408 INSECTA. 



underneath ; those of the first pair iDarticularly are very remarkable 

 in the larger species, these three joints forming there a large palette, 

 the inferior surface of Avhich is covered by little bodies, some in the 

 form of papillae, 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, particu- 

 larly behind, where the last ainiuli form an elongated cone furnished 

 on the sides Avith 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 moveable fdaments. Between 

 them arc two small cylindrical bodies, perforated at their extremity 

 by a hole, Avhich are so many air-ducts, and in which the two 

 tracheae terminate ; stigmata, however, are observed on the sides of 

 tlie 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 

 Avith maxillK? and a labium with palpi. Each of the three first annuli 

 bears a pair of tolerably long logs, the tibiae and tarsi of which are 

 bordered with hairs, Avhich afford them additional aid in swimming. 

 The first ring is the largest or longest, and is defended above as avcU 

 as underneath by a squamous plate. 



These larvae suspend themselves on the surface of the water by 

 means of two 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 larvae of the Libellulae, and those of the Culices and Asclli. 

 When the period of their metamorphosis has arrived, they issue 

 from the water, and having gained the shore penetrate into the 

 earth, Avhich must, however, be constantly moistened, or very 

 humid. They then excavate an oval cavity, and shut themselves up 

 in it. 



According to Roesel, the eggs of the D. marginalis 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 under- 

 goes its first change of tegument. The second ensues at the expi- 

 ration of a similar period, and the animal is then double its former 

 size. Its final length is two inclies. They have been observed, in 

 summer, to enter into their pupa state at the end of fifteen days, and 

 to become perfect Insects in fifteen or twenty more. Besides the 

 cloaca of the Insects of this family, the Dytisci have a tolerably long 

 caecum, which is perceptible even in the larva. 



This great genus is subdivided as follows : 



Some have antennte composed of eleven distinct joints, the exte- 

 rior palpi filiform or somewhat larger at the extremity, and the base 

 of their posterior feet as Avell as that of the others exposed. 



Sometimes the thickness of the antennae gradually diminishes from 



