Last night the smaller juvenile Honduran curly hair tarantula (Brachypelma albopilosum) molted. The molt began normally and nearly everything proceeded within a normal time frame of about an hour once the actual molting began.
Near the end of the molt, the distal half (part furthest away from the body) of the right hind leg remained partially enclosed within the old exoskeleton. It seemed that slipping the last leg out of the old exoskeleton shouldn’t be a problem. After all, every molt I’ve observed up to this point had gone smoothly.
Because tarantulas are so soft-bodied and delicate while molting, I knew to leave the spider alone and let nature take its course. It was 2 AM. The worst case scenario might be loss of an end of one of that leg, which should regrow after the next molt. I went to bed.
This morning I awoke to a healthy, active, significantly larger spider with seven legs. The entire leg of concern was shed entirely sometime during the evening.
So, what happened? Bear with me on this, the photograph will tell the story, and it will all make sense.
I photographed and outlined the molting process last spring in an entry entitled “Juvenile Mexican red knee tarantula (Brachypelma smithi) molt sequence.”
After emerging from the old exoskeleton, the tarantula remains on its back for up to a half hour, alternately flexing and pulling the legs back in tightly as seen in the fifth and six images in “Nine image curly haired tarantula (Brachypelma albopilosum) molt sequence.”
The coordinated collective extension and contraction of the legs occurs as the spider pushes blood (hemolymph) into the legs, stretching them to a significantly increased length during that time period.
So, what happened during the six hours between 2 AM and 8 AM? And why did the spider shed the entire leg?
I think I can answer the first question anyway by closely examining the photograph below, taken several hours after the molt.
If you double-click the image, you will see the freshly shed, damaged previous exoskeleton with the partially thickened new leg held firmly in place.
The distalmost end of the retained new leg (part furthest away from the body) appears to have expanded, rupturing the part of the old exoskeleton that originally housed it. The tight ring of the original exoskeleton’s coxa and trochanter was too heavily reinforced to allow the next region of the new leg to expand, effectively trapping the new leg and preventing its expansion.
The regions of the new leg which had been closest to the body are clearly fully expanded, and the break between the coxa of the spider’s new leg and the remaining thickened trochanter, which has expanded to about 1/3 of the width of the original cephalothorax, is evidenced as an open wound at the proximal end of the retained leg.
Kind of gory, but not entirely unexpected.
Shedding of limbs, automy, is certainly not uncommon, as evidenced by the frequency by which we encounter arthropods missing legs.
Rainer Foelix, author of Biology of Spiders, states that one can easily induce a spider to perform automy by pulling or squeezing the femur, a region toward the middle of the leg. Interestingly, doing this to an anesthetized spider does not result in automy, leading to the conclusion that automy within spiders is a voluntary act.
Page 282 of Foelix’s book explains in detail why automy typically occurs at the joint between the coxa and trochanter (close to the body), rather than a joint nearer the area of compression or damage. Essentially, the single muscle traversing the joint between the coxa and trochanter readily detaches itself from the trochanter and withdraws into the coxa. All other muscles in that area act together to close off the end of the coxa similar to the way that the lid closes on a small pedal-based, hinged-lid garbage can. Additionally, the hemolymph (blood) pressure forces the joint membranes to bulge forward, sealing the wound.
Leg automy occurs frequently in nature. In fact, samples of spiders typically show 5 to 20% of a typical field catch of spiders to be missing at least one leg. Most often this occurs in male spiders when an aggressive female tries to consume them, within seconds of a bee or wasp caught in a web successfully stinging the web’s owner in the soft membrane between leg joints, or when a spider is grabbed by a predator.
Another unexpected biology lesson arising from tarantula keeping!