Estereotipia e plasticidade na seqüência predatória de Theridion evexum keyserling 1884 (Araneae: Theridiidae)

Abstract

Web-spiders' foraging behaviour starts with the selection of a suitable site for web construction. The predatory sequence is initiated following spider contact with or receipt of some stimulus from a potential prey. The sequence continues with an approach or pursuit followed by attack and capture of the prey item; spider predatory behaviour shows varying degrees of plasticity. The present study describes the web of Theridion evexum (Theridiidae), and its predatory sequence upon the ant Atta sp. and Tenebrio molitor beetle larvae. The level of stereotypy present in the predatory sequences of T. evexum was evaluated and compared with other three theridiid species. The predatory sequences of T. evexum were tape-recorded, transferred from the camera to a computer and analyzed with the assistance of EthoSeq, resulting in an ethogram for each of the prey types, and in a list of behavioural routines. The web of T. evexum has viscid elements all around. The selection of microhabitat with high humidity levels could be adaptive in this species, since the humidity conserves the viscid thread properties for a longer period of time, and thus reduces energy expenditures due to thread reposition. Preying upon ants requires more bites and partial transports than preying upon tenebrio larvae. This suggests that the immobilization of the ant is more difficult than the beetle larvae immobilization. The predatory behaviour is equally stereotyped to both prey types. This is probably related to dietary and web structure changes in the evolution from orbweavers to theridiids, which lead to a reduction in prey capture repertoire size in this family. Among the theridiids, T. evexum has the most stereotyped predatory sequence, and this seems to be associated with the low density of threads present in its web.