From the above detailed account of the alimentary canal of the animals whose internal anatomy we are considering, it appears that M. Cuvier's observation—that the length and complication of the intestines indicate a less substantial kind of nutriment—does not hold universally: thus, in Necrophorus and Silpha, carnivorous insects, the intestinal canal in its length and convolutions exceeds those of most herbivorous ones, and in Cassida viridis and some others of the latter tribe are not longer than those of the predaceous beetles. In herbivorous larvæ also, in general, the length of the alimentary canal does not exceed that of the body, but in those of some flesh-flies (Musca vomitoria) it very greatly exceeds it[582]. So true is the observation—that there is no general rule without exceptions.
In this letter it may not be out of place to say a few words upon the excrements of insects; which, strange as the observation may seem, but it is no less true than strange, are sometimes pleasing to the eye, from their symmetry, and to the taste, from their sweetness. In those that masticate their food they are solid, and in those that take it by suction, fluid or semi-fluid. In the caterpillars of Lepidoptera they are of the former description, and every grain wears some resemblance to an insect's egg: as the passage in many of these consists of six fleshy parts separated by channels, so the excrement represents six little prisms separated by six channels[583]. The Aphides all secrete a fluid excrement as sweet as honey, of which the ants are so fond[584], which is ejected not only at the anal passage, but, in many, by two little siphonets also above it[585]. A semi-fluid excrement is produced by some species of Chermes, as that which inhabits the Box, which often comes from the animal in long convoluted strings resembling vermicelli. Reaumur says its taste is agreeable, much more so than that of manna[586]. Under this head should be included the abundant spume with which the larva of Cercopis spumaria envelopes itself[587].
I am, &c.
[LETTER XLI.]
INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED.
SECRETION.
Having given you so full an account of the system of digestion in insects, I am now to say something concerning their secretions, and the organs by which they are elaborated. Though no individual amongst them perhaps secretes so many different substances as the warm-blooded animals; yet in general the Class abounds in secretions perhaps as numerous and extraordinary as in the last-mentioned tribes, to some of which a few of them are analogous, while others are altogether peculiar. We know little or nothing of the mode in which the process of secretion in insects is accomplished; in most cases we cannot even discover, except in general, whence the secreted substance originates; and in others, though we are able to trace the vessels that contain it, we are often in the dark as to their structure.—Cuvier, as has been before hinted, from not being able to detect any thing in them like glands, and from their being constantly bathed in the blood or nutritive fluid, conceives that they separate the peculiar substances they contain, by imbibition or infiltration, through the pores of the skin[588]; a circumstance which seems to indicate a certain conformation of the pores both as to size and figure, so as to enable them to admit only one peculiar product.
In treating on this subject, I shall first consider the organs of secretion, and next their products.