[Illustration: 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 {211}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. [Illustration: Fig. 97—Hind- or swimming-leg of _Cybister tripunctatus_. A, The whole leg detached; B, the movable parts in the striking position. _a_, Coxa; _b_, trochanter; _c_, femur; _d_, tibia; _e_, last joint of tarsus.]