176 AN INTRODUCTION TO ENTOMOLOGY 



adult are essentially the same; the two living in the same situation, 

 and feeding on the same food. The adult has increased power of loco- 

 motion, due to the completion of the development of the wings; this 

 enables it to more readily perform the functions of the adult, the spread 

 of the species, and the making of provision for its continuance; but 

 otherwise the life of the adult is very similar to that of the young. 



The development of a locust or short-horned grasshopper will 

 serve as an example of gradual metamorphosis. Each of the instars 

 of our common red-legged locust, Melanoplus femur-rubrum, is repre- 

 sented in the accompanying series of figures. The adult (Fig. 199) 

 is represented natural size; each of the other instars, somewhat 

 enlarged; the hair line above the figure in each case indicates the 

 length of the insect. 



The young locust just out from the egg-shell can be easily recog- 

 nized as a locust (Fig. 194). It is of course much smaller than the 

 adult; the proportion of the different regions of the body are some- 

 what different ; and it is not furnished with wings ; still the form of the 

 body is essentially the same as that of the adult. In the second and 

 third instars (Fig. 195 and 196) there are slight indications of the 

 development of wing-rudiments; and these rudimentary wings are 

 quite conspicuous in the fourth and fifth instars (Fig. 197 and 198). 

 The change at the last ecdysis, that from the fifth instar to the adult r 

 is more striking than that at any preceding ecdysis ; this is due to the 

 complete expansion of the wings, which takes place at this time. 



The Paurometabola. Those orders of insects that are characterized 

 by a gradual metamorphosis are grouped together as the Paurometa- 

 bola. This is not a natural division of the class Hexapoda but merely 

 indicates a similarity in the nature of the metamorphosis in the orders 

 included. [This group includes the Isoptera, Dermaptera, Orthop- 

 tera, Corrodentia, Thysanoptera, Homoptera, and Heteroptera. 



The term nymph. An immature instar of an insect that undergoes 

 a gradual metamorphosis is termed a nymph. 



In old entomological works, and especially in those written in the 

 early part of the last century, the term nymph was used as a synonym 

 of pupa ; but in more recent works it is applied to the immature instar 

 of insects that undergo either a gradual or incomplete metamorphosis . 

 In this book I restrict the use of this term to designate an immature 

 instar of an insect that undergoes a gradual metamorphosis. 



Deviation from the usual type. It is to be expected that within so 

 large a group of organisms as the Paurometabola there should have 



