224 INSECTA. 
alule. One of these pieces is united to the wing and partt- 
cipates in all its motions, but then the two parts are nearly in 
the same plane. The size of these alule is in an inverse ratio 
to that of the halteres. ‘The prothorax is always very short 
and frequently we can merely discover its lateral portions. 
In some, such as the Scenopini, certain Culices, and Psycho- 
dx, they are prominent and tuberculous. ‘The greater part 
of the trunk or thorax is composed of the mesothorax. Be- 
fore, on each side, or behind the prothorax are two stigmata 5 
two others may be observed near the origin of the halteres ; 
those of the mesothorax, as in the Hymenoptera, are con- 
cealed or obliterated. 
The abdomen is frequently attached to the thorax bya 
portion only cf its transversal diameter. It is composed of 
from five to nine apparent annuli, and usually terminates in a 
point in the females; in those where the number of annuli is 
less, the last ones frequently form a sort of ovipositor present- 
ing a succession of little tubes sliding into each other like the 
joints of a spy-glass. The sexual organs of the males are ex- 
terior in many species, and bent under the abdomen. Their 
usually long and slender legs are terminated by a tarsus of 
five joints, the last of which has two hooks, and very often 
two or three vesicular or membranous pellets. 
All the Diptera dissected by M. Leon Dufour were provided 
with salivary glands, a character, according to him, common 
to all Insects furnished with a sucker; their structure, how- 
ever, varies according to the genus(1). 
Many of these Insects are noxious, both by sucking our 
blood and that of our domestic animals, by depositing their 
eggs on their body in order that their larve may feed on 
them, and by infecting our preserved meats and cerealia. 
Others in return are highly useful to us by devouring noxious 
Insects, and consuming dead bodies and animal substances left 
(1) See his “ Recherches Anatomiques sur l’Hippobosque des Chevaux,” Ann. 
des Sc. Nat., VI, 301. . 
