kinds attached to the tail. Those of the chamelion fly, the rat-tailed worm, and many of the common, gnats, exhibit beautiful examples of ingenious natural mechanism. It is to the larvæ of Diptera that we apply the common term maggots or maggots; sometimes also they are termed grubs, but that appellation should be confined, for the sake of distinction, to the larvæ of the Coleoptera. They are often very destructive 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 larvæ of the flesh-flies, in particular, (Sarcophaga, and certain species of Musca,) 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 enclosed 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 regard 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,
