Class 3. 



IXSECTA. 



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. 



The Myriapoda are the only species of which the mouth exhibits another type of con- 

 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 

 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 

 far the largest, separated from the following by an articulation, moveable, and alone 

 exposed ; which alone appears, at first sight, to compose the trunk, and bears the name 

 of the thorax, or corselet. Sometimes, as in the Hymenoptera, Lepidoptera, he, it is 

 much shorter than the following, and constitutes, with the two others, a common 

 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 insufficient, and often ambiguous, as they 

 did not rest upon a ternary structure of the thorax, as I had clearly noticed in the first 

 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 oi 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 

 used them. Some naturalists have since named the prothorax, or anterior thoracic 

 segment which bears the anterior pair of legs, collar (collare). Wishing to preserve 

 the name corselet, but to restrain its application in proper limits, we shall employ it in 

 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 

 mass, the trunk, composed of the three segments together, will retain the denomination 

 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 \_anttpectus}, 

 middle breast [medipectus], and hind breast \_postpectus] . The middle line is the 

 sternum, which we also divide into three: — 7'he fore sternum [joros^erwMm], middle 

 sternum [mesosternum], and hind sternum [nietasternwri]. 



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 

 laterally by means of a soft and flexible membrane, which is indeed but a less solid 

 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 Linneeus wliich have more thait six leijs, 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 mij^ht take the name of 

 thoracida, and in the Arachnida, cephalothnrax, being here still more 

 simple, with fewer appendages, that of thorax being reserved for the 

 hexup'irl iiisicts. 



t This segment ought not to be restricted, in the Hymenoptera, to 

 the upper, very short, transverse division of the thorax, at the aides of 

 which the second pair of wiiigs are inserted, being further composed 

 of that portion of the tliorax which extends to the base of the abdo- 

 men, as is proved by the position of tlie two last spiracles of the trunk. 

 1 even think this obicrvatiou 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- 

 titue 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 ntesothorax, scarcely distinct, 

 or obsolete, in the Hymenoptera and Diptera, and the two metathn- 

 racic, situated upon the segmi>nt 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 geueratiou. 



