312 



INSECTA. 



Xenos, Ross. 

 Here the two branches of the antennae are inarticulated. The ab- 

 bomen, with the exception of the anus, which is fleshy and retractile, 



IS rT>rnf>^i id 



IS corneous 



Two species of this genus are known, one of which lives on 

 the Wasp, called gallica, and the other on an analogous Wasp 

 of North America, the Polistes fucata, Fab. * 



ORDER XII. 



DIPTERA f. 



The distinguishing characters of dipterous Insects consist in six 

 feet; two membranous, extended wings, with, almost always, two 

 movable bodies above them called halteres | ; a sucker composed of 

 squamous, setaceous pieces, varying in number from two to six, and 

 either enclosed in the superior gx'oove of a probosciform sheath ter- 

 minated by two lips, or covered by one or two inarticulated laminae, 

 which form a sheath for it §. 



Their body, like that of other Hexapoda, is composed of three 

 principal parts. The number of ocelli, when they are present, is 

 always three. The antennae are usually inserted on the front, and 

 approximated at base; those of the Diptera of our first family 

 resemble those of the Nocturnal Lepidoptera in form and compo- 



* See the Memoir of M. Kirby. Lin. Trans., XI. 



t Antldafa, Fab. 



X In order to be convinced that these organs do not represent the second wing?, 

 we must compare the thorax of a large Tipula with that of some Hymenopterous 

 Insect, and particularly of a female Cryptocerus, where the posterior stigmata are 

 very apparent. Here, as in all the Hymenoptera, the segment bearing the second 

 pair of wings is but very slightly developed, or incomplete, and merely follows a 

 small, very narrow, transverse, linear, and extremely short piece, immediately under 

 the scutellura. Next follows the metathorax, which forms that semi-segment, 

 v/hich in my Memoir on the articulated appendages of Insects I have called mediate. 

 Oueach side of it is a spine with two stigmata, more exterior than the spines, and 

 situated at but a little distance from them. The thorax of these Tipula; exhibits 

 the same disposition, except that the semi-segment, which in the Hymenoptera 

 gives insertion to the second wings, is here somewhat less distinct, and that no 

 trace of wings can be perceived at either of the ends. The halteres (balanciers) 

 occupy the precise situation of the spines, and the stigmata, in like manner, are 

 exterior. It is evident, then, that this posterior extremity of the thorax bearing 

 the halteres, corresponds to the mediate segment, that in which the musical organs 

 of the male Cicadas are placed, and which in several Acrydia of the same sex presents 

 analogous peculiarities. 



§ This proboscis is elongated, in several species of the same family, in the manner 

 of a long siphon. 



