DEVELOPMENT 1 67 



negative reaction to light (negative phototropism ) or a positive 

 reaction to contact (positive thigmotropism) . 



Exposed, sedentary larvae, as those of many Lepidop- 

 tera and Coleoptera, often exhibit highly developed protective 

 adaptations. Caterpillars may be colored to match their sur- 

 roundings and may resemble twigs, bird-dung, etc. ; or larvae 

 may possess a disagreeable taste or repellent fluids or spines, 

 these odious qualities being frequently associated with warn- 

 ing colors. 



Larvae need protection also against adverse climatal condi- 

 tions, especially low temperature and excessive moisture. 

 The thick hairy clothing of some hibernating caterpillars, as 

 Isia (Pyrrharctia) Isabella, doubtless serves to mollify sudden 

 changes of temperature. Naked cutworms hibernate in well- 

 sheltered situations, and the grubs of the common " May 

 beetles," or " June bugs," burrow down into the ground below 

 the reach of frost. Ordinary high temperatures have little 

 effect upon larvae, except to accelerate their growth. Exces- 

 sive moisture is fatal to immature insects in general conspicu- 

 ously fatal to the chinch bug, Rocky Mountain locust, aphids 

 and sawfly larvae. The effect of moisture may be an indirect 

 one, however; thus moisture may favor the development of 

 bacteria and fungi, or a heavy rain may be disastrous not only 

 by drowning larvae, but also by washing them off their food 

 plants. 



As a result of secondary adaptive modifications, larvae 

 may differ far more than their imagines. Thus Platygastcr in 

 its extraordinary first larval form (Fig. 218) is entirely unlike 

 the larvae of other parasitic Hymenoptera, reminding one, 

 indeed, of the crustacean Cyclops rather than the larva of an 

 insect. As Lubbock has said, the characters of a larva depend 



1 i ) upon the group of insects to which the larva belongs and 



(2) upon the special environment of the larva. 



Pupa. The term pupa is strictly applicable to holometabo- 

 lous insects only. Most Lepidoptera and many Diptera have 

 an obtect pupa (Fig. 212), or one in which the appendages 



