DEVELOPMENT 



59 



As there is no distinction between larva and pupa in most 

 heterometabolous insects, it is customary to use the term 

 nymph during the interval between egg and imago. 



Ametabola. The most generalized insects, Thysanura and 

 Collembola, develop to sexual maturity without a metamor- 

 phosis ; the form at hatching is retained essentially throughout 

 life, there are no traces of wings even in the embryo, and there 

 is no change of habit. These two orders form the group 

 Ametabola, All other insects have a metamorphosis in the 

 .broad sense of the term, and are therefore spoken of as Metab- 

 ola. In this we follow Packard, rather than Brauer, who 

 uses a somewhat different set of terms to express the same 

 ideas. 



Stadium and Instar. During the growth of every insect, 

 the skin is shed periodically, and with each moult, or ecdysis, 

 the appearance of the insect changes more or less. The inter- 

 vals between the moults are termed stages, or stadia. To 

 designate the insect at any particular stage, the term instar 

 has been proposed and is growing in favor ; thus the insect at 

 hatching is the first instar, after the first moult the second 

 instar, and so on. 



Egg. The eggs of insects are exceedingly diverse in form. 

 Commonly they are more or less spherical, oval, or elongate, 

 but there are innumerable special forms, some of which are 



FIG. 207. 



Eggs of various insects. A, butterfly, Polygonia interrogationis; B, house fly, 

 Musca domestica; C, chalcid, Bruchophagus funebris; D, butterfly, Papilio troilus; E, 

 midge, Cecidomyia trifolii; F, hemipteron, Triphleps insidiosus; G, hemipteron, 

 Podisus spinosus; H, fly, Drosophila ampelophila. Greatly magnified. 



