BEFORE THE WEB: NEOICHNOLOGY OF BURROWING SPIDERS

True spiders (Order Araneae) first appear during the Carboniferous, but fossils of infraorders Mygalomorphae and Araneomorphae do not appear until the Triassic. Although burrowing is considered to be a primitive trait, fossil burrows attributed to spiders are known mainly from the Pleistocene. This may be due to a failure to recognize fossil spider burrows. While many modern mygalomorph and some araneomorph spiders burrow in soil, there are few studies on the morphology of their burrows. This study aims to determine the range of burrow morphologies produced by spiders and to determine if burrow morphology is changed in response to environmental stresses. Two species of burrowing spiders were studied: the mygalomorph Gorgyrella inermis (South African trapdoor spider) and the araneomorph Hogna lenta (field wolf spider). Both species are obligatory burrowers that exhibit different predatory behaviors; G. inermis ambushes prey from its burrow entrance whereas H. lenta leaves its burrow to actively hunt. The morphology of each species’ burrows was determined by placing individuals in sediment-filled terraria for intervals of 14 and 30 days. The composition and moisture content of the sediment was modeled after the natural conditions of each species. Subsequent 30-day trials exposed each spider to sediments of increasing clay content and density and increased or decreased moisture content. At the end of each trial the spiders were removed from their burrows and the burrows were cast. Burrow casts were described and measured for the number of surface openings, maximum depth, tunnel width and height, width-to-height ratio, circumference, slope, total length, complexity, and tortuosity. Both species produced vertical shafts, but H. lenta burrows had one to two surface openings whereas G. inermis had only one. The burrows of G. inermis were deeper and lined with more silk than the burrows of H. lenta. Increasing sediment density produced shallower burrows for H. lenta and no changes for G. inermis. Both species were unable to burrow when sediment was drier than 35%. The results of this study may be used to assist in the recognition of fossil burrows produced by early spiders in units where body fossils are absent and to enable better paleoecological and paleoenvironmental interpretation of units containing fossil burrows.