INSECTA 



457 



Larvae of the Holometabola on the other hand possess, for the 

 most part, mouth parts having a form and mode of working different 

 from that of their aduks, their legs are reduced in size and complexity 

 or even absent, and they show no sign of external wing growth. It 

 is in the pupal stage that adult appendages appear for the first time 

 on the surface. 



Though the many forms of larvae may be regarded as adaptive 

 modifications of a primitive type (for example the eruciform larva as 



Fig. 318. Types of coleopterous larvae. A, Campodeiform larva of Ptero- 

 stichus, Caraiiidae (original). B, Eruciform larva of Melolontha, Scarabaeidae 

 (original). C, Legless larva oi Phyllobius urticae, Curculionidae. After Rymer 

 Roberts. 



an adaptation to a sedentary life among abundant food) their origin 

 may be explained by reference to embryology. In the development 

 of insects a germ band lies ventrM to the yolk and this undergoes 

 development from before backwards progressively, into segments 

 which bear limbs. At an early stage (Fig. 3 19 A), the cephalo-thoracic 

 segments and appendages may be present while the abdomen is as 

 yet unsegmented. A little later (Fig. 319 B), the abdomen becomes 

 segmented and later still (Fig. 319 C), on these segments embryonic 



