INSECTA. 
335 
the semi-annulus to which they belong, the extent and direction of 
that semi-annulus varying, furnish the thorax, thus considered, with 
a combination of characters, which in a systematic point of view are 
of great importance. Some naturalists, Knoch in particular, had 
already employed them, but on no fixed principle, and under arbitrary 
denominations. A necessary preliminary step was the careful and 
comparative study of the thorax, as it exists in all the orders of the 
class of Insects. This was undertaken at my request, by the late 
Lachat. His friend, M. Victor Audouin, has prosecuted his re- 
searches and presented to the Academic des Sciences an excellent 
memoir on the subject. All that is yet known of it, however, is 
from the general sketch given by the Baron Cuvier in his report *, 
* The exposition of the parts of the thorax, and a fixed nomenclature created 
for them, says the Baron in his report, should naturally be placed at the head of 
the work. The trunk of Insects is always divisible into three annuli, each of which 
bears a pair of legs, called by M. Audouin, from their position, the prothorux, the 
mesothorax, and the metathorax. Besides these legs, the mesothorax bears the first 
pair of wings, and the metathorax the second. Each of these three segments is 
composed of four parts; one inferior, two lateral (forming the pectus), and a 
fourth superior, which constitutes the back : the inferior is called the sternum ; the 
lateral portion, or the flank, is divided into three principal parts, one which is 
attached to the sternum, called episternum, another behind the first, and to which 
the coxa is articulated, the epimera (^pimere). A little moveable piece, hitherto 
unknown, which serves to unite the epimera and the coxa, is named trochantinus, 
(trochantin) by way of distinguishing it from trochanter. The third piece of the 
flank, which in the mesothorax and metathorax is situated before the episternum 
and under the wing, is called the hypotherci. Sometimes there is also a small cor- 
neous piece round the stigma, styled the peritrema. The superior portion of each 
segment, which the author calls tergum, is divided into four pieces, named, from 
their position in each ring, prcescutum, scutum, and postscuteUum. The first is fre- 
quently, and the fourth almost always, concealed in the interior. Naturalists have 
seldom distinguished any other part of tlie mesothorax but the scuteltum, which is 
frequently remarkable for its large size and its configuration, although an analogous 
piece is found in the three segments. Thus the trunk of Insects may be divided 
into thirty-three principal parts, and, if we count the hypothera, the number will 
amount to forty-three, more or less visible in the interior. From these pieces, 
besides, arise various internal productions, which, on account of their uses and im- 
portance, require to be named : thus, from the posterior portion of the sternum of 
each segment, a vertical apophysis arises internally, sometimes shaped like a Y, 
called by M. Audouin the entothorax. It furnishes insertions to muscles and pro- 
tects the medullary cord ; an analogous one is seen in the head and sometimes in 
the first annuli of the abdomen. Other internal prominences result from the pro- 
longation of the external neighbouring pieces that are soldered together. M. Au- 
douin names them apodema (apod^mes). Some of them give insertion to muscles, 
others to the wings ; — finally, there are other small moveable pieces either internally 
and between the muscles, or at the base of the wings, which our author styles the 
(^piddmes) epidema. We have stated that the principal pieces, or vestiges of them, 
are always to be found, but they are frequently far from being separable. In par- 
ticular genera, or in certain orders, many are only to be distinguished by traces of 
sutures. M. Audouin — Diet. Class. d’Hist. Nat., art. Insectes — has since substi- 
tuted the name of paraptera for that of hypoptera. That of entothorax will also be 
changed, in some situations, into entucephala, relative to the head — and into ento- 
yaster, as respects the abdomen. He remarks that the head of Insects is composed 
of several segments. We have also observed, that the rostrum of the Cicadae, repre- 
senting the lower lip, is not attached to the head but to the membrane which 
