Note the character of the thoracic segments. Examine the wings and legs. The fore wings are modified into strong horny sheaths, or _elytra_, which completely cover and protect the folded hind wings. The hind wings are large and membranous. How are they folded? Note the adaptation of the middle and hind legs for swimming. Determine the various segments of the legs, i.e. _coxa_, _trochanter_, _femur_, _tibia and tarsus_. Note the long longitudinal median keel on the ventral aspect of the thorax. The abdomen articulates with the metathorax by the full width of the broad first abdominal segment. It is composed of a series of segments without appendages, of about equal length but decreasing in width from in front backwards. Of how many segments does the abdomen seem to be composed when viewed from the ventral aspect? From the dorsal? 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. [Illustration: FIG. 40.--Dissection of female great water-scavenger beetle, _Hydrophilus_ sp., the heart and tracheæ being cut away.]