Make a drawing of the ventral aspect of the whole body.

Technical Note.—After examining the abdomen thus far, remove it from the rest of the body, and boil it in dilute potassium hydrate (KOH) in a test-tube. This will soften and partially bleach the body wall.

Examine the softened specimen, and note that at least two additional segments are to be found retracted or telescoped into the apparently last segment. The character of these terminal abdominal segments differs in male and female individuals, and specimens of both sexes should be examined. (The males can be distinguished from the females by the peculiar pad-like expansion of the last tarsal segment of the fore legs.) Pull out the retracted segments, and note that they are unevenly chitinized, parts of their surface being simply membranous. Projecting backwards are several long-pointed processes. The female has but one retracted segment. Though the females of many insects possess more or less elaborately developed egg-laying organs, this is not the case with the beetles. Look for spiracles near the lateral margins of the dorsal surface of the abdomen. How many pairs are present?

Internal structure (fig. [40]).—Technical Note.—If fresh specimens are to be had, kill by dropping into the cyanide bottle (see p. [463]). Specimens preserved in a 5% solution of chloral hydrate may be used if necessary. When putting specimens into this solution a small slit should be cut through the body wall to allow the preservative to enter the body cavity. When ready to dissect a specimen cut off the elytra and wings close to the base, and carefully remove all of the dorsal wall of the abdomen and thorax and the median portion of the dorsal wall of the head. Pin out, ventral side down, under water in a dissecting-dish.

Fig. 40.—Dissection of female great water-scavenger beetle, Hydrophilus sp., the heart and tracheæ being cut away.

Note in the median dorsal line of the abdomen a pale transparent longitudinal vessel, the heart or dorsal vessel. Note on each side of it six prominent triangles or "Vs" with apex of each directed laterally, the posterior three smaller than the anterior three of each side. These triangles are formed by respiratory tubes or tracheæ. From each spiracle or breathing-pore there extends into the body a respiratory tube or trachea. These lateral tracheæ join a main longitudinal trachea on each side, from which are given off branches, which in turn repeatedly subdivide, until all parts of the body are ramified by tracheæ, large and small, bringing air to all the tissues. The oxygen is taken up from this air, and carbonic-acid gas is given up to it, when it passes out of the body again through the spiracles. Thus in the insects oxygen and carbonic-acid gas are not carried by the blood but by special air-tubes. The respiratory system of insects is very different from that of other animals.

Mount a bit of trachea in glycerine on a glass slide and examine under the microscope. Note the fine spiral line (looking like transverse annular striations) which is a thickening of the chitinous inner wall of the tube and which by its elasticity keeps the tracheal tubes open.