Spitting Performance Parameters and Their Biomechanical Implications in the Spitting Spider, Scytodes Thoracica

Spitting spiders Scytodes spp. subdue prey by entangling them at a distance with a mixture of silk, glue, and venom. Using high-speed videography and differential interference contrast microscopy, the performance parameters involved in spit ejection by Scytodes thoracica (Araneae, Scytodidae) were measured. These will ultimately need to be explained in biomechanical and fluid dynamic terms. It was found that the ejection of "spit" from the opening of the venom duct (near the proximal end of the fang) was orderly. It resulted in a pattern that scanned along a lateral-medial axis (due to fang oscillations) while traversing from ventral to dorsal (due to cheliceral elevation). Each lateral-to-medial sweep of a fang produced silk-borne beads of glue that were not present during each subsequent medial-to-lateral sweep. The ejection of "spit" was very rapid. A full scan (5-57 fang cycles, one upsweep of a chelicera) typically occupied less than 30 ms and involved fang oscillations at 278-1781 Hz. Ejection velocities were measured as high as 28.8 m/s. The "spit" was contractile. During the 0.2 s following ejection, silk shortened by 40-60% and the product of a full scan by both of the chelicerae could exert an aggregate contractile force of 0.1 - 0.3 mN. Based on these parameters, hypotheses are described concerning the biomechanical and fluid dynamic processes that could enable this kind of material ejection.