Class 3. INSECTA. 477 



straight, or are folded up transversely; in others they are doubled up, or folded longi- 

 tudinally, like a fan; sometimes they are horizontal, sometimes inclined like a roof; in 

 many they meet upon the back, and in others they are wide apart.* The two- winged 

 insects, of the dipterous order, have also, beneath their wings, two small moveable 

 threads, terminated by a mass, and which, according to the ordinary opinion, replace 

 the pair of wings which are deficient; they are called balancers j {halteres). Other 

 two-winged, very extraordinary insects, have also two balancers, but situated at the 

 anterior extremity of the thorax, and which we name, to distinguish them from the 

 others, prebalancers (prehalteres) . Above the true balancers is a small membranous 

 scale, formed of two pieces, united at one of the edges, and resembling the two shells 

 of a bivalve mollusc: this is the alulet, or cueilleron {alula]. Some aquatic Beetles 

 also exhibit it beneath the elytra, inserted at their base. 



Many insects, such as the Cockchafers, Cantharides, &c., have, instead of the two 

 upper or anterior wings, two scale-like pieces, more or less thickened, and more or less 

 solid and opaque, which open and shut, and beneath which the wings are transversely 

 folded in repose. These scale-like pieces have received the name of elytra.X The in- 

 sects which are furnished with these organs are called Coleoptera, or insects in a sheath. 

 These pieces are never wanting §, but this is not always the case with respect to the 

 wings themselves. In other kinds of insects, the extremity of these scales is entirely 

 membranous, like the wings; and in this case these pieces are called hemelytra : [hence 

 the name of the Linnfean order Hemiptera] . 



The scutellum, or escutcheon, is ordinarily a triangular piece, situated upon the back 

 of the mesothorax, between the places of insertion of the elytra, or wings. It is some- 

 times very large, and then covers the greater portion of the upper side of the abdomen. 

 Various Hymenoptera exhibit behind it, upon the metathorax, a small space called the 

 false escutcheon (post-scutellum). 



The legs are composed of a haunch of two joints \_coxa and trochanter], a thigh 

 \_femur], a shank of a single joint [tihia], and a finger, commonly called the tarsus, 

 which is divided into several phalanges, or joints, the number of which varies from 

 three to five, depending chiefly upon the changes which the first and penultimate joints 

 suff^er in their relative proportions. Although the counting of these joints may some- 

 times prove difficult [from their minuteness] , and the numerical series may not always 

 be in relation with the natural system, it nevertheless forms a good character for the 

 distinction of genera : the last joint is generally terminated by two hooks. The form 

 of the tarsi is subject to some modifications, according with the habits of the insects. 

 Those of the aquatic species are generally flattened, very much fringed, and resemble 

 oars. II 



The abdomen, which forms the third and last part of the body, is confounded with 



* That is, when the insect is in inaction. The rapidity of the vibra- 

 tions of the wings appears lo us to be one of the chief causes of tiie 

 humming noise which many make. The explanations which have been 

 given of it are not satisfactory. [Burmeister, and some others, have 

 considered, more recently, that it is by the action of the air passing 

 rapidly through the metathoracic spiracles, during flight, that this 

 noise is produced]. 



(• These are appendages, in my opinion, of the tracheae of the first 

 abdominal segment, and correspond to the space pierced with a small 

 hole adjacent to the anterior edge of an orifice, with a membranous, 

 internal diaphragm on each side of the same segment in the Locusts. 



sisted upon by Latreille, these balancers will necessarily become 

 metathoracic, and, as such, must be considered analogous to the 

 posterior pair of wings. The large size of the true wings, and of the 

 mesothorax, is in favour of this view of the subject, the alula, as it 

 seeins to me, being nothing but a portion of the fore-wing.] 



t See M. Odier's memoir on the chemical composition of these 

 organs, inserted in the M^m, Societi d'Hiat. Nat. de Pa'v ^translated 

 in the Zoological Journal']. 



§ [Latreille has evideutly overlooked the female of the Glow-worm, 

 that ni Drilus Jinvetcen3, and of Pachypua escavatua, all of which have 

 neitV-er elytra nor wings, although belonging to the order Coleoptera.) 



(See my memoir on the articulated appendage* of insects in the A/^m. | i Mr. Kirby, in his monograph of the Bees of England, and in hia 

 dii Mua. d'Hist. Nat.) [On the supposition that the terminal part of eicellent Introduction to Entomology, calls the tarsi of the fore-legl 

 the thorax of the Diptera is in fac^ thoracic, al.d not abdominal, as in- | the hand, the first joint being the palm. 



