DEVELOPMENT 145 



five molts; many Heteroptera, as the chinch bug and squash bug, have 



ive (with six instars) ; the periodical cicada, six (Marlatt) ; the larva of 



the Colorado potato beetle, three; Lepidoptera usually four or five, but 



)ften more, as in Isia Isabella, which molts as many as ten times (Dyar) ; 



house fly, Musca domestica, two molts (three larval- instars) . Pack- 

 ard suggests that cold and lack of food during hibernation in arctians 

 (as /. Isabella) and partial starvation in the case of some beetles, cause 

 a great number of molts by preventing growth, the hypodermis cells 

 meanwhile retaining their activity. 



The appearance of the insect often changes greatly with each molt, 

 particularly- in caterpillars, in which the changes of coloration and 

 armature may have some phylogenetic significance, as Weismann has 

 attempted to show in the case of sphingid larvae. 



Adaptations of Larvae. Larvae exhibit innumerable conformities 

 of structure to environment. The greatest variety of adaptive struc- 

 tures occurs in the most active larvae, such as pred'aceous forms, ter- 

 restrial or aquatic. These have well-developed sense organs, excellent 

 powers of locomotion, special protective and aggressive devices, etc. 

 In insects as a whole, the environment of the larva or nymph and that 

 of the adult may be very different, as in the butterfly or the dragon fly, 

 and the larvae are modified in a thousand ways for their own immediate 

 advantage, without any direct reference to the needs of the imago. 



The chief purpose, so to speak, of the larva is to feed and grow, and 

 the largest modifications of the larva depend upon nutrition. Take as 

 one extreme, the legless, headless, fleshy and sluggish maggot, embedded 

 in an abundance of food, and as the other extreme the active and 

 " wide-awake" larva of a carabid beetle, dependent for food upon its 

 own powers of sensation, locomotion, prehension, etc., and obliged 

 meanwhile to protect or defend itself. Between these extremes come 

 such forms as caterpillars, active to a moderate degree. The great 

 majority of larval characters, indeed, are correlated with food habits, 

 directly or indirectly; directly in the case of the mouth parts, sensory 

 and locomotor organs, and special structures for obtaining special food; 

 indirectly, as in respiratory adaptations and protective structures, 

 these latter being numerous and varied. 



Larvae that live in concealment, as those that burrow in the ground 

 or in plants, have few if any special protective structures; active larvae, 

 as those of Carabidae, have an armor-like integument, but owe their 

 protection from enemies chiefly to their powers of locomotion and their 

 aversion to light (negative phototropis.m) ; various aquatic nymphs (Zaitha, 

 10 



