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. (i) 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^ i.e. they have no 

 marked change or metamorphosis. 



(2) In cicadas there are slight but most instructive 

 differences between larvae 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 (e.g. 

 Perla), for in these the larvae are aquatic, with closed 

 respiratory apertures, and with tracheal gills or folds, while 

 the adults are winged and aerial, and breathe by open 

 tracheae. 



These insects are called hemimetabolic, i.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 



