THE METAMORPHOSIS OF INSECTS 183 



Occasionally atavistic individual larva; are found which have 

 external wing-buds. 



As to the causes that brought about the internal development of wings we 

 can only make conjectures. It has occurred to the writer that this type of wing- 

 development may have arisen as a result of boring habits, or habits of an analogous 

 nature, of the stem forms from which the orders of the Holometabola sprang. 

 Projecting wing-buds would interfere with the progress of a boring insect; and, 

 therefore, an embedding of them in the body, thus leaving a smooth contour, 

 would be advantageous. 



In support of this theory attention may be called to the fact that the larvae 

 of the most generalized Lepidoptera, the Hepialidae, are borers; the larvae of the 

 Siricidae, which are among the more generalized of the Hymenoptera are borers; 

 so too are many Coleoptera; most larva; of Diptera are burrowers; and the larvae 

 of Trichoptera live in cases. 



The retarding of the development of the compound eyes. One of the 

 most distinctively characteristic features of larvse is the absence of 

 compound eyes. The life of most larva? is such that only limited 

 vision is necessary for them ; and correlated with this fact is a retard- 

 ing of the development of the greater portion of the compound eyes; 

 only a few separate ommatidia being functional during larval life. 



In striking contrast with this condition are the well -developed eyes 

 of nymphs and naiads. 



The larvse of Corethra are the only larvse known to me that 

 possess compound eyes. 



The invaginated conditions of the head in the larva of the more 

 specialized Diptera. The extreme of side wise development is exhibited 

 by the larvas of the more specialized Diptera. Here not only are the 

 legs and wings developed internally but also the head. This phe- 

 nomenon is discussed later. 



The different types of larvae. As a rule, the larvas of the insects of 

 any order resemble each other in their more general characteristics, 

 although they bear but little resemblance to the adult forms. Thus 

 the grubs of Coleoptera, the caterpillars of Lepidoptera, or the mag- 

 gots of Diptera, in most cases, can be recognized as such. Still in 

 each of these orders there are larvas that bear almost no resemblance 

 to the usual type. As examples of these may be cited the water- 

 pennies (Parnidac, Coleoptera), the slug-caterpillars (Cochlidiida?, 

 Lepidoptera), and the larvas of Microdon (Diptera). 



To understand the variations in form of larvas it should be borne 

 in mind that the form of the body in all larvas is the result of secondary 

 adaptations to peculiar modes of life; and that this modification of 

 form has proceeded in different directions and in varying degrees in 

 different insects. 



