Tuesday, October 18, 2005
Bugs leave stuff behind as they enact their little lives. We call it frass. If there was human frass, it could be stuff like my four pairs of converse caked with mud, grass seeds, gromulch and dog hairs sitting in a heap on the back porch. Some frass is ridiculously easy to find, like the webs you stumble into. Some frass is distressing, like the pile of termite droppings under your picnic table. But other very cool frass, shed exoskeletons, is kind of illusive. It would seem most bugs seek a concealed spot to molt.
The exoskeleton, aka cuticle, is secreted by the bug's epidermis in an amazing process in which the inner layers of the current exoskeleton are digested and recycled into the new exoskeleton. The structure of the exoskeleton is really nifty. One interesting factoid is that it's made of chitin, which also makes up the cell walls of fungi. Hmmmm, bugs and 'shrooms . . . what is the connection? Speaking of mushrooms, my brother-in-law (not the one who resembles the candidate for congress; the other one) and his wife once had dinner with us at Claim Jumper. They both ordered the prime rib (which mushrooms remind me of), ate most of it, and then each swallowed a chitosan tablet. Chitosan is interesting stuff made of chitin, but the jury is way out on whether it prevents fat absorption. Wings, I'd say the most significant evolutionary development in insects, are part of the exoskeleton, which enables flight without sacrificing a set of limbs. The same might be said for mushrooms, and also flying would burn a lot of calories, so it's all connected in a vague sort of way.
One thing the exoskeleton can't do is grow. Young insects'--larvae and nymphs--function is to grow so molt they must in an exquisite paradox: the insect (or spider) must shed its efficient protective layer to emerge as a larger version of its former self. The new covering takes a while to cure, and during this period the insect is pale, soft and vulnerable. Kind of like my feet when I kick off those tennies, although there the vulnerability would lie mostly in the nose of the beholder, not the feet. That must be why the molting creatures seek a hiding spot in which to complete the process. This process is repeated a number of times until the insect matures. Then, their mission is to disperse, mate and procreate. You'll never see frass with wings.
Some exoskeletons have color, but most I've found are pale and transparent. As species with incomplete metamorphosis (like stink bugs) grow, the color patterns in the instars change. The pigments must reside in the two inner layers (the procuticle) which are then reabsorbed and reorganized at each molt. Some insects and spiders change color in response to environmental conditions. What process accomplishes chemical change in the non-living cuticle?
Thanks to the entomology pages by John R. Meyer at NC State University for the info on morphogenesis.