246 



THE RISE OF ANIMAL LIFE 



Fig. 11-44. Insects show wide variation in the manner in which they cJevelop from egg to the adult. The grass- 

 hopper shows one type, namely, gradual metamorphosis. Note that the young resemble the parents in most 

 respects when hatched. As they grow, however, they acquire adult structures such as functional wings. 



metamorphosis as it appears in other ani- 

 mals will be discussed in a later chapter. 



It has recently been learned that insects 

 as well as Crustacea possess certain hor- 

 mones that influence development and 

 probably profoundly affect other phases of 

 their lives. It has been demonstrated that 

 a gland, the corpus allatum, lying behind 

 the brain, is honnonal in function. When it 

 is removed from the bug Rhodnius, molting 

 does not occur. If the gland is transplanted 

 to other distantly related insects, it is still 

 effective, hence the substance secreted is 

 apparently non-specific. 



OTHER ARTHROPODS 



There are several other groups of less im- 

 portant arthropods which will be considered 

 briefly: the spiders, ticks, and scorpions 

 ( Arachnoidea ) ; the centipedes (Chilo- 

 poda ) ; and the millipedes ( Diplopoda ) , Of 



Fig. 11-45. Insects such as beetles undergo complete 

 metamorphosis during their development. The larva 

 shows no resemblance to the adult and frequently 

 lives in quite a different environment. This is a 

 larva, "grub worm," of the June beetle which ma- 

 tures in the soil, feeding on underground vegetation. 

 Note the nine dark spiracles. 



