138 



EVOLUTION AND GENETICS 



all represent the growing stage of the insect and all transform 

 ultimately into a more or less different resting or pupal stage. 

 During the pupal stage the transition from larval structure to the 

 very different adult structure is accomplished. The pupa trans- 

 forms in due course into the adult, which undergoes no change of 

 form and does not grow. The development of metamorphosis 

 is evidently a factor which enabled the insects to meet the in- 

 creasing rigors of climate during the period of their evolution, 



Fig. 76. — A may-fly, Ephemera varia. A, adult; B, naiad. (After Needham, 

 from Comfstock's Inlroduclion to Entomology, with the permission of the 

 Comstock Publishing Company.) 



since it provides for periodical growth and dormant stages in a 

 very effective manner. 



Diversity of Insects. Structural modifications of insects are, 

 as might he expected in a class containing more than 500,000 

 species, extremely diverse. Every part of the body displays some 

 modification, but the main changes are in the mouth parts and 



wmgs. 



The Mouth Parts. As in all Arthropoda, the mouth parts of the 

 insects are apparently modified jointed appendages. In the primi- 

 tive state they are formed for biting and chewing, but various 

 modifications give rise to suctorial and lapping mouths of the kinds 

 found in true bugs, flies, butterflies and moths, and bees. The 

 biting mouth consists of a pair of mandibles behind which lies a 

 pair of maxillae, and behind the maxillae there is a labium, origi- 

 nally paired. The mouth parts of a cockroach illustrate this type 

 (Fig. 77). Suctorial mouths consist of a trough-like structure 



