Fig. 21.—Upper surface of head of Dytiscus. a, labrum, or upper lip; b, clypeus or shield; c, mandible dissected out, and (d) reversed; e, eye; f, antennae.
Fig. 21a.
Fig. 21 a.—Under surface. a, mentum or chin; b, ligula or tongue; c, labial palp; these three together forming the labium, or lower lip; e, eye; f, antennae. Above the maxillae, or lower jaws (d d), are shown dissected out: d1, inner or palpiform lobe; d2, maxillary palp; d3, lacinia or blade; d4, the palpifer or piece that bears the palp (d2); d5, stipes or stalk; d6, the cardo or hinge.
Now we may pass to the mouth parts. It will be good practice to dissect these out, either in air or in water. We may hold a Beetle between the finger and thumb of the left hand, and separate all the parts with a needle held in the right. It is a good plan to gum these parts on a card, for comparison with the figures in our favourite book—whatever that may be—on Natural History, and also with the mouth parts of insects of other Orders. For however much these may differ in form, and in the uses to which they are put, they are really modifications of the same parts.
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).
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.