INSECTA. 337 



doubled transversely. Those of others are folded or plaited longi- 

 tudinally like a fan. Sometimes they are horizontal, and sometimes 

 inclined in the manner of a roof : in several they cross on the back, 

 and in others they are distant *. Directly under them, in the Diptera 

 are two small moveable threads with a claviform termination, which, 

 according to the general opinion f , seem to replace the two wings 

 that are wanting. They are called (balanciers) halteres. Other 

 two-winged and more extraordinary Insects have also two halteres, 

 but situated at the anterior extremity of the thorax, which to distin- 

 guish from the others we will call prohal teres. Above these append- 

 ages is a little membranous scale formed of two pieces united by one 

 of their edges and resembling a bivalve shell — it is the alula or 

 cueilleron. The same appendage is also observed under the elytra (at 

 their base) of some aquatic Coleoptera. 



Many Insects, such as the Melolonthae, Cantharides, &c., in lieu of 

 the two superior or anterior wings, are furnished with two species of 

 scales, more or less solid and opaque, which oj)en and close, and be- 

 neath which, Avhen at rest, the Avings are transversely folded. These 

 scales or wing cases are called elytra I. The Insects provided with 

 them are named Coleoptera, and in such they are never absent, 

 though this is sometimes the case with respect to the wings. In 

 other Insects the extremity of the scale is completely membranous, or 

 like the wing : they are styled Herniptera. 



The scutel or scutellum is usually a small triangular piece, situated 

 on the back of the mesothorax, and between the insertions of the 

 elytra, or of the wings. Sometimes it is very large, and then it 

 covers the greater part of the superior portion of the abdomen. 

 In various Hymenoptcra, behind the scutellum and. on the meta- 

 thorax, Ave find a little space called the post-scutellum. 



The ambulatory organs of locomotion consist of a coxa formed of 

 two pieces, a femur, an nniarticulated tibia, and of a tarsus, Avhich is 

 divided into several phalanges. The number of its articulations 

 varies from three to five, a difference Avhich greatly depends upon 

 the proportional changes experienced by the first and penultimate 



* The Insect is supposed to be at rest. The rapid vibration of these organs 

 appears to us to be one of the principal causes of the liumminrj produced by these 

 animals. The explanations hitherto given of it are not satisfactory. 



f They are, in my opinion, appendages of the trachejc of the first abdomiual 

 segment, and correspond to that space, perforated with a small hole, adjacent to 

 the antei-ior side of an opening, with a membranous and internal diaphragm, that 

 is seen on each side in the same segment in several species of Acrydium. See my 

 Mem. sur les Append. Artie, des Insect., in the Mem. du Mus. d'Hist. Nat. 



X For their chemical composition, see Odier, Mem. cit. in the Mem. de la See. 

 d'Hist. Nat. ; and the article Insectcs of the same work. 



VOL. I. Z 



