Animal Kingdom V.3

Page 134

234 INSECTA. covered with small scales, which at the first glance resemble dust, and give them the magnificent col?urs in which they are drest. They are easily removed With the finger, and that portion of the wing becomes' transparent. By the aid of glasses we discover that these scales are of various figures, and implanted in the wing by means of a pedicle, arrang~d gra. dually and in ser·ies, like tiles on a roof. Before the superior wirigs of these Insects are two species of epaulcttes-ptery· goda-which extend posteriorly along a I)Ortion of the back on which they are laid. The wings of some Insects remain straight, or· are doubled transversely. Those of others are folded or plaited longitudinally like a fan. Sometimes they are horizontal, and sometimes inclined iu the manner of a roof; in several they cross on the back, and in others they are dis· tant(l ). Directly under them, in the Diptera, are two small movable threads with a claviform termination, which, accord· ing to the general opinion(~) 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 distinguish from the others we will call pro· halteres. Above these appendages 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 Melolonthre, Cantharides, &c., in lieu of the two superior or anterior wings, are furnished with two species of scales, more or less solid and opaque, ( 1) 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 humming produced by these animals. The explanations hitherto given of it are not satisfactory. (2) They are, in my opinion, appendages of the trachere of the first abdominal segment, and correspond to that space, perforated with a small hole, adjacent to the anterior side of an opening, with a membranous and internal diaphragm, tlut is seen on each side in the same segment in everal species of Acrydium. See my Mem. sur les Append. Artie. des Insect., in the Mem. du Mus. d'Hist. Nat. INSECTA. 235 which open and close, and beneath which, when at rest, the wings are transversely folded. These scales or wing cases are called elytra( 1 ). 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 Hemiptera. 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 Hymenoptera, behind the scutellum and on the metathorax, we find a little space called the post-scutellum. The ambulatory organs of locomotion consist of a coxa formed of two pieces, a femur, an uniarticulated tibia, and of a tarsus, which is divided into several phalanges. The num­ber of its articulations varies from three to five, a difference which greatly depends upon the proportional changes expe­rienced by the first and penultimate joints. Although their supputation may sometimes prove embarrassing, and this nume­rical series may not always be in exact accordance with the na­tural order, it furnishes a good character for the distinction of genera. The last joint is usually terminated by two hooks. The form of the tarsi is subject to some modifications, according to the habits of the animal. Those of aquatic species are usually strongly ciliated and flattened, and resemble oars(2). The abdomen, which forms the third and last part of the body, is confounded in the Myriapoda, with the thorax: but in all other Insects, or those which have but six feet, it is dis­tinct. It contains the viscera and the sexual organs, present­ing nine or ten segments or annuli, some of which, however, (1) For their chemical composition, see Odier, Mem. cit., in the Mem. de la Soc. d'IIist. Nat., and the article Imectea of the same work. (2). M. Kirby, in his Monograph of the Bees of Englan~ designates the two anterior tarsi by the name of hands. The first joint is the palm,-palma· This gentleman, in conjunction with M. Spence has published a very complete and d ta•l , e 1 ed work on the elements of Entomology.