408 
INSECTA. 
underneath ; tliose of the first pair particularly are very remarhahle 
in the larger species, these three joints forming there a large palette, 
the inferior surface of vhich is covered hy little bodies, some in the 
form of papilla?, and others, larger, in that of cups or suckers, &c. 
Some of the females are distinguished from their males by their 
sidcated elytra. The body of the larva is composed of from eleven 
to twelve anmdi, and covered with a squamous plate ; this larva is 
long, ventricose in the middle, and slender at each end, particu- 
larly behind, Avliere the last annuli form an elongated cone furnished 
on the sides witli a fringe of floating' hairs, with which the animal 
acts on the water, and propels its body forwards; the latter is usually 
terminated hy two conical, bearded and moveable filaments. Between 
them are two small cylindrical bodies, perforated at their extremity 
by a hole, Avhich are so many air-ducts, and in Avhich the tAvo 
tracheae terminate ; stigmata, hoAVOver, are obsei’A'cd on the sides of 
the abdomen. The head is large, oA^al, attached to the thorax by 
a neck, and furnished Avith strongly arcuated mandibles, under the 
extremity of Avhich De Geer percewed a longitudinal slit, .so that, in 
this respect, these organs resemble the mandibles of the lai’A^a of the 
Myrmeleon, and serve as suckers ; the mouth, hoAvever, is provided 
Avith maxilla? and a labium Avith palpi. .Each of the three first annuli 
bears a i?air of tolerably long legs, the tibiae and tarsi of Avhich are 
bordered AA’ith hairs, Avhich afford them additional aid in sAvimming. 
Tlie first ring is the largest or longest, and is defended aboAm as Avell 
as underneath by a squamous plate. 
Tliese larvae suspend themselves on the surface of the Avater by 
means of tAvo lateral appendages at the extremity of their body, 
Avhich they keep aboAm it. When they Avish to change their position, 
they communicate a sudden Amrmicular motion to their body, and 
strike the AA'ater Avith their tail, They feed more particularly on 
the larvae of the Libellulae, and those of the Culices and Aselli. 
\Then the period of their metamorphosis has arriAmd, they issue 
from the AA'atcr, and haAung gained the shore penetrate into the 
earth, Avhich must, hoAveA'er, be constantly moistened, or A^ery 
humid. They then excaA^ate an oval cavity, and shut themselves up 
in it. 
According to Roesel, the eggs of the D. marginalis are hatched 
from ten to tAA^elve days after they are laid. In four or five days 
after this epoch, the larva is already fiAm 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 tAVo inches. They haAm been obseiwed, in 
summer, to enter into their pupa state at the end of fifteen days, and 
to become perfect Insects in fifteen or tAventy more. Besides the 
cloaca of the Insects of this family, the Dytisci haA'c a tolerably long 
caecum, Avhich is perceptible even in the larva. 
This great genus is subdivided as folloAVS ; 
Some have antenna; composed of elcAmn distinct joints, the exte- 
rior palpi filiform or somcAvhat 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 
