RHIPIPTERA. 479 



selves from between the abdominal scales of the Insects on which 

 they have lived. They are a sort of CEstri to Insects, and we shall 

 soon find a species of Conops that undergoes its metamorphosis in 

 the abdomen of the Bombi. The Rhipiptera form two genera. 



STYLOPS, Kirb. 



The first one observed and instituted by M. Kirby. The superior branch 

 of the last segment of the antennae is composed of three little joints. The 

 abdomen is retractile and fleshy. 



But a single species is known; it lives on the Andrenae. 



XENOS, Ross. 



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

 men, with the exception of the anus, which is fleshy and retractile, is cor- 

 neous. 



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 

 Polistesfucata,Tab. 







ORDER XII. 

 DIPTERA(l). 



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 groove of a probosciform sheath ter- 

 minated 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 antennas are usually inserted on the front and 

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

 ble 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 



(1) Two winged. 



