706 TEXT-BOOK OF ENTOMOLOGY 



wings, though having the same habits ; in holometabolous insects, 

 the larva became adapted to entirely different habits and environ- 

 ments, so that in Hymenoptera, and especially Diptera, the larva 

 became remarkably unlike the imago. 



5. Until the Mesozoic age, or late in the Carboniferous period, 

 there were, so far as we now know, only ametabolous and heterometab- 

 olous insects, and these orders (Orthoptera, Dermaptera, Hemiptera, 

 Plectoptera, Odonata, and Neuroptera) were not numerically rich in 

 genera and species, while since early Mesozoic times geological ex- 

 tinction has reduced their numbers. 



6. During the Mesozoic age, and since then, the number of species, 

 genera, families, and orders has greatly increased, and insects have be- 

 come more and more holometabolous. The orders of Coleoptera, Lepi- 

 doptera, Hymenoptera, and Diptera are many fold greater in number 

 of species and variety of form than the heterometabolous orders. 



The rapid increase in the number and variety of types of insects 

 evidently is correlated with the profound geological changes which 

 took place at the end of the Paleozoic age, involving the appearance 

 of larger continental masses, or a greater land area, thus opening new 

 regions for settlement. Also the origin of flowering plants at about 

 this time undoubtedly had much to do with the genesis of new 

 adaptive structures, such as the changes in the mouth-parts and 

 wings. 



7. The process of metamorphosis, at least in the subtropical, tem- 

 perate, and polar regions, is largely dependent on the change from 

 summer to winter, and, in the tropics, from the rainy to the dry 

 season. 



As regards the organization of larval (nepionic) as compared with 

 imaginal forms, the nymphs and larvae of insects are, with the excep- 

 tion of many Diptera, nearly as perfectly developed as the adult. In 

 this respect the immature insect differs fundamentally from the 

 larvae of certain worms (for example, the pilidium of Nemerteans) 

 and from the pluteus and brachiolaria stages of echinoderms, which 

 possess only digestive and water-vascular organs. 



Insect nymphs and larvae also differ from the nauplius and zoea of 

 Crustacea in having at birth all the most important systems of 

 organs (digestive, circulatory, respiratory, nervous, muscular, with 

 sometimes a nearly perfected reproductive system) of the imago, 

 also the same number of cephalic, thoracic, and abdominal segments 

 and appendages. Metamorphism in insects involves (except in the 

 Diptera) rather modifications in the form and functions of organs 

 and appendages already present than the formation of new ones. In 



