INSECTA. 333 



proboscis. The labrum is triangular and arched, and covers the base 

 of the sucker. 



In the second modification, the labrum and mandibles are nearly 

 obliterated, or are extremely small : the labium is no longer free, 

 and is only distinguishable by the presence of two palpi, to which it 

 gives insertion : the jaws have acquired a most extraordinary length, 

 and are transformed into tubular filaments, which, being united at 

 their edges, compose a sort of spiral proboscis called the tongue, but 

 which, to avoid all equivocation, it would be bettor to call spirigna- 

 tha ; its interior exhibits three canals, the intermediate of which is 

 the duct of the alimentary juices. At the base of each of these fila- 

 ments is a palpus, usually very small, and but slightly apparent. 



The Myriapoda are the only insects in which the mouth presents 

 another mode of organization — it will be explained in treating of 

 that order. 



The trunk * of insects, or that intermediate portion of their body 

 which bears the legs, is generally designated by the term thorax, or 

 corselet by the French. It is composed of three segments, not well 

 distinguished at first, the relative proportions of which vary consider- 

 ably. Sometimes, as in the Coleoptera the anterior, much the largest, 

 separated from the following one by an articulation, moveable, and 

 alone exposed, appears at the first glance to constitute the entire 

 trunk, and is called the thorax or corselet ; sometimes, as in the Hy- 

 menoptera, Lepidoptera, &c., it is much shorter than the ensuing 

 one, has the appearance of a collar, and, with the two others, forms a 

 common body, attached to the abdomen by a pedicle, or adhering 

 closely to it across its whole posterior width, and is also called thorax. 

 These distinctions were insufficient, and frequently ambiguous, inas- 

 much as they were not based on a ternary division, distinctly an- 

 nounced by me in the first edition of this work, as a character pro- 

 per to the Hexapoda. M. Kirby having already employed the deno- 

 mination of metathorax, to designate the after-thorax f , that of 



* This term, here, is synonymous with that of thorax. In order to avoid confusion, 

 I think it would be better to restrict the application of the former to the Linnaean 

 Aptera with more than six legs, and where those organs are borne by particular seg- 

 ments, that is, where the head is distinct from the trunk. With respect to the 

 Crustacea in which these parts of the body are confounded, the thorax might be 

 called thoracida ; and cephalo-thorax in the Arachnides, animals presenting the same 

 character, but in which the trunk or thorax is more simple and provided with fewer 

 appendages. The Entomostraca, in this respect, approach the latter, but as they 

 belong to another class, the term thoracida should still be applied to them ; that of 

 thorax would then be exclusively appropriated to the Hexapoda. 



t This segment should not be restricted, in the Hymenoptera, to this superior, 

 very short, and transverse division of the thorax, on the sides of which the second 



