Any mantis psychologist would tell you, Entomologic, that your cast-skin eating nymphs suffer from Pica (cf), a well known human disorder in which those afflicted eat objects that have no food value, such as dirt and pecils, etc, etc.
The composition of chitin, an acetyl glucosamine has been described at painful length by some of us on this forum in the past. What is important here is that though this polypeptide is regularly broken down by the enzyme chitinase in order to allow the old skin to be loosened before being discarded, it appears to be indigestible. I have said in the past, wrongly, I now think, that there is evidence that the same enzyme or a close relative, breaks down chitin in the mantid's gut. In fact, I can only find an actual reference to this in regard to an ant (c.f. R.F. Chapman,
The Insects, 4th ed., p.35) that can break down the chitin in fungi.
The problem with using such an enzyme in the mantid's gut is that the food bolus is wrapped in a net of chitin (peritrophic matrix/membrane) that protects the gut wall from physical injury and which would presumably be destroyed by chitinaase.
I have often puzzled over the fact that such a large proportion of the prey animals passes unchanged through the mantis and appears mixed in the tiny pellets of uric acid that the mantis excretes. But the massive elytra of a beetle, say, are not composed just of chitin but by salts like calcium carbonate that strengthen the polypeptide matrix and are, perhaps, leached out of the chitin before it is excreted. Certainly, all (well, most) jokes aside, this is a very interesting phenomenon.
@Deby: That sure is a strong reaction to onions!
