The heart, already noted, is composed of a longitudinal series of very thin-walled chambers, each with a pair of lateral openings into the body-cavity and with terminal openings into the adjacent chambers. The blood, which is colorless or greenish or yellowish, is sent forward through the successive heart chambers by regular contractions until it finally pours from the most anterior chamber freely into the body-cavity. Here it bathes the body-tissues, flowing perhaps in regular paths, giving up food to the tissues and taking up food from the alimentary canal, until it finds its way through the lateral openings into the heart chamber again. There are no arteries or veins.
Note the large mass of muscles in the metathorax. Note, by attempting to remove it, that the anterior part of the muscle mass is attached to a chitinous partition-wall between the meso- and meta-thorax. Remove this partition-wall (and one between the metathorax and abdomen) and note that certain muscles run deeply down into the body. By pulling on the bits of chitin to which the muscles are attached, the muscles (if they have not been cut) can be stretched to the length of three-quarters of an inch. When released they will contract. (This stretching and contracting takes place only in fresh specimens.) What are these large and numerous muscles of the thorax for?
Remove the thin membrane stretching over the abdomen and in which the heart and tracheal "Vs" lie, and note immediately underneath it the large coiled intestine with a knot of greenish yellow threads in the centre. Carefully uncoil and pin out the intestine, cutting away the tying tracheæ, but being careful not to cut other structures. Work out the full length of the alimentary canal, noting the œsophagus, the widened crop behind it, and the long intestine. From the intestine arise several greenish yellow threads, the Malpighian tubules. These are the excretory organs of the insect. What is the total length of the alimentary canal?
The reproductive organs, consisting of a pair of glands (egg-glands or sperm-glands) with a pair of tubes which unite before reaching the body-wall and have a common external opening, may now be seen. These should be removed, thus exposing the ventral nerve-chain in the abdomen. To expose the chain in the thorax it will be necessary to pick away carefully the muscles. As in the crayfish, the central nervous system in the beetle consists of a ventral nerve-chain, a brain or supra-œsophageal ganglion and a pair of circum-œsophageal commissures connecting the brain and the foremost ganglion (infra-œsophageal) in the ventral chain. There are, in the ventral chain, four ganglia in the thorax and four in the abdomen. The large nerves running from the brain to the compound eyes and to the antennæ can be traced.
Make a drawing showing the nervous system.
Life-history and habits.—The eggs, usually about one hundred, are deposited in a silken sac or case which is spun by the female, and either floats freely or is attached to the under sides of the leaves of aquatic plants. This egg-case is not wholly filled with eggs but has a considerable air-chamber in it, causing it to float. It is oval in shape, and has a peculiar curved horn-like projection at the upper end. In sixteen or eighteen days the young water-scavenger beetles hatch as elongate, wingless, active larvæ, provided with three pairs of legs and strong jaws. They remain for a short time after hatching in the egg-case, feeding on each other! After they issue from the case they feed on flies or other insects which fall into the water, and on snails. They breathe through a pair of spiracles situated at the posterior tip of the abdomen, coming to the surface and thrusting this tip up so that the spiracles are out of water. They grow rapidly, molting three times before becoming full grown. They attain a length of nearly three inches. When full grown they leave the water, crawling out on the damp shore of the pond or stream, and burrow into the soil for a few inches. Here they molt again, or pupate as it is called, changing to a non-feeding, quiescent stage called the pupal stage. The pupa is the stage in which the great changes from wingless, crawling and swimming, short-legged, long, slender-bodied larva to winged, swimming and flying, long-legged, compact, broad-bodied adult are completed. Late in the summer or in the fall the pupal skin breaks and the adult issues. It works its way to the surface of the ground, and betakes itself to the nearest water.
The water-scavenger beetle shows in its post-embryonal development a "complete metamorphosis" as contrasted with the "incomplete metamorphosis" of the locust. Wherever among insects similar changes occur, the young issuing from eggs as larvæ only remotely resembling the parent, and these active feeding larvæ changing finally into more or less quiescent, strictly non-feeding pupæ, which finally change into the active adults, a complete metamorphosis is said to exist. All the beetles, the butterflies and moths, the two-winged flies, the ants, bees and wasps, and certain other groups of insects undergo in their post-embryonic development a complete metamorphosis. The crickets, katydids, the sucking bugs, the May-flies, the white ants and numerous other insects have, like the locust, an incomplete metamorphosis, that is, the young when hatched resemble in most respects, except in the absence of wings, their parents.
The adult water-scavenger beetle feeds chiefly on decaying vegetation in the water, but instances of the taking of other insects and of snails have been noted. Although an aquatic insect the beetle, like its larva, has no gills for breathing the air which is mixed with the water, but has to come to the surface occasionally to obtain air. This it does in an interesting way, which should be carefully observed by the pupils. The air is received and held by a covering of fine hairs on the ventral surface of the body, so that a considerable supply may be carried about by the beetle while underneath the surface. The beetles often leave the water by night, flying abroad to other ponds or streams. In winter the beetles hibernate, burying themselves in the banks of the ponds which they inhabit.
For a good account, with illustrations, of the water-scavenger beetle's life-history see Miall's "Natural History of Aquatic Insects," pp. 61-87.