132 ENTOMOLOGY 



ingly reduced. These characteristics are interpreted as being results of 

 partial or entire disuse, the amount of reduction being proportional to 

 the degree of inactivity. Extreme reduction is seen in the maggots of 

 parasitic and such other Diptera as, securing their food with almost no 

 exertion, are simple in form, thin-skinned, legless, with only a mere 

 vestige of a head and with sensory powers of but the simplest kind. 



Transitional Forms. The eruciform is clearly derived from the 

 thysanuriform type, as Brauer and Packard have shown, the continuity 

 between the two types being established by means of a complete series 

 of intermediate stages. The beginning of the eruciform type is found 

 in Neuroptera, where the campodeoid sialid larva assumes a quiescent 

 pupal condition. The key to the origin of the complete metamorphosis, 

 involving the eruciform condition, Packard finds in the neuropterous 

 genus Mantis pa (Fig. 212), the first larva of which is truly campodea- 

 form and active. Beginning a sedentary life, however, in the egg-sac of 

 a spider, it loses the use of its legs and the antennae become partly aborted, 

 before the first moult. In Packard's words, "Owing to this change of 

 habits and surroundings from those of its active ancestors, it 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. 218) afford other excellent examples of the tran- 

 sition from the thysanuriform to the eruciform condition during the life 

 of the individual. 



Thysanuriform characters become gradually suppressed in favor of 

 the eruciform, 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 sup- 

 ply 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 (^V 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 



