OF INSECTS. 
325 
kinds attached to the tail. Those of the chameleon 
flv, the rat-tailed worm, and many of the common 
gnats, exhibit beautiful examples of ingenious natural 
mechanism. It is to the larvee of Diptera that we 
apply the common term maggots or mawks ; some- 
times also they are termed grubs, hut that appellation 
should be confined, for the sake of distinction, to the 
larvee of the Coleoptera. They are often very de- 
structive to corn and meadow grasses by eating the 
roots, and many of them, as is well known, rapidly 
consume animal substances, both in a dead and living 
state. The larvee of the flesh-flies, in particular, 
( Sarcophaga , and certain species of Mitsca,) infest 
living sheep, and frequently prove fatal to them. 
In the greater number of instances, the larva is 
changed into a pupa without shedding the skin ; the 
latter merely hardens, changes its form somewhat 
by contracting, and thus becomes a case for the en- 
closed insect. Sometimes, however, the skin is cast 
off, and even a kind of cocoon formed ; and the nymph 
occasionally retains the power of locomotion. This 
takes place only with such kinds as are aquatic. 
This order is a most extensive one, — indeed there 
is every reason to believe that it falls very little short 
of the Coleoptera in this respect, — and, if we re- 
gard the number of individuals belonging to many 
of the species, they will be found greatly to exceed 
all others. Clouds of musquitoes are common in 
Northern Lapland, and in other countries, so dense 
and extensive as to intercept the rays of the sun ; 
and, when we consider the small size of these insects, 
