LIFE-HISTORY OF INSECTS 397 



meet and fuse, first ventrally and later dorsally, thus constituting the 

 mid-gut. The yolk nuclei previously mentioned have meanwhile 

 increased rapidly, forming yolk cells which absorb the yolk. These 

 cells are included in the endodermic mid-gut, and there break up. As 

 the endoderm grows round the yolk, it is accompanied by a layer 

 (splanchnic) of the mesoderm. Fore- and hind-gut are formed by 

 invaginations which fuse with the mid-gut. 



In the last stages of development the primitive ccelomic pouches 

 lose their cross partitions, become filled with mesenchyme cells, and 

 practically obliterated. The body cavity of the adult is formed by the 

 appearance of lacunae amid the cells of the mesenchyme. 



The tracheae arise as segmentally repeated invaginations of the ecto- 

 derm. The openings of the invaginations form the stigmata. From 

 the hind-gut arise the Malpighian tubules, which are therefore ecto- 

 dermic. The development of the other organs is similar to that of 

 the Crustacea. 



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. 



Life-history 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 hce, aphides, termites, and bugs, are 

 very like the parents, except that they are sexually im- 

 mature, 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. 

 Per la) y for in these the larvae are aquatic, with closed 



