DEVELOPMENT 



127 



This type of transformation, without a true pupal period, is character- 

 istic of the more generalized of the metamorphic insects, namely, Orthop- 

 tera, Platyptera, Plecoptera, Ephemerida (Fig. 19), Odonata (Fig. 20), 

 Thysanoptera and Hemiptera (Fig. 206). These orders constitute the 

 group Heterometabola. Within the limits of the group, however, various 

 degrees of metamorphosis occur; 

 thus Plecoptera, Ephemerida and 

 Odonata undergo considerable 

 change of form; a resting, or 

 quiescent, period may precede the 

 imaginal stage, as in Cicada (Fig. 

 207); while male Coccidae have 

 what is essentially a complete 

 metamorphosis. In fact, the vari- 

 ous kinds of metamorphosis grade 

 into one another in such a way as 

 to make their classification to some 



extent arbitrary and inadequate. 



iic. 205. Pnormia rcgi.na. A, larva; B, 



As there is no distinction be- puparium; C, imago, x 5. 



tween larva and pupa in most 



heterometabolous insects, it is customary to use the term nymph during 

 the interval betw r een egg and imago. 



FIG. 206. Six successive instars of the squash bug, Aiiasa tristis. X 2. 



Ametabola. The most generalized insects, Thysanura and Collem- 

 bola, develop to sexual maturity without a metamorphosis; the form 



