DIPTERA. 



615 



behind, the space between them being very ample, and divided by a longitudinal impression 

 ill 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 Andreuae 

 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 larvae 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 CEstri of insects. We shall 

 subsequently see that a species of Conops under- 

 goes its changes in the interior of the abdomen of 

 Bombi. 



They compose [ four genera ] Xf-nos, Rossi ; 

 Stylops, Kirby [and Elenchus and Halictophagus, 

 Curtis]. They chiefly vary in the form of the 

 antennae. The species of the first-named genus live 

 in Wasps, and those of Stylops in Andrence. See 

 on these insects the memoir of Kirby, in the 

 eleventh volume of the Linnaan Transactions ; [also the work of Curtis, and several memoirs 

 which I have published in the Entomological Transactions^. 



■"it:. ISO. — A, Stylops Dftlii, nat. size; b. mHtfiiified ; c, An- 

 (Irena, with the heads ot two of its Urva exserted between 

 the abdomiual rings a \ n, larva extracted and magnified. 



THE TWELFTH ORDER OF INSECTS,— 



THE DIPTERA (Antliata, Fab.),- 



Ilas 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 Hymenoptcra, 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 Lepidoptcra, but in the 



