260 INSECT TRANSFORMATION 



From these considerations it may be gathered that the 

 metamorphic insects have developed their specialized type 

 of life-history from the simpler and more straightforward 

 course of growth seen among insects of the more primitive 

 groups. It has also been possible to frame suggestions as to 

 the progress of change from the outward to the hidden manner 

 of wing-growth. The question now naturally arises whether 

 this change took place once in an early stock ancestral to 

 all the Endopterygota ; or whether it may have occurred several 

 times various orders or groups of orders arising independently 

 from the Exopterygota, perhaps at various periods. In later 

 pages of this chapter evidence of true relationship between 

 the various orders of the higher insects will be brought forward ; 

 but it may be pointed out, how a comparative study of the 

 larval and pupal stages in different orders, supports the view 

 that the Endopterygota form a natural group which had a 

 single and not a multiple origin in the course of the evolution 

 of the insectan class. 



The development of the wings and other organs of the 

 imago from ingrowing buds follows a specialized method 

 common in its main features to all the orders of metamorphic 

 insects. This community in the fundamental process of 

 growth indicates community of origin. Then similar types of 

 larvae are found in different orders. The active, armoured, 

 relatively long-legged larva that is characteristic of many 

 beetles is seen also as the early stage in the life-history of 

 megalopteroid Neuroptera, and the larvae of the Planipennia 

 are modifications of the same type, with a special adaptation 

 of the jaws for sucking, analogous to the condition found also 

 in some beetles such as the Dyticidae. The legless grub with 

 well-developed head is common to certain beetles (such as the 

 weevils and bark-beetles), to the great majority of the Hymen- 

 optera, and to the more primitive Diptera, while the caterpillar 

 is the characteristic larval form among the Mecoptera, 

 the Lepidoptera and the Hymenopterous saw-flies. These 

 similarities may indeed be explained as convergences due 

 to adaptations to special conditions of life, independently 

 acquired, yet such close likeness could hardly have been 

 reached had there not been a common inheritance of similarly 

 modifiable structures. 



