314 INSECTA. 
in those where the number of annuli is less, the last ones frequently 
form a sort of ovipositor, presenting a succession of little tubes sliding 
into each other like the joints of a spy-glass. The sexual organs of 
the males are exterior 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 wath 
salivary glands, a character, according to him, common to all Insects 
furnished with a sucker; their structure, however, varies according 
to the genus *. 
Many of these Insects are noxious, both by sucking our blood, and 
that of our domestic animals, by depositing their eggs on their bodies, 
in order that their larvee 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 on the surface of the earth, that poison the air 
we breathe, and by accelerating the dissipation of stagnant and 
putrid water. 
The term of life assigned to the perfect Aptera is very short. They 
all undergo a perfect metamorphosis, modified in two principal ways. 
The larve of several change their skins to become nymphs. Some 
even spin a cocoon, but others never change their tegument, which 
becomes sufficiently solid to form a case for the nymph, resembling a 
seed or anegg. The body of the larva is first detached from it, leav- 
ing on its internal parietes the external organs peculiar to it, such as 
the hooks of the mouth, &c. It soon assumes the form of a soft or gela- 
tinous mass, on which none of the parts which characterize the perfect 
Insect can be seen. After the lapse of a few days, those organs be- 
come defined, and the Insect is a true nymph. It extricates itself 
from confinement by separating the anterior extremity of its case, 
which comes of like a cap. 
The larvee of the Diptera are destitute of feet, though appendages 
that resemble them are observable in some. This order of Insects is 
the only one in which we find larvee with a soft and variable head. 
This character is almost exclusively peculiar to the larvee of those 
which are metamorphosed under their skin. Their mouth is usually 
furnished with two hooks, that enable them to stir up alimentary 
substances. The principal orifices of respiration, in most of the larvee 
of the same order, are situated at the posterior extremity of their 
* See his ‘‘ Recherches Anatomiques sur l’Hippobosque des Cheyaux,’’ Ann. des 
Sc. Nat., VI, 301. 
