The unique buckspoor webs of Seothyra optimize silk use and capture efficiency

Abstract

Buckspoor-web spiders of the genus Seothyra are southern African endemics predominantly occurring in arid and semi-arid habitats. They have evolved a uniquely shaped web, comprising a burrow in which the spider rests, linked to a slightly asymmetrical two- to four-lobed web on the soil surface. We investigated the association between three web parameters (periphery, surface area and compactness) and three body size parameters (carapace length, total length and mass) of S. schreineri in central South Africa. Using a novel pixel-seeding image analysis, we were able to accurately determine the surface area of individual webs and compare their area:periphery ratio with that of hypothetical circular webs with the same perimeter length to determine the isoperimetric quotient of each. Isoperimetric quotients across all life stages indicated an average reduction of approximately 25% in silk investment, being highest in juveniles (∼35%). Web periphery was related to web area by a power function, but web area increased at a progressively faster rate than periphery with increasing web size, indicating that larger spiders build less compact webs, i.e. web shape is affected by spider size. Web area and periphery scaled positively and allometrically with the three measures of spider body size, but no significant effect of body size on web compactness was found. We conclude that the unique shape of Seothyra webs evolved as a mechanism to optimize the periphery and concurrently reduce the surface area, thereby enhancing their potential to capture prey and reduce silk investment in the web structure.