MOORE, A.M.F.*; ARGINTEAN, S.; Univ. of the Pacific, Stockton, CA; Univ. of the Pacific, Stockton, CA: Kinematics of prey capture by gumfoot threads of the black widow (Latrodectus hesperus)

We recorded the kinematics of prey capture by the black widow�s gumfoot (gf) threads in order to determine how the trap functions and where energy is stored. Vertical gf lines run from cobweb to substratum. When walking prey disturbs them, the gf threads abruptly spring the prey upward toward the web. Since vertical kinetic energy is generated without action by the spider, energy must have been stored in the silk of the web and then released. Although spider silk is often thought of as an energy absorbing material, we have recently shown that gf silk is capable of 94% energy return provided it is not stretched beyond 2.5% of its original length. Beyond 2.5% strain, the material yields and dissipates the majority of energy. If stretched beyond its yield point, only about 35% of the energy is returned as the thread recoils to its original length. We, therefore, hypothesized that gumfoot threads functioned entirely within the pre-yield region. To test this hypothesis, we videotaped 10 trials of pseudoprey capture by gumfoot threads at 180 frames per second. The prey oscillated up to 2 s after leaving the ground. Pseudoprey consisted of 7-8 mg of insulated copper wire coated in WiteOut. The oscillations were compound, indicating that there is more than one spring in the system. The predominant frequency in each trial was between 40-60 Hz. After the trap was sprung, gf threads were an average 5% shorter than before the trap was sprung. Thus, gumfoot threads are acting in their low efficiency, post-yield range. Additional energy appears to be stored in the rest of the web, as the top of the gf thread rose an average of 2.4 mm in each trial.