i8o 



ENTOMOLOGY 



FIG. 221. 



indirect, however, the internal metamorphosis is nevertheless 

 continuous and gradual, without the abruptness that charac- 

 terizes the external transformation. In the larval stage ima- 

 ginal organs arise and grow ; in the pupal stage the purely 

 larval organs gradually disappear while the imaginal organs 

 are continuing their development. 



Phagocytes. The destruc- 

 tion of larval tissues, or his- 

 tolysis, is due often to the 

 amoeboid blood corpuscles, 

 known as leucocytes or phago- 

 cytes, which attack some tis- 

 sues and absorb their mate- 

 rial, but later are themselves 

 food for the developing imagi- 

 nal tissues. The construction 

 of tissues is termed histo- 

 genesis. 



In Coleoptera, however, the 

 degeneration of the larval mus- 

 cles is entirely chemical, there 

 being no evidence of phago- 

 cytosis, according to Dr. R. S. 

 Breed. Berlese-, indeed, goes 

 so far as to deny in general 

 the destructive action of leuco- 

 cytes on larval tissues. 



Imaginal Buds. The wings 

 and legs of a fly originate in 

 the larva in the form of cellu- 

 lar masses, or imaginal buds, 

 as Weismann discovered. Thus 

 in the larva of Corethra, there 

 a pair of dorsal buds and a pair 

 each bud being clearly an evagi- 

 the bottom of a previous invagi- 



Imaginal buds of full grown larva 

 of Pieris, dorsal aspect, b, brain; 

 m, mid intestine; s 1 , prothoracic 

 spiracle; s*, first abdominal spiracle; 

 sg, silk gland; I, prothoracic bud; 

 II, bud of fore wing; 777, bud of 

 hind wing. After GONIN. 



are in each thoracic segment 

 of ventral buds (Fig. 219), 

 nation of the hypodermis .at 



