Fig. 20.—To show fold of (right) wing of Dytiscus.
The cross-mark in the diagram represents a joint in the chitinous rod that forms the wings. This lies just above the cell (which is left white in Fig. 18). The shorter part of the rod is bent down, forming an acute angle (Fig. 20); of course, carrying with it the membranous part of the wing.
This may seem a little difficult. But if it be tried on a specimen, no real difficulty will be experienced. When the wing has been unfolded, it will, if let go, spring back to its old position, the shorter part lying underneath, and the chitinous rod fitting into a groove formed by the projecting sides of the segments of the abdomen.
To this point the sum of our knowledge about Dytiscus amounts to this: It is aquatic in habits; its body is divided into three regions; and it has a pair of membranous wings, covered by chitinous wing-cases, or sheaths, technically called el´ytra (each being an el´ytron). Wing-cases of this kind are the distinguishing mark of the Beetles, or Coleop´tera, though they are not always so well developed as in the specimen with which we are dealing. This we can discover for ourselves by examining all the Land Beetles met with in a country ramble or in a stroll round the garden.
Now let us unpin our Beetle, turn it on its back, and examine it from the under side. Head, thorax, and abdomen may be made out more clearly than before, and we can see that the last two regions are divided into segments.
Let us deal with the head first. This may be easily separated from the thorax with a dissecting needle, or with a pocket-knife—an exceedingly handy tool. The huge goggle-eyes cannot escape observation; and, even without a magnifier, they may be seen to be compound—that is, made up of a number of facets, which show like a fine network.
Just in front of the eyes are the antennae, which serve as organs of touch and perhaps also of other senses.
Kirby has recorded facts which seem to show that the antennae (in some cases) are also organs of hearing. Other authorities, after many observations, have come to the same conclusion. The matter, however, is beset with difficulty. It is certain that some Insects have their ears in their legs; and for the present, at any rate, we may be satisfied to know that the antennae are sense-organs, certainly of touch, probably of smell, and, in some cases, of hearing. An excellent authority on the subject is Sir John Lubbock’s book, The Senses of Animals[11], which contains references to very many original papers.
Fig. 21.