DIPTERA. 225 
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 
skin to become nymphs. Some even spin a cocoon, but 
others never change their tegument, which becomes sufli- 
ciently solid to form a case for the nymph, resembling a seed 
er anegg. ‘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 characterize 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 larve of the Diptera are destitute of feet, though ap- 
pendages that resemble them are observable in some. This 
order of Insects is the only one in which we find larve with 
a soft and variable head. This character is almost exclusively 
peculiar to the larve 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 larvie 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 re- 
places it. 
Messrs Fallen, Meigen, Wiedemann, and Macquart have 
lately rendered great service to this part of entomological 
science, both by establishing various new geaera, by describ- 
ing a vast number of new species, and by rectifying errors 
relative to several of those previously known. They have also 
employed the characters presented by the disposition of the 
nervures of the wings, to which I first resorted, with a corres- 
ponding nomenclature in my Genera. M. Macquart, in par- 
Vout. IV.—2 D 
