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 sexvial 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 with 

 salivary glands, a cliaracter, according to him, common to all Insects 

 furnished M'ith a sucker; their structure, however, varies according 

 to the genus *. 



Many of these Insects are noxious, both by sucking our blood, and 

 that of cur domestic animals, by depositing their eggs on tlieir bodies, 

 in order that their larvae 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 larvee 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 an egg. The body of the larva is first detachtd from it, leav- 

 ing on its internal parietes the external organs peculiar to it, such as 

 the hooks of the movith, &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 daj's, those organs be- 

 come denned, 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 larvae with a soft and variable head. 

 This character is almost exclusively peculiar to the larvae of those 

 which are metamorphosed under their skin. Their mouth is usually 

 furnished with two hocks, that enable them to stir up alimentary 

 substances. The principal orifices of respiration, in most of the larvae 

 of the same order, are situated at the posterior extremity of their 



* See his " Recheic.lics Anatomiques sur I'Hippobosqiie des Chevaux," Ann. des 

 Sc. Nat., VI, 3 01. 



