These divisions are well shown in Fig. 17, where other parts are also marked. It will pay to go over our own specimen with this figure before us, and so make acquaintance with the several parts, to some of which we shall return in greater detail. [Illustration: FIG. 18.--Male Dytiscus in flight.] At this point, if we have not done so before, it will be convenient to fasten our Beetle, in the position figured, by a stout pin driven between the thorax and the abdomen, just above the suture (_h_). We want to raise one of the wing-cases. If a needle be taken in each hand, between the thumb and first two fingers, and that in the left hand be used to steady the creature, the wing-case on the right may be raised with the needle in the right hand, and then cut off. The small filmy membrane, of somewhat triangular shape, which comes off with the wing-case, is the winglet. There is one on each side; and their vibration causes the humming noise made by these insects in flight. When the water dries up in one pond, or food becomes scarce, they will leave and fly off to another. The wing lies folded upon the abdomen. A good deal of very interesting matter has been written on the way in which Insects fold their wings, but we can see for ourselves how this Beetle folds them. All we have to do is to take the wing, and draw it gently away from us, and so unfold it. We may use finger and thumb, or a small pair of forceps. When let go, it will spring back to its old position. Reference to the expanded wing in Fig. 18, and to the diagrams Figs. 19 and 20, will show how the wing is folded. [Illustration: FIG. 19.--To show fold of (right) wing of Dytiscus.] [Illustration: 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.