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. 



The Myriapoda are the only species of vi^hich 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, &c., 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 apphcation 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 [a?itipectus], 

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

 sternum, wliich we also divide into three : — The fore sternum [^prosternwni] , middle 

 sternum [mesosternum'], and hind sternum [nietasternuni]. 



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 

 tnmk to those Aplera of Linnxus which have more than six le^s, and 

 where tlicse limbs are borne upon distinct se)^ments, with the head 



distinct from the trunk. In the Crustacea, w 

 the body are soldered together, the thorax k 

 thoracida, and in the Arachnida, cephalothora 

 simple, with fewer appendages, that of thoras 

 htxiipocl iiisicts. 



t This set^ient ought not to be restricted, 

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

 which the second pair of wings are inserted, bein^ 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 winijed insects, the 



here these two parts of 

 light take the name of 

 X, being here still more 

 i being reserved for the 



in the Hymenoptera, to 



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 divisiiin of 

 the metathorax, the medial segment. Thus, all the segniejits 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. 



