564 DIPTERA. 



sometimes are mistaken for worms. They vary a good deal 

 In their forms, structure, habits, and transformations, so that 

 It Is somewhat dlfficvilt to give any general description of 

 them. Their breathing-holes are usually situated near the 

 extremities of the body. Aquatic maggots often have a 

 tubular tall, through which they breathe, and the orifice of 

 this tube Is sometimes svirrounded with beautiful feather- 

 formed appendages. The larva? or maggots of the gnats, 

 and of nearly all those flies which have four or six bristles 

 In the proboscis, have a distinct head covered with a horny 

 shell. Larvoe of this kind, when fully grown, cast off their 

 skins to become pupse or chrysalids. These pujose are usu- 

 ally of a brown color, and somewhat resemble the chrysa- 

 lids of certain moths, or more nearly those of Hymenopte- 

 rous Insects ; for their short and Imperfect legs and wings, 

 though folded on the breast, are not Immovably fastened to 

 It. They commonly have several small thorns on each end 

 of the body, and a row of smaller prickles across each of 

 the rings of the back. By the help of these thorns and 

 prickles they work their way out of the places wherein 

 they had previously lived, just before they bui'st open their 

 pupa-skins to come forth In the perfected or winged state. 

 The pupge of mosqultos are not prickly, but they possess 

 the power of swimming or tumbling about In the water, by 

 the help of two little fins on their tails.* 



The larvffi of the Dipterous Insects in general do not make 

 cocoons ; those of some gnats (^Blycetopldloi), which live In 

 tree mushrooms, or holeti, not only cover themselves with 

 a silken web, under Avhlch they live, but also spin cocoons, 

 wherein they undergo their transformations. Some of the 

 Cecldomylans also make silken cocoons. The larva; of the 

 other flies are not so variable in their forms as the foregoing. 

 They are commonly plump, whitish maggots, obtuse behind, 

 and tapering before, with a small and soft head, that can be 

 drawn within the fore part of the body. They take their 



* See pages 4 aud 5. 



