Fig. 93—Amphizoa lecontei. North America. A, Larva; B, imago.
Fig. 94—Pelobius tardus. Britain. A, Young larva; B, adult larva; C, imago. (A and B after Schiödte.)
Fam. 7. Pelobiidae.—Antennae destitute of pubescence: outer lobe of maxilla jointed, metasternum with a short transverse impressed line on the middle behind. Hind legs rather slender, formed for swimming, the tarsi longer than the tibiae. This family is limited to the one genus Pelobius (Hygrobia of some authors). Like Amphizoa, to which it is in several respects analogous, it has a singular geographical distribution; there are only four known species, one lives in Britain and the Mediterranean region, one in Chinese Tibet, two in Australia. Pelobius may be briefly described as a Carabid adapted to a considerable extent for living in and swimming about in water; differing thus from Amphizoa, which has no special adaptation for swimming. The larva of Pelobius is remarkable; it breathes by means of branchial filaments on the under surface of the body, the spiracles being present, though those of the abdomen are very minute and the others small. The head is very large, the mandibles are not tube-like, the food being taken after the manner of the Carabidae; the 8th abdominal segment ends in three long processes; the small 9th segment is retracted beneath them. The adult Pelobius tardus is remarkable for its loud stridulation. The sound is produced by an apparatus described correctly by Charles Darwin;[[91]] there is a file on the inside of the wing-cases, and the Insect turns up the tip of the abdomen and scrapes the file therewith. The Insects are called squeakers in the Covent Garden market, where they are sold.
Fig. 95—Cnemidotus caesus. England. A, Imago; B, larva, highly magnified. (After Schiödte.)
Fam. 8. Haliplidae.—Antennae bare, ten-jointed; metasternum marked by a transverse line; posterior coxae prolonged as plates, covering a large part of the lower surface of the abdomen; the slender, but clubbed, hind femora move between these plates and the abdomen. The Haliplidae are aquatic, and are all small, not exceeding four or five millimetres in length. The ventral plates are peculiar to the Insects of this family, but their function is not known. The larvae are remarkable on account of the fleshy processes disposed on their bodies; but they exhibit considerable variety in this respect; their mandibles are grooved so that they suck their prey. In the larva of Haliplus, according to Schiödte, there are eight pairs of abdominal spiracles, but in Cnemidotus (Fig. 95, B), there are no spiracles, and air is obtained by means of a trachea traversing each of the long filaments. The Insects of these two genera are so similar in the imaginal instar that it is well worthy of note that their larvae should be distinguished by such important characters. Haliplidae is a small family consisting of three genera, having about 100 species; it is very widely distributed. We have 13 species in Britain, all the genera being represented.
Fig. 96.—Cybister roeseli (= laterimarginalis De G.) Europe. A, Larva (after Schiödte); B, ♂ imago.
Fam. 9. Dytiscidae (Water-beetles).—Antennae bare; hind legs formed for swimming, not capable of ordinary walking: metasternum without a transverse line across it; behind closely united with the extremely large coxae. Outer lobe of maxilla forming a two-jointed palpus. The Dytiscidae, or true water-beetles, are of interest because—unlike the aquatic Neuroptera—they exist in water in both the larval and imaginal instars; nevertheless there is reason for supposing that they are modified terrestrial Insects: these reasons are (1) that in their general organisation they are similar to the Carabidae, and they drown more quickly than the majority of land beetles do; (2) though the larvae are very different from the larvae of terrestrial beetles, yet the imaginal instars are much less profoundly changed, and are capable of existing perfectly well on land, and of taking prolonged flights through the air; (3) the pupa is, so far as known, always terrestrial. The larvae and imagos are perfectly at home in the water, except that they must come to the surface to get air. Some of them are capable, however, when quiescent, of living for hours together beneath the water, but there appears to be great diversity in this respect.[[92]] The hind pair of legs is the chief means of locomotion. These swimming-legs (Fig. 97) are deserving of admiration on account of their mechanical perfection; this, however, is exhibited in various degrees, the legs in the genera Dytiscus and Hydroporus being but slender, while those of Cybister are so broad and powerful, that a single stroke propels the Insect for a considerable distance.