GROWTH AND TRANSFORMATION 189 



scapularis described by F. Silvestri (1905), who considers 

 that in this species the final larva with external wing- 

 rudiments is a normal prepupal stage in the Ufe-history. 

 The outward appearance of part of a wing- rudiment on the 

 thoracic segment of a beetle or other metamorphic insect 

 may be most reasonably interpreted as a reversion towards 

 the primitive condition found in young exopterygote insects, 

 indicating that from such conditions the metamorphosis 

 has been elaborated by the postponement of the outward 

 appearance of the wing-buds until successively later stages 

 of the life-history. This postponement is clearly correlated 

 with the structural divergence between larva and imago to 

 which reference has already been made. 



It has also been noted that such divergence is commonly 

 associated with difference in feeding-habits. Comparison 

 of larva and imago from this point of view furnishes an 

 interesting and instructive study. Among beetles and the 

 Alderfly group of the Neuroptera, the larva as well as the 

 imago bites soUd food by means of typical insectan 

 mandibles ; in the details of its feeding, however, the larva 

 usually differs from the adult, devouring roots, for example, 

 while the latter eats leaves, or during its life in pond or 

 stream pursuing aquatic prey while the perfect insect 

 attacks inhabitants of the land and air weaker than itself. 

 In most families of Neuroptera as well as the carnivorous 

 Water-beetles (Dyticidae) the perfect insect has normal 

 biting mandibles, while in the larva the slender curved jaws 

 are modified for piercing the bodies of insects which serve 

 as prey and sucking their juices. The very remarkable 

 divergence as regards feeding shown by the Lepidoptera, 

 among which the caterpillar has strong biting mandibles, 

 while butterflies and nearly all moths have vestigial mandibles 

 and elongate flexible maxillae adapted for sucking nectar 

 or other fluid, is familiar to all students of insect life. In 

 connection with our contention as to the mutual divergence 

 of imago and larva among metamorphic insects it is note- 

 worthy that the Micropterygidae, the most primitive of all 

 moths, have still, when adult, small functional mandibles, 



