494 SYNOPTIC COLLECTION. 



The great class of Insecta may be considered as the 

 most specialized of invertebrates. We have briefly traced 

 the evolution of this class from the ancestral type as 

 expressed by the Thysanura to the extremely differen- 

 tiated forms of the Hymenoptera and Diptera. The 

 primitive development of the simplest, wingless species 

 and the straightforward, direct development of the more 

 generalized winged forms are seen to be processes which, 

 if understood, throw strong light on those complex meta- 

 morphoses that characterize the method of indirect devel- 

 opment, and that make the study of insects at once most 

 attractive and most difficult. 



In no class of invertebrates are there such varied 

 adaptations of structure to the favorable and adverse 

 conditions of environment with the correlative increase 

 or decrease in organs as in the Insecta. The Hymenop- 

 tera and Diptera challenge each other as demonstrators 

 of the law of specialization, the one of specialization 

 by addition and the other of specialization by reduction. 

 These forces we have found operative in most of the 

 subkingdoms of invertebrates, but nowhere to such an 

 extreme degree as in these two remarkable and intensely 

 interesting orders of insects the intelligent, progressive 

 Hymenoptera, and the adaptive, reduced Diptera. 



