180 ENTOMOLOGY 
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. 
Imaginal buds of full grown larva 
of Pieris, dorsal aspect. b, brain; 
m, mid intestine; s!, prothoracic 
spiracle; s*, first abdominal spiracle; 
sg, silk gland; J, prothoracic bud; 
II, bud of fore wing; JIJ, bud of 
hind wing.—After Gonrn. 
Phagocytes.—The destruc- 
tion of larval tissues, or /iis- 
tolysis,. 1s 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 /isto- 
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,” coes 
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 wnaginal buds, 
as Weismann discovered. Thus 
in the larva of Corcthra, there 
are in each thoracic segment a pair of dorsal buds and a pair 
of ventral buds (Fig. 219), each bud being clearly an evagi- 
nation of the hypodermis at the bottom of a previous invagi- 
