44-8 1NSECTA. 



One of these pieces is united to the wing and participates in all its motions, 

 but then the two parts are nearly in the same plane. The size of these alula? 

 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. 



The abdomen is frequently attached to the thorax by a portion only of 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 presenting 

 a succession of little tubes sliding into each other like the joints of a spy-glass. 

 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. 



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 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 dissipa- 

 tion 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 larva) 

 of several change their skin 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 detached from it, leaving 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 gelatinous mass, on which none of the parts that characterise the 

 perfect insect can be seen. After the lapse of a few days, those organs become 

 defined and the insect is a true nymph. It extricates itself from confinement 

 by separating the anterior extremity of its case which comes off like a cap. 



The larva 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 larvre 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 hooks that 

 enable them to stir up alimentary substances. The principal orifices of respi- 

 ration, in most of the larvae of the same order, are situated at the posterior 

 extremity of their body. Several of them, besides, present two stigmata on 

 the first ring, that which immediately follows the head or replaces it. 



We will divide the Diptera into two principal sections. In those which 

 compose the first, the head is always distinct from the thorax, the sucker is 

 enclosed in a sheath, and the hooks of the tarsi are simple or dentated. The 

 metamorphosis of the larvae into nymphs is always effected after they have left 

 the mother. In the first subdivision we find Diptera whose antennas are 

 multi-articulated. 



