THE METAMORPHOSIS OF INSECTS 179 



This mode of development is termed cenogenisis .* It contrasts 

 strongly with gradual metamorphosis, where there is a direct develop- 

 ment from the egg to the adult. 



In each of the orders that are characterized by an incomplete 

 metamorphosis, the adaptive characteristics of the young insects fit 

 them for aquatic life; while the adults lead an aerial existence. The 

 transformations of a May-fly (Fig. 201) will serve to illustrate this 

 type of metamorphosis. 



The primitive insects were doubtless terrestrial ; this is shown by 

 the nature of the respiratory system, which is aerial in all insects. In 

 the course of the evolution of the different orders of insects, the 

 immature forms of some of them invaded the water in search of food. 

 This resulted in a sidewise development of these immature forms to 

 better fit them to live in this medium ; while the adult continued their 

 development in, what may be termed by contrast, a direct line. In 

 some of the Plecoptera, as Capnia and others, the results of the ceno- 

 genetic development are not marked except that the immature forms 

 are aquatic. 



In the three orders in which the metamorphosis is incomplete, the 

 cenogenetic development of the immature instars involved neither a 

 change in the manner of development of the wings nor a retarding of 

 the development of the compound eyes ; consequently these immature 

 forms, although sidewise developed, constitute a class quite distinct 

 from larvas. 



The Hemimetabola. The three orders in which the development is 

 a hemimetabolous one are grouped together as the Hemimetabola; 

 these are the Plecoptera, Ephemerida, and Odonata. This grouping 

 together of these three orders is merely for convenience in discussions 

 of types of metamorphosis and does not indicate a natural division of 

 the class Hexapoda. The radical differences in the three types of 

 aquatic respiratory organs characteristic of the three orders indicate 

 that they were evolved independently. 



The term naiad. The immature instars of insects with an incom- 

 plete metamorphosis have been termed nymphs; but as a result of 

 their sidewise development they do not properly belong in the same 

 class as the immature instars of insects with a gradual metamorphosis . 

 I, therefore, proposed to designate them as naiads (Comstock '18, 6). 



The adoption of the term naiad in this sense affords a distinctive 

 term for each of the three classes of immature insects corresponding to 

 the three types of metamorphosis, i. e., nymphs, naiads, and larvae. 



*Cenogenisis : kainos (KU^OS), new; genesis. 



