vi HEXAPODA PARTHENOGENESIS P^DOGENESIS 501 



FIG. 356. A and B, Diagrammatic representa- 

 tion of the formation of the imaginal hypodermis 

 in the abdomen of the Muscidae, proceeding from 

 the centres of the imaginal hypodermis (islands) 

 (hi). Ih, Larval hypodermis. 



In trying to understand these metamorphosic processes we must always keep in 

 view that ecdysis only affects the chitinous cuticle of the body, from which the sub- 

 jacent hypodermis withdraws, secreting a new chitinous integument under the old one. 



The formation of the head is very peculiar. It is (as oesophagus and optic vesicles 

 connected with the oesophagus) in- 



vaginated into the thorax, and is later A 



evaginated anteriorly out of the thorax 

 during the pupal stage. The anterior 

 part of the oesophagus becomes in this 

 process the neck, which after the 

 evagination of the head connects it 

 with the thorax. 



Of the inner organs, the heart and 

 the rudiments of the sexual organs 

 seem to pass 'direct into the corre- 

 sponding organs of the pupa. 



The whole musculature of the larva 

 except a few muscles of the second 

 thoracic segment disappears. The 

 imaginal connective tissue and the 

 greater part of the imaginal musculature 

 are formed anew from the mesoderm 



elements, which early appear on the inner side of the imaginal discs. Perhaps the 

 imaginal discs themselves yield, besides the thoracic hypodermis, the mesodermal 

 layers belonging to them as well ; this point, however, needs further investigation. 

 Certain dorsal muscles of the second thoracic segment of the larva do not disappear, 

 but are transformed during a temporary loss of their transverse striation into the 

 wing muscles of the imago. 



A large part of the larval tracheal system disappears. The imaginal tracheal 

 system seems to be regenerated out of scattered cells and cell groups of the larval 

 tracheal hypodermis. In the digestive tract the greater part of the mid-gut dis- 

 appears. The imaginal mid-gut forms anew out of persistent epithelial islands of the 

 larval mid -gut. Parts of the fore- and hind-guts proceed direct from the fore- and 

 hind-guts of the larva, while other parts arise out of circular islands or formative 

 centres, the so-called imaginal rings, of the larval fore- and hind-guts. 



The central portion of the nervous system (brain and ventral chord) and prob- 

 ably also the beginnings of the larger peripheral nerves proceed by means of peculiar 

 alterations and transformations from the corresponding parts of the larva. 



The salivary glands of the larva fall to pieces and disappear, falling victims to 

 the leucocytes. The imaginal salivary glands are regenerated out of imaginal rings 

 of the larval glands. The larval fat body is gradually devoured by the leucocytes. 



As already pointed out, the disintegration of the larval organs and the new 

 formation of the imaginal organs do not belong to two distinct periods. Both pro- 

 cesses go on side by side, so that in general there is'no discontinuity either in the out- 

 ward form or in the structure of the organs. Physiological discontinuity prevails 

 only in this sense, that the organs cannot, during transformation, perform their 

 respective functions. 



Parthenogenesis Cyclic Reproduction Paedogenesis. 



Parthenogenesis occurs in many insects, and especially frequently in the plant 

 lice (Rhynchota) and in many Hymenoptera, though here also, most probably, 

 it is not the only method of reproduction, but merely alternates with reproduction 



