352 PHYLUM ARTHROPODA. 
In summarising the development of Insecta, one must 
specially note the peripheral segmentation, the formation of 
the two-layered germinal streak, the presence of an over- 
arching blastodermic fold, the segmentation of the meso- 
derm, and the formation of the mid-gut by the union of 
endodermic bands. : 
Metamorphosis of Insects.—(1) In the lowest Insects, 
namely, in the old-fashioned wingless Thysanura and 
Collembola, the hatched young are miniatures of the adults. 
By gradual growth, and after several moultings, they attain 
adult size. 
Similarly, the newly hatched earwigs, young of cock- 
roaches and locusts, of lice, aphides, termites, and bugs, are 
very like the parents, except that they are sexually immature, 
and that there are no wings, which indeed are absent from 
some of the adults. 
These insects are called ametabolic, te. they have no 
marked change or metamorphosis. 
(2) In cicadas there are slight but most instructive 
differences between larve and adults. The adults live 
among herbage, the young on the ground, and the diversity 
of habit has associated differences of structure, as in 
the burrowing fore-legs of the larva. Moreover, the larva 
acquires the characters of an adult after a quiescent period 
of pupation. 
The differences between larva and adult are more striking 
in may-flies, dragon-flies, and the related Plecoptera (eg. 
ferla), for in these the larva are aquatic, with closed 
respiratory apertures, and with tracheal gills or folds, while 
the adults are winged and aerial, and breathe by open 
tracheze. 
These insects are called Lemimetabolic, z.e. they have a 
partial or incomplete metamorphosis. 
(3) Very different is the life history of all other sets of 
Insects—ant-lions, caddis-flies, flies, fleas, butterflies and 
moths, beetles, ants, and bees. From the egg there is 
hatched a larva (maggot, grub, or caterpillar), which lives a 
life very different from the adult, and is altogether unlike 
it in form. The larva feeds voraciously, grows, rests, and 
moults. Having accumulated a rich store of reserve 
material in its “fatty body,” it finally becomes for some 
