METAMORPHOSIS OF INSECTS. 27 1 



arily an insinking from the surface. The eyes arise in part at least as 

 invaginations of the skin. 



The gut has a three-fold origin. The fore-gut (stomatodoeum), and the 

 hind-gut (proctodeeum), are fore and hind invaginations of ectoderm ; the 

 mid-gut (mesenteron) is lined by endoderm, which seems to originate 

 in the invagination of the blastoderm. It is doubtful, however, whether 

 the yolk does not to some extent help to form the endoderm. The heart 

 is formed from two dorsal rows of mesoderm cells. The trachese arise 

 as ingrowths of the epidermis. The Malpighian tubules are outgrowths 

 from the ectodermic hind-gut. 



Metamorphosis of Insects. 



(i) In the lowest Insects, namely, in the old-fashioned 

 wingless Thysanura and Collembola, the hatched young are 

 miniature adults. By gradual growth and after several moult- 

 ings, they attain adult size. 



Similarly the newly hatched earwigs, young of cockroaches 

 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 exhibit no 

 marked change or metamorphosis. 



(2) In Cicadas there is slight but most instructive differ- 

 ence between larvse and adults. The adults live among 

 herbage, the young on the ground, and the diversity of habit 

 has associated differences of structure, witness the bur- 

 rowing 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 larvse are aquatic, with closed 

 respiratory apertures, 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 ot 

 Insects — ant-lions, caddis-flies, flies, fleas, butterflies and 

 moths, beetles, ants and bees. From the egg there is 



