THE LARVA. 41 



Besides these, the larvae of several Diptera have been described by 

 different writers, as having, some, prolegs upon all their segments, 

 and others only upon their first and last. Much irregularity appears to 

 prevail in this Order with respect to the feet of the larvae, which 

 is clearly evinced from the descriptions of those of the different 

 families of the Order. The preceding sketch of their distribution 

 must, consequently, suffice for the present, until we proceed to 

 their detailed description. A precise, and, at the same time, natural 

 division of them, is scarcely possible, from their multitudinous differ- 

 ences; but what we have remarked above, we hope will serve, in 

 some measure, as a guide. 



57- 



We now proceed to the consideration of what still remains to be 

 observed upon the construction of the body of the larvae. 



It has already been remarked, that it properly consists of twelve 

 segments, which are separated from each other by slight constrictions. 

 Beyond this, there are but few generalities to notice in it. For the 

 most part, each of the segments, with the exception of the second, third, 

 and last, has, on each side, a small longitudinal aperture, which is 

 surrounded by a broad callous margin, and is called SPIRACLE, or 

 STIGMA (spiraciila, stigma}, and by means of it the air is accessible to 

 the respiratory organs distributed throughout the body. Many of the 

 larvae which live in water, have, instead of spiracles, membranous 

 laminae, or plates, throughout which the trachete, or AIR TUBES, are 

 distributed, and which thus supply the function of gills, and may, 

 therefore, be very properly called gill plates (branchiae, aer'iductus, 

 of Kirby and Spence). They are distinctly observable in the larvae 

 of many May-flies (Ephemera Phryganea). A similar respiratory 

 apparatus is observable in the larvae of many Diptera, although seated 

 at a different part. Some bear, like the larva of Slrntiomys and gnats 

 (Culex), a coronet of a plumose form at their anus, by means of which 

 they more easily sustain themselves at the surface of the water. In the 

 middle of this coronet, or close to very similar appendages, are found 

 the orifices of the tracheae (compare the larva of Dy(iscux); in others 

 (Erisialifi, PI. II. f.8) a pair of thin tracheae run parallely the whole 

 length of the body, and their orifice remains at the surface of the 

 water, while the larvae themselves repose at the bottom of the puddles 

 and pools. 



