INSECTA. 475 
the canal of the nutritive fluids. At the base of each of these filaments there is a palpus 
! ordinarily very minute, and scarcely visible. 
j The Myriapoda are the only species of which the mouth exhibits another type of con- 
1 struction, which I shall describe when treating upon those insects. 
The trunk* of insects, or that intermediate portion which bears the feet, is generally 
designated by the Latin name thorax, which the French term corselet. It is formed of 
I three segments, which were not at the first carefully distinguished, and of which the 
relative proportions greatly vary. Sometimes, as in the Coleoptera, the anterior is by 
|i far the largest, separated from the following by an articulation, moveable, and alone 
j! exposed ; which alone appears, at first sight, to compose the trunk, and bears the name 
ij of the thorax, or corselet. Sometimes, as in the Hymenoptera, Lepidoptera, &c., it is 
I much shorter than the following, and constitutes, with the two others, a common 
I body, attached to the abdomen by a peduncle, or closely united to it throughout its 
[ entire posterior breadth, and which is called the thorax. 
These distinctions, thus established, were insufldcient, and often ambiguous, as they 
did not rest upon a ternary structure of the thorax, as I had clearly noticed in the first 
I edition of this work, as a character proper to hexapod insects. Mr. Kirby has em- 
! ployed the name of metathorax for the hind part of the thorax.f Those of prothorax 
' and mesothorax naturally presented themselves to the mind when the ternary division 
! of the thorax was once adopted, and the celebrated Professor Nitzsch was the first who 
I used them. Some naturalists have since named the prothorax, or anterior thoracic 
I segment which bears the anterior pair of legs, collar (collare). Wishing to preserve 
I the name corselet, but to restrain its application in proper limits, we shall employ it in 
j all those cases where this segment greatly surpasses the others in size, and where the 
! latter are united to the abdomen so as to appear to constitute an integral part of it, — 
a peculiarity proper to the Coleoptera, Orthoptera, and many Hemiptera. When the 
' prothorax is short, and forms, with the succeeding segments, a common and exposed 
I mass, the trunk, composed of the three segments together, will retain the denomination 
i of thorax. We shall also continue to call the inferior surface of the trunk the breast 
(poitrine), dividing it, according to the segments, into the fore-breast [antipectus] , 
i middle breast [medipectus'] , and hind breast [^postpectus']. The middle line is the 
sternum, which we also divide into three: — The fore sternum [^prostermm'], mididlQ 
sternum [mesosternum'] , and hind sternum [metasternum~\ . 
The teguments of the thoracic segments, as also those of the abdomen, are generally 
divided into rings or semi-rings : one dorsal, or superior, the other inferior, and united 
I laterally by means of a soft and flexible membrane, which is indeed but a less solid 
I portion of the same teguments in many insects, especially the Coleoptera. We also 
observe, at the reunion of these rings, a small space, more solid, or of the substance of 
• To avoid all confusion, it would be better to restrict the term 
trunk to those Aptera of Linnaeus which have more than six legs, and 
where these limbs are borne upon distinct segments, with the head 
distinct from the trunk. In the Crustacea, where these two parts of 
the body are soldered together, the thorax might take the name of 
thoracida, and in the Arachnida, cephalothorax, being here still more 
simple, with fewer appendages, that of thorax being reserved for the 
hexapiid insects. 
+ This segment ought not to be restricted, in the Hymenoptera, to 
the upper, very short, transverse division of the thorax, at the sides of 
which the second pair of wings are inserted, being further composed 
of that portion of the thorax which extends to the base of the abdo- 
men, as is proved by the position of the two last spiracles of the trunk. 
I even think this observation is applicable to all winged insects, the 
metathorax being divided, on the upper side, into two parts, one 
bearing, in the four-winged species, the second wings, and being des- 
titute of spiracles, and the other being furnished with the latter. This 
second part appears to be dependent upon the abdomen, as in nearly all 
insects, except the petiolated Hymenoptera, Rhipiptera, and Diptera. 
Sometimes it is incorporated with the thorax, and closes it posteriorly, 
as in these last insects : hence I have named this second division of 
the metathorax, the medial segment. Thus, all the segments would 
have a pair of spiracles, but those of the mesothorax, scarcely distinct, 
or obsolete, in the Hymenoptera and Diptera, and the two metatho- 
racic, situated upon the segment which immediately follows that which 
bears the second wings. The abdomen will thus be composed of nine 
segments, of which the last three compose the organs of generation. 
