1918] Nymphs, Naiads, and Larvce 223 



The fundamental fact that should be understood is that 

 larvae, i. e., the young of insects with a complete metamorphosis, 

 exhibit a cenogenetic or side wise development. Here the form 

 of the body bears but little relation to the form to be assumed 

 by the adult, the nature of the larval life being the controlling 

 factor. While the ontogeny of an orthopterous insect can be 

 taken as an indication of the course of the phylogeny of the 

 race to which it belongs, this obviously is not the case with 

 representatives of the Diptera or the Lepidoptera. In these 

 orders, and in others with a complete metamorphosis, the larvae 

 have been sidewise developed while the adults have continued 

 their specialization in a direct line. This is an illustration of 

 the fact pointed out by Darwin in his "Origin of Species," that 

 at whatever age a variation first appeared in the parent it tends 

 to reappear at a corresponding age in the offspring. 



The more striking of the adaptive characteristics of larvae 

 are: the form of the body; the internal development of wings; 

 and the retarding of the development of the compound eyes. 

 It is obvious that the restriction of the application of the term 

 larva to the designating of this type of immature insect is a 

 useful one. 



Let us now study those immature insects to which the term 

 nymph has been applied, those that were formerly classed as 

 having an incomplete metamorphosis. 



It is now quite generally recognized that this group of 

 insects included those exhibiting two quite distinct types of 

 metamorphosis: first, those, as the Orthoptisra and Hemiptera 

 for example, in which the development is direct; and second, 

 those in which there is a cenogenetic development. The second 

 group includes the Plecoptera, the Odonata, and the Ephemerida. 



In this second group the ontogeny of the individual does 

 not represent the phylogeny of its race. The recognition of this 

 fact has led to the introduction of a term indicating a third 

 type of metamorphosis. We have, therefore, in the more recent 

 textbooks the following categories, in addition to development 

 without metamorphosis, which is characteristic of the Thysa- 

 nura and Collembola. 



First, gradual metamorphosis or paurometabolous develop- 

 ment. This is characteristic of the Orthoptera, Hemiptera, et al. 



Second, incomplete metamorphosis or hemimetabolous devel- 

 opment. This is characteristic of the Plecoptera, Odonata, and 

 Ephemerida. 



