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Show 320 INSECTA. They pass their first and last stage of existence in the fresh d placid waters of lakes, marshes, ponds, &c. They are and sw·im mers, an d ri·s e occas1· 0na II y to t l1 e surface of their lig oo'd a1 > o de s m. or de r to resp.i re; t Iu 's t h ey eas.il y effect by keq UI . ing their legs motionless, and permitting themselves to fl e~ Their body being reversed, they elevate its posterior extr~a ~ ity a little above the water, raise the extremity of their e]~. tra, or hdep~ess the endh'ofhthe abdomen, in order that air may enter.t e stigmata, w IC are covered by them, whenceit finds Its way to the trachere. They are excessively voracious, and feed on small animals inhabiting the same element which they never leave excepting during the night, or at i~ approach. When taken from the water they diffuse a nause· ating odour. They are frequently attracted into houses b the light of candles, &c. y Their larvre have a long and narrow body composed of twelve rings, the first of which is the largest; a stout head provided with two powerful mandibles, curved into an arc' a.nd perforate~ near the point; small antennre, palpi, and si~ Simple approximated eyes on each side. They have six tol· erably long legs, frequently fringed with hairs, and termi. nated by two small nails. They are active, carnivorous and respire. either by the anus or by a kind of fins resembling branch1re. When about to enter into their pupa state they leave the water. This tribe consists of two principal genera. DYTiscus, Geoff. The Dytisci have filiform antenna! longer than the head two eyes t h e anten.o r legs shorter than the following ones, and the' last most' commonly termi_nated by. a compressed tarsus ending in a point(!). By means of the1r legs frmged with long hail·s, the two last particu· (1) According to M. Leon Dufour, their crop is terminated behind by an annu· lar roll (hom-relet) a character not found in the preceding tribe. Their czcum forms a natatory bladder. Their pectus contains two pneumatic sacs, while the trachece of the other parts are tubular. The adipose splanchnic tissue possesses the characters of a true epiploon or mesentery. Their stigmata also differ from those ofthe Terrestrial Carnivora. COLEOPTERA. 321 I ly they are enabled to swim with gt·eat velocity. They dart ar, • W on other Insects, aquatic orms, &c. In most of the males the ~:ree first joints of the four anterior tarsi are widened and spongy underneath; those of the first pair particularly are very remarkable in the larger species, these three joints forming there a large palette, the inferior surface of which is covered by little bodies, some in the form of papillre, and others, larger, in that of cups or suckers, &c. Some of the females are distinguished from their males by their sulcated elytra. The body of the larva is composed of from eleven to twelve annuli, and covered with a squamous plate; this larva is long, ventricose in the middle, and slender at each end, particularly behind, where the last annuli fot·m an elongated cone furnished on the sides with a fringe of floating hairs, with which, the animal acts on the water, and propels its body forwards; the latter is usually terminated by two conical, bearded and movable filaments. Between them are two small cylindrical bodies, perforated at their extremity by a hole, which arc so many ail'-ducts, and in which the two trachere terminate; stigmata, however, are observed on the sides ofthe abdomen. The head is large, oval, attached to the thorax by a neck, and furnished with strongly arcuated mandibles, under the extremity of which De Geer perceived a longitudinal slit, so that, in this respect, these organs resemble the mandibles of the larva of the Myrmeleon, and serve as suckers; the mouth, however, is provided with maxillre and a labium with pal pi. Each of the three first annuli bears a pair of tolerably long legs, the tibire and tarsi of which are bordered with hairs which afford them additional aid in swimming. Thefirst ring is the largest or longest, and is defended above as well as underneath by a squamous plate. These larvre suspend themselves on the surface of the water by means of twt> lateral appendages at the extremity of their body, which they keep above it. When they wish to change their position, they communicate a sudden vermicular motion to their body, and strike the water with their tail. They feed more. particularly on the larvre of the Libellulre, and those of the Culices and Aselli. When the period of their metamorphosis has arrived, they issue from the water, and having gained the shore penetrate into the earth, which must, however, be constantly moistened, or very humid. They then excavate an oval cavity, and shut ~hemselves up in it. According to Rresel, the eggs of the D. marginalia are hatched from ten to twelve days after they are laid. In four or five days after this epoch, the larva is already five lines in length, and undergoes its first change of tegument. The second ensues at the expi- VoL. lll.-2 Q |
