DIPTERA. 447 



XENOS, Ross. 



Here the two branches of the antennae are in articulated. The abdomen, 

 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 

 fitcata, Fab. 



ORDER XII. 



DIPTERA*. 



THE distinguishing characters of dipterous insects consist in six feet ; two 

 membranous, extended wings, with, almost always, two 

 moveable bodies above them called halteres ; a sucker com- 

 posed of squamous, sectaceous pieces, varying in number 

 from two to six, and either enclosed in the superior groove 

 of a probosciform sheath terminated by two lips, or covered 

 by one or two inarticulated lamina? which form a sheath 

 for it. 



Their body, like that of other hexapoda, is composed of 

 three principal parts. The number of ocelli, when any 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 composition, and frequently in their 

 appendages ; but in the following and greater number of families they consist 

 of but two or three joints, the last of which is fusiform or shaped like a lenti- 

 cular or prismatic palette, furnished either with a little styliform appendage, 

 or a thick hair or seta, sometimes simple and sometimes hairy. Their mouth 

 is only adapted for extracting and transmitting fluids. When these nutritive 

 substances are contained in particular vessels with permeable parietes, the 

 appendages of the sucker act as lancets, pierce the envelope, and open a 

 passage to the fluid, which, by their pressure, is forced to ascend the internal 

 canal to the pharynx, situated at the base of the sucker. The sheath of the 

 latter, or the external part of the proboscis, merely serves to maintain the 

 lancets in situ, and when they are to be employed it is bent back. The base 

 of the proboscis frequently bears two filiform or clavate palpi, composed, in 

 some, of five joints, but in the greater number of one or two. The wings 

 are simply veined, and most frequently horizontal. 



The use of the halteres is not yet well known ; the insect moves them very 

 rapidly. In many species, those of the last families particularly, and above 

 the halteres, are two membranous appendages resembling the valves of a shell, 

 and connected by one of their sides, called (ailerons or cuillerons) alulae. 



* Two- winged. 



