Two weeks ago I received six Mexican red knee tarantula spiderlings. Five of the six had their full intact complement of eight legs.
One had seven legs. Its full size eighth leg was under the paper towel at the bottom of the shipping cup. That’s not a major problem. I’ve seen it numerous times before, and a quick search of the term ‘regeneration’ in this blog will take you to numerous accounts and images dealing with partial/complete leg regeneration in Mexican red knee (Brachypelma smithi) and Honduran curly hair (Brachypelma albopilosum) tarantulas.
Last week I removed the top of the container to give fresh water to that spiderling. Stretched along one of the walls of the container, the spider thrust forward, pushing its front legs toward me and then retreated, leaving its left front leg clinging to the side of the container. The leg pulsed rhythmically for close to an hour before all motion stopped.
At that point the newly severed leg remained in the position shown below, identical to the position of the leg I found at the bottom of the cup when I first unpacked the spiders one week earlier.
Voluntary separation of an extremity (automy) is common among arthropods. Most spiders have the capability to automize legs, and readily do so in response to potentially harmful stimuli. Juvenile spiders and long-lived spiders, such as tarantulas, fully regenerate automized legs and pedipalps over the next several molts.
Automy is a voluntary act. Anesthetized spiders can’t do it. Legs are typically automized between the coxa and trochanter. The coxa (the leg region at the base of the body) is jerked upward, while the femur maintains in its position, acting as a brace. The trochanter tilts sharply, causing the membrane of the joint to rupture dorsally.
The trochanter is traversed by a single muscle, which detaches from the trochanter and then withdraws into the coxal cavity, pulling surrounding sclerites together. The closing is likened to that of a hinge-topped garbage can. Hemolymph (blood) pressure forces the joint membrane to bulge forward, simultaneously sealing off the wound (see below). I noticed no fluid loss whatsoever.
In nature, automy occurs in defense against an aggressor or an attacking female. It may also occur after being stung in a more distal leg region, effectively preventing the venom from reaching the cephalothorax. The process occurs within seconds.
Under natural conditions most spiders lose one or two legs during their lifetime. Among long-lived species, the number of legs naturally lost over a life time can be considerable.
Source: Biology of Spiders, 3rd edition, Rainer Folix