Spinybacked Orb-Weaver

Gasteracantha cancriformis — the spinybacked orb-weaver, also called the crablike spiny orbweaver — is one of the most morphologically extraordinary spiders in the southeastern US and one of about 70 species in genus Gasteracantha worldwide. The species is widespread across the southeastern US (Florida, Georgia, the Carolinas, Texas), the Caribbean, Central America, and parts of South America. Adults (females — males are tiny and inconspicuous) are 5-10 mm body length with the species' defining body morphology: the abdomen is dramatically WIDENED LATERALLY into a hardened crab-like shell shape, with SIX SHARP POINTED SPIKES projecting from the margins of the abdomen — two pairs of spikes on the lateral edges and one pair at the rear. The spikes are reinforced sclerotized cuticle and may protect against predator attacks (birds, lizards, larger spiders) or stabilize the spider in the web during prey capture. The species' coloration is extraordinarily variable across individuals: typical 'morphs' include white-with-red-spikes, white-with-black-spikes, bright orange overall, bright yellow overall, and entirely black with red-tipped spikes. The variable coloration pattern is genetically controlled and may aid in confusing predator search images. The species' webs are typical orb webs (radial threads + spiral capture threads), but uniquely include conspicuous TUFTS OR FLAGS of silk attached to the radial threads at intervals. The silk tufts are believed to function as VISUAL WARNINGS to birds to prevent the birds from accidentally flying through the web and destroying it (Eberhard 1973 documented similar functions in related orb-weavers). The species is harmless to humans — small jaws, mild venom that produces only a brief mild stinging sensation similar to a small bee sting if the spider is squeezed. The species is one of the most-photographed small spiders in southeastern US and Caribbean macro nature photography because of the dramatic body shape and bright coloration.