IV. INSECTA: HEXAPODA. 



473 



The amnion is a thin layer of cells which covers the ventral surface 

 and arises in a manner similar to the vertebrate amnion; folds aris= 

 ing from the blastoderm in front and behind, right and left of 

 the embryo, fuse with one another and produce a double envelope, 

 an inner amnion, an outer serosa. 



With the rupture of the amnion and egg shell, the postembry- 

 onic development begins. This differs so in the different orders 

 that ametabolous, hemimetabolous, and holometaboloiTS insects are 

 recognized, i.e., insects with direct development withoiit meta- 

 morphosis, those with partial and those with complete metamor- 

 phosis. The ametabolous young is closely like the adult, so that 

 it only has to grow, with periodic ecdyses, and to mature its re- 

 productive organs. Since no insect has wings when it leaves the 

 egg, this direct development is joossible only in wingless forms like 

 the Apterygota and Aptera. 



Air winged insects, on the other hand have a more or less pro- 

 nounced metamorphosis, the final cause of which is the necessity 

 of developing wings. This view holds although there are wing- 

 less insects with a complete metamorphosis. These forms (fleas, 

 wingless moths, and ants) have undoubtedly sprung from winged 

 species and have inherited from them the metamorphosis which 

 has been retained after the wings were lost. In incomplete 

 metamorphosis the differences between the newly hatched young 

 and the adult, or imago, gradually 

 disappear (fig. 409). At the second 

 molt the wings often appear as small 

 ff)lds in the chitinous wall of meso- 

 and metathorax ; they grow with each 

 ecdysis, until at last, in size, form, 

 and movability, they are functional 

 wings. The chitinous coat of each 

 wing pad (fig. 490, B, 1, 2) encloses 

 the compressed and folded wing of 

 the next stage. Since the larvae by 

 their lack of wings are placed in 

 different circumstances from the 

 adult, the differences between the two 

 may be increased Ijy the development of special larval organs. 

 Thus the aquatic larvfe of the May flies and dragon flies differ from 

 the adults not only in the absence of wings, but by the different 

 form and the tracheal gills, which are almost always lost at the last 

 molt (fig. 495). 



Fie. 499.— Hemimetabolous develop- 

 ment of Pcrla niiira. (From Hux- 

 ley.) A^ wingless larva; B. larva 

 with winK pads, 1. 2; C, adult; /, 

 //, III, thoracic segments. 



