In Fig. 21 we have the upper side and in Fig. 21A the under side of the head represented, so that we may easily get acquainted with the different parts, and the names given to them. The cut should be gone over several times, and the parts in the picture compared with those in the specimen under consideration. It is good practice to endeavour to draw what is seen from the specimen itself, and then to compare the result with the work of the trained artist. And the mouth parts of Dytiscus may be compared with the mouth parts of the Cockroach (Fig. 33). [Illustration: FIG. 22.--Disposition of mouth parts.] Returning to practical work, the first thing is to separate the _labrum_, or upper lip, from the head. Then the large _mandibles_ should be dissected out, and cleaned (by soaking in caustic potash) from the muscles which will come away with them. Behind these are a smaller pair of jaws, the _maxillae_, furnished with a pair of palps, called maxillary palps from their position. These are to be dissected out; and then the lower lip, or _labium_, may be separated by passing a sharpened needle along the line where it joins the chin. The palps on the lower lip are called labial palps. When these parts are cleaned and dried, they should be gummed on card, as shown in Fig. 22, where the long lines represent the upper and lower lips respectively, and the shorter ones the mandible and maxilla of each side. So much for the head. Now we discover that what appeared to be the thorax, when we were looking at the upper surface of the Beetle, and what is called the thorax in descriptions of Beetles, is really but a portion of that region, which is seen to be divided into segments. The covering on the upper surface protects only the first segment, the middle and hinder ones being covered by the wing-cases and the _scutellum_ (a triangular piece jutting backward from the second segment, and meeting the suture). This is not represented in Fig. 17; but we may put in with our pen a tiny triangle, with its base towards the head, and its apex towards the tail--this will meet the case. The first segment bears no appendage above, but to the under side is attached the first pair of legs. The middle segment also carries a pair of legs, and on its upper surface are the wing-cases, to the under side of which, and to the body, the winglets are joined. The last segment bears the wings above, and the last pair of legs below, these being placed very far back, so as to give them greater power in propelling the animal through the water.