SPIDERS No.38 – A Discarded Exoskeleton Part 2 – Chelicerae

[Walter Piorkowski]

Image 1
An overall view of the cephalothorax exoskeleton including the chelicerae and fangs.
There is no spider here, just the remains of its former exoskeleton. The view is similar to seeing a live spider from above but the chelicera have been raised up at a 90 degree angle. After popping off the carapace cover, the spider has pulled its legs clear of the 8 openings without disrupting the old exoskeleton features.
The two rear leg openings appear slightly cut off at the top of the image. The opening above the chelicerae mold is where the carapace (seen in detail in the previous post) was located. Now instead we see the underside of the sternum, the center of the cephalothorax complex.
Leitz 2.5x Plan Achromat, 35 images at .005 inch increments.

Image 2
Details of the exterior view of the chelicerae with of course a good view of the now discarded fangs.
Leitz 4x Plan Fluorite 50 images at .002 inch increments.

Image 3
A rear leg mold that cracked off while working with the whole exoskeleton. The image provides some idea of how thin the discarded exoskeleton is.
Leitz 4x Plan Fluorite 24 images at .002 inch increments.

Leitz Ortholux microscope
4X Leitz projection eyepiece plus 1/3x relay lens
Canon 50D
Zerene PMax stacking.
Processing in Photoshop, Bibble Pro5

If replacing your whole face including the skin of your eyes isn’t strange enough this installment shows that the spider also gets new legs, palpi and even the jaws or chelicerae. How it extracts its chelicerae, keeping the entire assembly intact is simply amazing.

I can only imagine how difficult it must be to extract yourself from these closefitting copies of your external self. My only similar situation would be the equivalent of pulling off elbow length rubber gloves. However, I have read that a molting fluid is somehow excreted between the two surfaces.

This spider accomplishes this extraction without breaking much, if any of its former exoskeleton. A nice short review of this process can be read on the web site HowStuffWorks ”Spider Molting: the Exoskeleton”.

Walt

[rjlittlefield]

Nice images!

However, I have read that a molting fluid is somehow excreted between the two surfaces.

Quoting from "Biology of Spiders" by Rainer F. Foelix, 2nd edition, page 226*:

The epidermal cells secrete certain enzymes (chitinases and proteases) into the gap that develops between the epithelium and the cuticle, the exuvial space. These enzymes gradually dissolve the endocuticle but attack neither the exocuticle nor the nerve fibers associated with the sensory hairs. The dissolved components of the endocuticle are largely resorbed, which means that very little material is actually lost in the molting process (Fig.188). For a large "tarantula" weighing 80 g, the dry weight of the shed skin was found to be 2 g (Melchers, 1964).

Partially addressing the issues raised in your other thread, Foelix makes clear that the sensory hairs are completely regrown underneath old cuticle. However, he notes (again, page 226) that

It seems particularly interesting that the sensory hairs remain functional during most of the molting process. The reason is that the innervation of the old sensilla is maintained while the new hairs develop on the surface of the hypodermis (Figs 188, 189). The long dendrites innervating each "old" hair sensillum lie freely in the exuvial space and become wrapped by a sheath cell (cell 2 in Fig. 189). This cell starts to secrete cuticle on its outside, thus producing the new hair shaft. Another sheath cell (cell 3 in fig. 189) is responsible for the formation of the socket in which the hair shaft is articulated. About 24 hours before ecdysis all new hair sensilla are completely developed, and only then is the continuity between the old and the new sensory hair broken. When the exuvium is cast off, only the tips of the dendrites are lost, and the new hair sensilla are thus functional immediately after molting (Harris, 1977).

--Rik

* According to Google's Search Inside, corresponding material begins on page 278 of the newer Third Edition.

[Mitch640]

Been finding old castoffs since I was a little kid. I am glad I finally get to see them like this.

[Walter Piorkowski]

Hi Rik. Thank you for filling in these details. I neglected to purchase Mr. Foelix’s book last year, waiting for the third edition. I must get a copy soon.

Thanks again Mitch.

Walt