144 ENTOMOLOGY 



changes its form, and the fully grown larva becomes cylindrical, with 

 small slender legs, and, owing to the partial disuse of its jaws, acquires 

 a small, round head." Meloidae (Fig. 220) afford other excellent 

 examples of the transition from the thysanuriform to the cruciform 

 condition during the life of the individual. 



Thysanuriform characters become gradually suppressed in favor of 

 the cruciform, until, in most of the highly developed orders (Mecoptera, 

 Trichoptera, Lepidoptera, Diptera, Siphonaptera and Hymenoptera) , 

 they cease to appear, except for a few embryonic traces an illustration 

 of the principle of " acceleration in development." 



Growth. The larval period is pre-eminently one of growth. In 

 Heterometabola, growth is continuous during the nymphal stage, but 

 in Holometabola this important function becomes relegated to the larval 

 stage, and pupal development takes place at the expense of a reserve 

 supply of food accumulated by the larva. 



The rapidity of larval growth is remarkable. Trouvelot found that 

 the caterpillar of Telea polyphemus attains in 56 days 4,140 times its 

 original weight (J^o grain), and has eaten an amount of food 86,000 

 times its primitive weight. Other larvae exceed even these figures; thus 

 the maggot of a common flesh fly attains 200 times its original weight 

 in 24 hours. 



Ecdysis. The exoskeleton, unfitted for accommodating itself to the 

 growth of the insect, is periodically shed, and along with it go not only 

 such integumentary structures as hairs and scales, but also the chitinous 

 lining, or intima, of the stomodaeum, proctodaeum, tracheae, integumen- 

 tary glands, etc. The process of molting, or ecdysis, in caterpillars is 

 briefly as follows. The old skin becomes detached from the body by an 

 intervening fluid of hypodermal origin; the skin dries, shrinks, is pushed 

 backward by the contractions of the larva, and at length splits near the 

 head, frequently under the neck; through this split appear the new head 

 and thorax, and the old skin is worked back toward the tail until the 

 larva is freed of its exuvice. The details of the process are, however, by 

 no means simple. Ecdysis is probably something besides a provision 

 for growth, for Collembola continue to molt long after growth has 

 ceased, and the winged May fly sheds its skin once after emergence. 

 The meaning of this is not known, though ecdysis has an excretory 

 importance in the case of Collembola, which are exceptional among 

 insects in having no Malpighian tubes. 



Number of Molts. The frequency of molting differs greatly in 

 different orders of insects. Locustidae (formerly " Acridiidae ") have 



