Complex and surprising molt of adult female Brachypelma klaasi - Report

[OldFlash]

New knowledge has been assimilated. Wasn't too bad to research, I just forgot to reply.

Turns out, that dark spot in the shed exuvium is more like the arthropod equivalent of a scab than scar tissue. When a terrestrial arthropod experiences an injury such as a puncture or tear in the exocuticle, the arthropod's immune system kicks in and quickly clots the wound with cells in the hemolymph called hemocytes. Following the initial plug, further encapsulation of the wound occurs, and a large amount of chemical reactions take place to produce antimicrobial compounds and to produce a more stable and secure plug. The most important of which is melanin production which turns the affected area dark in coloration and provides the majority of the antimicrobial and antifungal properties. Depending on the stage of new cuticle production between ecdysis the injury, immune reaction, and new melanin rich plug can affect the formation of the new cuticle making the injury appear on the new cuticle even after the molt. Best case scenario is that an injury occurs before new cuticle creation and a spider can shed off the "scab"; worst case scenario is that the immune response happens late in the growth cycle and affects the new formation of cuticle to the point it basically can't heal and the spider bleeds out or an infection sets in.

What is particularly interesting in spider immunology is that their immune system is not adaptive. If the built in biochemical processes that encapsulates the wound, and the resulting antimicrobial and antifungal mechanisms don't work, the spider will get sick and die; they do not produce antibodies. Also what is kind of cool is that the whole process of wound healing is fast and something of an all-in-one solution.

After reading about the process of wound healing in terrestrial arthropods, it all matches up to your own observations. Mainly how quickly you noticed the wound healed itself and how the appearance of a dark area that looks burned matches the description of what a melanin rich plug would look like.

This right here is what this forum is all about. Thanks, @AphonopelmaTX. Thank you. I've been closely watching her, even having almost no idea no idea what could've caused it.
Your post gave me at least a starting point on what to look for, and if that’s really what’s going on with her, I can now focus on preventing it instead of being clueless about whether I need to tweak my husbandry or not.
Stuff happens all the time in this hobby, and after years of experience, we usually know how to handle most issues. What’s super frustrating is when you’re completely in the dark with no clue what’s wrong.
I’m not a fan of the “It happens” excuse some folks get when they lose a spider, so I dig for answers however I can.

Thanks again for sharing the knowledge

After all, you can’t fix a problem if you don’t know what’s broken.