146 ENTOMOLOGY 



Odonata) are often coated with mud and therefore difficult to distin- 

 guish so long as they do not move; caddis worms are concealed in their 

 cases, and caterpillars are often sheltered in a leafy nest. There is no 

 reason to suppose that insects conceal themselves consciously, however, 

 and one is not warranted in speaking of an instinct for concealment in the 

 case of insects since everything goes to show that the propensity to 

 hide, though advantageous indeed, is simply a reflex, inevitable, nega- 

 tive reaction to light (negative phototropism) or a positive reaction to 

 contact (positive thigmotropism). 



Exposed, sedentary larvae, as those of many Lepidoptera and Cole- 

 op tera, often exhibit highly developed protective adaptations. Cater- 

 pillars may be colored to match their surroundings 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 warning colors. 



Laryae need protection also against adverse climatal conditions, 

 especially low temperature and excessive moisture. The thick hairy 

 clothing of some hibernating caterpillars, as Isia 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. Excessive moisture is fatal 

 to immature insects in general conspicuously fatal to the chinch bug, 

 Rocky Mountain locust, aphids and sawfly larvae. The effect of mois- 

 ture 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 Platygaster in its extraordinary first 

 larval form (Fig. 221) 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 (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 holometabolous 

 insects only. Most Lepidoptera and many Diptera have an oUect 

 pupa (Fig. 215), or one in which the appendages and body are compactly 

 united; as distinguished from the free pupa of Neuroptera, Trichoptera, 



