414 



ARTHROPODA 



tions among the bees are especially interesting, since here the determination of 

 sex rests with the existence or non-existence of fertilization (pp. 130, 134). 

 Much rarer than the ordinary parthenogenesis is paedogenesis (p. 129), which 

 occurs only in certain Diptera like Miastor. In the female Miastor larva (fig, 

 456) the eggs develop before the appearance of the ducts, so that the young can 

 only escape by rupture of the mother. After several paedogenetic generations 

 there appear at last larvae which pupate and produce adult male and female flies. 



FIG. 456. Larva of a Cecidomyid with pcTclogenetic daughter larvae (from Hatschek 



after Pagenstecher). 



With the exception of these paedogenetic forms, the Pupipara, many Aphidae 

 and a few other viviparous species, the Hexapoda are oviparous. The develop- 

 ment begins, after oviposition, by a superficial segmentation of the egg. Later 

 there appear two embryonic structures, the yolk sac and the amnion; the first, in 

 contrast to the vertebrate structure with the same name, is dorsal. The amnion 

 is a thin layer of cells which covers the ventral surface and arises in a manner 

 similar to the vertebrate amnion; folds arising 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, enclosing the germinal area. 



The postembryonic development presents two important features, 

 i . As in other arthropods growth is possible only by periodic ecdyses so that 



the life cycle consists of several periods 

 separated by molts of the cuticle. 2. No 

 insect, as it escapes from the egg, has 

 wings. If present in the adult they must 

 arise during the larval stages. This 

 postembryonic development of the wings 

 is the starting-point of the metamor- 

 phosis, and forms the basis of a division 

 of the development into ametabolous (no 

 metamorphosis), hemimetabolous (in- 

 complete metamorphosis) and holometab- 

 olous (complete metamorphosis). An 

 ametabolous development is possible only 

 in wingless insects, the postembryonic 

 development consisting only of periodic molts. Some wingless forms 

 (fleas, wingless moths, ants, etc.), have a metamorphosis, because they have 

 inherited it from winged ancestors and have not lost it with the wings. 

 Hemimetabolous development is marked by a gradual change from 

 the newly hatched animal, the larva, to the sexually mature adult or 

 imago (fig. 457). There often appears with the second molt, the 



FIG. 457. Hemimetabolous de- 

 velopment of Perla nigra (from 

 Huxley). .4, wingless larva; B, 

 larva with wing pads, i, 2; C, adult; 

 I, II, III, thoracic segments. 



