414 



ARTHROrODA 



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

 sex rests with the existence or non-existence of fertihzation (pp. 130, IJ4). 

 Much rarer than the ordinary parthenogenesis is pedogenesis (p. 120), which 

 occurs onlv in certain Diptera Hke Miaslor. In llie female Miaslor larva (tig, 

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

 onlv escape bv rupture of tlie moilier. After several pa'dogenetic generations 

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



Fig. .156. — L,irva of a Cccidomyid with pa'dogenetic daughter larva^ (from Ilatschek 



after Pagenstechcr) . 



With the exception of these pa^dogenctic forms, the Pupipara, manv Aphid,-? 

 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 sat and the amniim: 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 enibrvo, 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. Xo 

 insect, as it escapes from the egg, has 

 wings. If present in tlie adult they must 

 arise during the hirval 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), hcmimetabolous (in- 

 complete metamorphosis) and holometab- 

 olous (complete metamorphosis). An 

 amelaholoiis dcvclopiiiciil is possible only 

 in wiiigless insects, the postembryonic 

 development consisting only of periodic molts. Some wingless forms 

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

 inherited it from winged ancestors and ha\-e not lost it with the wings. 

 Hcmbnclabolous development is marked by a gradual change from 

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

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



Fig. 457. — Heinimetaliolous de- 

 velopment of Peril iiigrd (from 

 Huxley). -1. wingless larva; P, 

 larva with wing pads, i, 2; c', ailult; 

 /, //, ///, thoracic scgnieiils. 



