414 
THE STUDY OF INSECTS. 
these rudimentary wings is not known; but they doubtless 
have some important use, for they are present in nearly all 
flies, even when the front wings are wanting. 
The mouth-parts of flies are formed for sucking, and 
sometimes also for piercing. Their structure differs greatly 
in different families; and 
in some cases it is exceed¬ 
ingly difficult to determine 
the correspondence of the 
different parts. In the 
more typical forms the 
mouth-parts consist of six 
bristle-like or lance-like 
organs enclosed in a sheath, 
and a pair of jointed palpi. 
A difference of opinion 
exists as to the corre- 
Fig. 482. —A crane-fly, showing wings and Spondence of these parts J 
halteres. 
but according to the most 
generally accepted view the six bristles represent the 
upper lip ( labruni ), the tongue ( hypopharynx ), the two man¬ 
dibles, and the two maxillae, and the sheath enclosing these 
bristles is the lower lip (labium). The palpi which are not 
enclosed in the sheath are the maxillary palpi. At the tip 
of the lower lip there is, on each side, a lobe-like appendage; 
these are the labial palpi. The labial palpi of certain flies 
are quite large; in the House-fly, for example, they are ex¬ 
panded into broad plates, which are fitted for rasping. 
In their transformations flies pass through a complete 
metamorphosis. The larvae are commonly called maggots. 
These are usually cylindrical and are footless; some possess 
a distinct head, others do not; the form of the mouth-parts 
varies greatly in the different families; and there arc re¬ 
markable variations in the form of the respiratory organs, 
especially as to the number and position of the spiracles. 
The pupae are usually either naked or enclosed in the last 
