NYMPHS AND LARVJS 55 



Nymphs and Larvae. It will be convenient at this stage to 

 emphasise the fundamental differences between nymphs and larvae. 

 In the strict zoological sense such differences scarcely hold good 

 and, whenever the young differ fundamentally from their parents, 

 the term larva becomes applicable. It has, however, long been 

 both customary and convenient to distinguish between these two 

 types of immature insect. 



An insect undergoing incomplete metamorphosis leaves the egg 

 in a relatively advanced condition of morphological development. 

 In its general structure and body-form it prefigures the imago. 

 Furthermore, it adopts a mode of life similar to that of its parents, 

 it frequents the same habitat, and feeds upon similar food. Its 

 mouth-parts exhibit but slight, if any, structural differences 

 from the final condition assumed by those organs, and compound 

 eyes, usually accompanied by dorsal ocelli, are the functional visual 

 receptors. Rudiments of wings and genitalia develop gradually 

 as external rudiments, and the final transformation comprises 

 little or no marked morphological change other than the acquisition 

 of those organs in their fully developed condition. Physiologically 

 the main change is involved in the increasing development of the 

 sexual organs up to maturity. Immature insects exhibiting these 

 gradual growth changes are termed nymphs. 



It is noteworthy that in the Plecoptera, Ephemeroptera and 

 Odonata the nymphs are exceptional in being aquatic, while their 

 imagines adopt an aerial mode of life. In the possession of special 

 respiratory organs and of other adaptive modifications, which fit 

 them for an aquatic existence, the nymphs of these three orders 

 exhibit more or less evident coenogenetic development. The 

 final transformation into the adult consequently involves more 

 drastic morphological change than usually obtains with typical 

 incomplete metamorphosis. This feature has been emphasised 

 by Comstock (1918), who proposed the term naiad for the 

 immature insect in each of the three orders named, and 

 adopted the expression in his subsequent text-book, " An 

 Introduction to Entomology " (1924). The distinctions between 

 nymphs and naiads, therefore, are of a purely adaptive nature 

 and are not absolute. In certain Plecoptera, for example, 



