926 



INSECTA. 



call it, ossification of the entire organs, and 

 not in any difference in their normal condition. 

 In every instance the anterior pair of wings, or 

 elytra, like the posterior pair, are formed of 

 numerous tracheae, accompanied by circulatory 

 canals extensively ramifying throughout their 

 whole structure, as may be well seen in the 

 imperfectly solidified wings of Orthoptera and 

 Ilemiptera, and in the perfectly formed ones of 

 many of the Coleoptera, although it has some- 

 times been supposed that the elytra are entirely 

 destitute of these structures.* Excepting in a 

 few instances, as in the Strepsiptera,f the elytra 

 are almost entirely motionless during flight, and 

 are either simply elevated or directed horizon- 

 tally in order that they may not impede the 

 motions of the true wings. Thus the number 

 and condition of the parts employed in flight 

 are seen to vary in different insects. In Co- 

 leoptera the posterior wings alone are actively 

 employed, in Neuroptera and Hymenoptera 

 both the anterior and posterior, but in Hymen- 

 optera the latter are smaller and less important 

 than the former, while in Diptera the posterior 

 are reduced to mere appendages of the atrophied 

 meta-thorax, and the office of flight devolves 

 entirely upon the anterior pair, which are the 

 only ones developed for such purpose. On the 

 other hand, in some species, instead of a reduc- 

 tion in the number of these parts, there is an 

 evident tendency to repetition, as is beautifully 

 shown in the existence of two circular mem- 

 branous appendages or winglets (alula) deve- 

 loped at the inner angles of the elytra, and 

 continuous with the delicate membrane that 

 lines the under surface in the great Hydrous 

 and the Dyticida. Similar appendages are 

 observed in the posterior wings of some Lepi- 

 doptera and Hymenoptera, and in the proper 

 wings of some Diptera. The non-developement 

 of the posterior wings in Diptera evidently 

 seems to be the natural result of the excessive 

 developement of the meso-thoracic segment, 

 which bears the proper wings, the analogues of 

 the anterior pair in Hymenoptera, and the con- 

 sequent atrophied condition of the adjoining 

 meta-thoracic segment, from which a posterior 

 pair ought to have been developed. But that 

 all insects, even the Diptera, have primarily 

 the same number of these organs, is exemplified 

 in this order in the existence of a pair of 

 appendages to the meta-thorax, in the form of 

 little club-shaped bodies denominated halteres 

 or poisers, and which exist modified in form 

 in every Dipterous insect. In the common 

 gnat they are simple footstalks surmounted by 

 a round knob, attached one on each side of the 

 atrophied meta-thorax. This is their form in the 

 house-fly and many other genera. That these are 

 the proper representatives of the posterior pair 

 of wings is now the opinion of the most recent 

 observers, and is most decidedly confirmed by 

 the results of our own examinations. They 

 are generally more or less concealed beneath 

 the winglets, from which they are perfectly dis- 

 tinct, being always connected with the meta- 



* Westwood, Text Book, p. 283. 



t Dale in Curtis 's British Entomology, fol. 226. 



thorax, while the winglets are attached to the 

 scutellum of the meso-thorax, and in some 

 instances, as in Tabanus bovinus, are continu- 

 ous with the margin of the meso-thoracic 

 wings. 



The articulations of the wings are formed 

 upon the same principles as those of the legs, 

 but are more simple in their construction. Those 

 at the proximal extremity of the cubital tier- 

 vures, or pterigostia, are of a somewhat cotyloid 

 form to allow of free motion in several direc- 

 tions, and often, as in those at the base of the 

 elytra, are furnished with a long spine or process, 

 to which some of the powerful muscles are 

 attached. Those by which the wings are 

 folded beneath the elytra are imperfectly formed 

 ginglymoid joints in the nervures, and seldom 

 allow of motion in more than one direction. 

 In most Coleoptera, as in Scarabaidte, Hydro- 

 philidtf, &c. there is only one of these joints in 

 each wing, but in the Brachelt/tra, in which 

 the wings are closely packed beneath very short 

 covers, there are often so many as four in each 

 wing,* while in other species, as in the Bu- 

 prestidce, in which the wings are not folded 

 but are only of the length of the abdomen, 

 these joints are entirely absent. In every 

 instance the membranous portions of the wings 

 are either plaited longitudinally or folded trans- 

 versely when the wings are concealed beneath 

 the elytra. 



In the rteuration, or distribution of the tra- 

 cheae in the wings, pterigostia, which by the 

 French entomologists are called nervitres, on a 

 casual inspection there appear to be many 

 remarkable variations. But when the wings 

 are attentively examined, it is found that there 

 is always a great uniformity in the distribution 

 of the principal nervures, and this is so precise 

 and regular in many orders that it has been 

 employed by some naturalists as strongly cha- 

 racterizing different groups. The irregularity 

 which at first is supposed to exist in the dis- 

 tribution of these nervures in some families 

 arises entirely from the greater or less relative 

 enlargement of the principal trunks or their 

 branches. The characters derived from these 

 parts were formerly employed by Frisch in 

 Germany and Harrisf in this country, but 

 have of late years been more particularly ap- 

 plied to the classification of Hymenoptera by 

 Jurine,} St. Fargeau, and Mr. Shuckard, the 

 first two of whom have founded their arrange- 

 ments of Hymenoptera upon characters de- 

 rived almost entirely from these structures, 

 each of which they have designated by a distinct 

 name. Mr. Shuckard, who has studied this 

 subject with much care, gives the following 

 description of the anterior wing in Hymenop- 

 tera^ " The contour of the wing is formed 

 by its anterior, apical, and posterior margins. 



* Straus, Considerat., &c. p. 127. 



t Exposition of English Insects, 4to. London, 

 1782. 



J Nouvelle Methode de Classes les Hymenop- 

 teres et les Dipteres, par L. Jurine, torn. i. 4to. 

 Geneve, 1807. 



Transactions of the Entomological Society of 

 London, vol. i. p. 209. 



