversal to a less social (solitary) state, and the first such reversal documented in spiders. Interestingly, linked with this is a reversal in other traits. Anelosimus pacificus web architecture, use of sticky silk, the ability of juveniles to emerge from the egg sac, and build and maintain webs without help from the mother, all more resemble traits in typical solitary theridiids than those in its social congeners. These observations may help elucidate what factors contribute to the evolution of sociality in spiders.