THE LARVA 599 



fissures, where the insects are driven to avoid their enemies, and hence do not 

 need wings. The growth of wings and consequently the development of a meta- 

 morphosis is suppressed, so that, as Lang says, "in contrast to the original 

 ametabola of the Apterygota, we have here an acquired ametabola.' 1 ' 1 



It is rare that, after the rudiments of wings have once appeared in the very 

 young, they should disappear in the late nymph stage ; this is, however, said 

 by Walsh to be the case with the Ephemerid Bsetisca (Fig. 440). This is a 

 case of retardation in an acquired ametabolesis. 



THE LAKVA 



The term larva is peculiarly applicable to the young of the 

 holonietabolous orders. The name (Latin, larva, a mask) was first 

 given to the caterpillar because it was thought by the ancients to 

 mask the form of the perfect insect. Swammerdam supposed that 

 the larva contained within itself " the germ of the future butterfly, 

 enclosed in what will be the case of the pupa, which is itself in- 

 cluded in three or more skins, one over the other, that will succes- 

 sively cover the larva." What led to his conception of the nature 

 of these changes was probably his observations on the semi- 

 transparent larva of the gnat, in which the body and limbs of the 

 pupa can be partially seen; for Weismann has shown that the great 

 Dutch observer's belief that the pupal and imaginal skins were in 

 reality already concealed under that of the larva is partially founded 

 in fact. Swammerdam states: "I can point out in the larva all the 

 limbs of the future nymph, or Culex, concealed beneath the skin," 

 and he also observed beneath the skin of the larvae of bees, just 

 before pupating, the antennae, mouth-parts, wings, and limbs of the 

 adult. But, as we shall see farther on, the discovery by Weismann 

 in the larva of the germs of the imago has completely changed our 

 notions as to the nature of metamorphosis, and revolutionized our 

 knowledge of the fundamental processes concerned in the change 

 from larva to pupa, and from pupa to imago. 



Not only are the larvae of each order of insects characteristic in 

 form, so that the grub or larva of "beetles is readily distinguished 

 from those of other orders, or the maggot of flies from the apodous 

 larva of wasps and bees, but within the limits of the larger orders 

 there is great diversity of larval forms, showing that they are the 

 result of adaptation to their surroundings. This is especially the 

 case with the larvae of the Coleoptera, Lepidoptera, Diptera, and 

 Hymenoptera. 



In general, the larvae of insects may be divided into two types, 



