4 CHARLES T. .BRUKS. 



The modifications of the imaginal insect naturally show much 

 greater complexity, and arc far less easily to be correlated with 

 any group of activities. It can be accepted as a general prin- 

 ciple also, that they are in great part seemingly less directly due 

 to the environment, excluding, of course, certain features like 

 those related to oviposition, hypoga'ic life, etc. In spite of 

 many apparently exuberant structural characters, frequent 

 greatly exaggerated development of certain parts, and occasional 

 extreme secondary simplification, the imaginal condition is far 

 more stable. This is most readily appreciated by taxonomists, 

 who experience far less difficulty in delimiting large groups on 

 the basis of adults than they do in attempting a similar treat- 

 ment of larva?. 



The wide applicability of recapitulation and the biogenetic 

 law to the ontogeny animals in general is so clear that its im- 

 portance as a fundamental principle cannot be discounted. The 

 exact nature of the parallelism between phylogeny and ontogeny 

 has always been the basis for divergent views on the part ot 

 biologists, who have readily accepted its existence, and its origin 

 as a part of the evolutionary process. It has been applied with 

 much general success to the embryogeny of the vertebrates and 

 of many invertebrates. In many cases of invertebrates that 

 exhibit striking metamorphosis, like crustaceans and echino- 

 derms, at least some features of the post-embryonic development 

 fall readily in line with the more continuous process of recapitula- 

 tion in the vertebrate embryo. 



It is with the metabola among insects that we find the intia- 

 tion of an almost entirely discontinuous process in development, 

 interpolated between the embryonic development and the sexually 

 mature condition. This is most strikingly illustrated by an 

 insect such as the common housefly. During the embryogeny, 

 the rudiments of antenna 1 , legs and other appendages appear, 

 and give every indication that they will pass over continuously 

 into the homologous parts of the imago. During these Mages 

 the embryo undoubtedly reproduces what we consider to have 

 been the original condition in the ancestors of insects, particularly 

 in reference to the metameric disposition of the parts .ind appen- 

 dages of the head, lu a more generalized used we hud the 



