564 
DIPTERA. 
sometimes are mistaken for worms. They vary a good deal 
in their forms, structure, habits, and transformations, so that 
it is somewhat difficult 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 tail, through which they breathe, and the orifice of 
this tube is sometimes suri'ounded with beautiful feather- 
formed appendages. The larvae 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. Larvae of this kind, when fully grown, cast off' their 
skins to become pupae or chrysalids. These pupae 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 burst open their 
pupa-skins to come forth in the perfected or winged state. 
The pupa; of mosquitos 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 larvae of the Dipterous insects in general do not make 
cocoons ; those of some gnats ( Mycetoplrilce ), which live in 
tree mushrooms, or boleti, not only cover themselves with 
a silken web, under which they live, but also spin cocoons, 
wherein they undergo their transformations. Some of the 
Cecidomyians also make silken cocoons. The larvae 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 and 6. 
