312 INSECTA. 
Xenos, Ross. 
Here the two branches of the antenne are inarticulated. The ab- 
bomen, with the exception of the anus, which is fleshy and retractile, 
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 Aalteres t; a sucker composed of 
squamous, setaceous pieces, varying in number from two to six, and 
either enclosed in the superior groove of a probosciform sheath ter- 
minated by two lips, or covered by one or two inarticulated lamine, 
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 antenne 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. 
+ Anthiata, Fab. 
t In order to be convinced that these organs do not represent the second wings, 
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 scutellum. Next follows the metathorax, which forms that semi-segment, 
which in my Memoir on the articulated appendages of Insects I have called mediate. 
On each 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 Tipule 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 Cicadz 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. 
