The mode of life differs very considerably in the two groups of larvæ. Although the Dipterous maggots are not as a rule quite incapable of locomotion like the grubs of the Hymenoptera (bees, ichneumons), the majority are nevertheless possessed of but little power of movement in the food-substance on which they were deposited as eggs. They do not go in search of food, either because they are parasitic in other insects in the same manner as the ichneumons (_Tachina_), or else they live on decaying animal or vegetable substances or amidst large swarms of their prey, like the larvæ of the _Syrphidæ_ amongst _Aphides_. They generally undergo pupation in the same place as that which they inhabit as larvæ and indeed in their larval skin which hardens into an oval pupa-case. Some few leave their feeding place and pupate after traversing a short distance (_Eristalis_). As in the case of the Hymenoptera the structure of the larvæ can here also be explained by peculiarities in their mode of life. Creatures which live in a mass of food neither require special organs of locomotion nor specially developed organs of sense (eyes). They have no use for the three pairs of jaws since they only feed on liquid substances, and the hooks within the mouth do not serve for the reduction of food but only for fastening the whole body. With the jaws and their muscular system there likewise disappears the necessity for a hard surface of attachment, _i.e._ a corneous head. The mode of life of the larvæ of the gnat-type is quite different in most points. The majority, and indeed the most typically formed of these, have to go in search of their food, whether they are predaceous, such as the _Culicidæ_ and many of the other _Nemocera_ (_Corethra_, _Simulium_), or whether they feed on plants, which they in some cases weave into a protective dwelling tube (certain species of _Chironomus_). Many live in water and move with great rapidity; others bury in the earth or in vegetable substances; and even those species which live on fungi sometimes wander great distances, as in the well-known case of the “army worm” where thousands of the larvæ of _Sciara Thomæ_ thus migrate. Now the two types of larvæ correspond generally with the two large groups into which, as it appears to me correctly, the Diptera (_genuina_) are as a rule divided. In this respect there is therefore an equality of form-relationship--the grouping is the same, and the incongruence depends only upon the form-divergence between the two kinds of larvæ being greater than between the two kinds of imagines.[203]