Ageing alters spider orb-web construction

Abstract

Ageing is known to induce profound effects on physiological functions but only a few studies have focused on its behavioural alterations. Orb-webs of spiders, however, provide an easily analysable structure, the result of complex sequences of stereotypical behaviours that are particularly relevant to the study of ageing processes. We chose the orb spider Zygiella x-notata as an invertebrate organism to investigate alterations in web geometry caused by ageing. Parameters taken into account to compare webs built by spiders at different ages were: the length of the capture spiral (CTL), the number of anomalies per cm, and four parameters of web regularity (the angle between radii, the number of spiral thread units connecting two successive radii, the parallelism and the coefficient of variation of the distances between silk threads of two adjacent spiral turns). All web parameters were related to ageing. Two groups of spiders emerged: short- and long-lived spiders (with a higher body mass), with an average life span of 150 and 236 days, respectively. In both short- and long-lived spiders' webs, the CTL and the silk thread parallelism decreased, while the variation of the distances between silk threads increased. However, the number of anomalies per cm and the angle between radii increased in short-lived spiders only. These modifications can be explained by ageing alterations in silk investment and cognitive and/or locomotor functions. Orb-web spiders would therefore provide a unique invertebrate model to study ageing and its processes in the alterations of behavioural and cognitive functions.