DIPTERA. 481 



The term of life assigned to the perfect Aptera is'very short. 

 They all undergo a perfect metamorphosis, modified in two princi- 

 pal ways. Tiie larvce of several change tlieir skin to become 

 nymphs. Some even spin a cocoon, but others never change tiieir 

 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 as- 

 sumes 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 larvas 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 larvas 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 

 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 afl^ected after they have left the mother. In the first sub- 

 division we find Diptera whose antennas are multi-articulated. 



FAMILY I. 



NEMOCERA. 



In this family the antennae usually consist of from fourteen to six- 

 teen joints, and from six, or nine, to twelve, in the others. They are 

 either filiform or setaceous, frequently hairy, particularly in the 

 males, and much longer than the head. The body is elongated, 

 the head small and rounded, the eyes large, the proboscis sahent, 

 3 L 



