Because of the ready adaptation of their labours to many of the conveniences of life. Thus, mead is prepared in many parts of Europe from the honey of bees; silk is employed for clothing: several insects, as cochineal, afford excellent dyes. Galls are employed for ink; wax, for lights, and other purposes. Lac, employed to make varnish, sealing-wax, etc. is produced by a certain Indian species of coccus. As medicines, we have Spanish flies, ants; and, adds Blumenbach, the oil-beetle, recommended for hydrophobia, and many beetles for relieving tooth-ache.
Because they consist of an aggregation of smaller eyes, or those which are termed simple; for their general convexity is divided into one immense number of small hexagonal or six-sided convex surfaces, which may be considered cornea. Simple eyes are formed in the larvae of many winged insects, which upon their last or complete metamorphosis, at the same time that they receive their wings, gain the large compound eyes. The late Mr. T. Carpenter, the optician, of Regent-street, paid more attention to this branch of entomology than any man of his time. By aid of a powerful microscope, he experimented upon upwards of 200 insects; the most familiar of which were the boat-fly, dragon-fly, ant, gnat, bee, wasp, ichneumon, cockchafler, peach-fly, earwig, grasshopper, locust, cricket, and cockroach. His results were a conviction that the whole of these insects did really possess numerous and distinct eyes, varying in number according to the species of insect; in some, upwards of 40 - in others, 1,000; and upwards of 30,000 in some species! The eyes of the libellula, or dragon-fly, Mr. Carpenter says, are, on account of their size, peculiarly well adapted for examination under the microscope. They are a couple of protuberances immoveably fixed in the head, and divided into a number of hexagonal cells, each of which contains a complete eye. The external parts of these eyes are so perfectly smooth, and so well polished, that when viewed as opaque objects, they will, like so many mirrors, reflect the images of all surrounding objects: each of these protuberances, in its natural state, is a body cut into a number of facets, like an artificial multiplying glass - but with this superiority in the workmanship, that as in that glass every facet is plane, here every one is convex; they are also much more numerous, and contained in a much smaller space. Each of the eyes is an hexagon, varying in size, according to its situation in the head; and each of them is a distinct convex lens, and has a similar effect of forming the image of an object placed be-fore it.
Blumenbach observes, compound eyes seem calculated for seeing at a distance ; simple eyes, for looking at near objects ; at least it may be supposed so, as we find that butterflies, in their perfect winged state, have such large compound, telescopic eyes ; whilst, as caterpillars, they have small simple ones. Only a few insects can move their eyes, and from this fact has been deduced a probable explanation of at least one object of the numerous facets or surfaces of which the compound eyes of insects are composed.
Leuwenhoeck has counted 17,235 facets in the cornea of a butterfly.