DIPTERA. 
615 
behind, the space between them being very ample, and divided by a longitudinal impression 
in the middle. The posterior extremity of the metathorax is prolonged into a large scutellum 
over the abdomen. 
These insects live in the larva state between the scales of the abdomen of some Andrenge 
and Wasps, belonging to the subgenus Polistes. They move their prebalancers at the same 
time as their wings. Although apparently far removed, in many respects, from the Hymen- 
optera, I nevertheless consider them nearest allied to some of these insects, such as the 
Eulophi. 
M. Peck has observed the larvse of Xenos Peckii, which is found in Wasps ; it is oval-oblong, 
without feet, annulated, with the anterior extremity dilated into a head, and the mouth formed 
of three tubercles. These larvae are transformed to pupae in the same situation, and beneath 
their own skin, as it appears to me from an ex- 
amination of Xenos Rossii, and without changing 
its form. (See the memoir of M. Jurine upon this 
insect.) Probably the two prebalancers are ser- 
viceable in enabling the insect to disengage itself 
from between the scales of the abdomen of the in- 
sects in which they have lived. 
They are a kind of (Estri of insects. We shall 
subsequently see that a species of Conops under- 
goes its changes in the interior of the abdomen of 
Bonibi. 
They compose [ four genera ] Xenos, Rossi ; 
Sty lops, Kirby [and Elenchus and Halictophagiis, 
Curtis]. They chiefly vary in the form of the 
antennae. The species of the first-named genus live 
in Wasps, and those of Sty lops in Andrence. See 
on these insects the memoir of Kirby, in the 
eleventh volume of the Linncean Transactions j [also the work of Curtis, and several memoirs 
which I have published in the Entomological Transactions^ 
Pis'. l.SO. — A, Stylops Dalii, nat. size ; b, matrnified ; c, An- 
drena, with the heads of two of its larva exserted between 
the abdominal rings a ; d, larva extracted and magnified. 
THE TWELFTPI ORDER OF INSECTS,— 
THE DIPTERA (Antliata, Fab.),— 
lias for its characters six feet, two membranous extended wings, having almost always beneath 
them two moveable slender bodies named halteres, or balancers, (which Latreille, in a note, 
endeavours to prove cannot be the representatives of hind wings, but rather of a pair of 
spines observed in the metathorax of some Hymenoptera, such as Cryptocerus). The sucker is 
composed of scaly, setiform pieces, of variable number (from two to six), and either inclosed in 
a canal on the upper side of the proboscis, which is terminated by two fleshy lip-like lobes, or 
covered by one or two inarticulated plates, which serve it for a sheath. 
The body is composed, as in other hexapod insects, of three principal pieces ; the ocelli, 
when present, are [almost] always three in number, [two in some Tipulidae]. The antennae 
are ordinarily inserted on the forehead ; those of our first family have much relation, both in 
their form, composition, and appendages, with those of the Nocturnal Lepidoptera, but in the 
