352 



IN SECT A. 



densation, so to speak, of the process of ordinary cell division. Such 

 condensations are probably represented in the histolysis in the case of the 

 internal organs, and in the formation of imaginal discs in the case of the 

 external ones, and I think it probable that further investigation will shew 

 that the imaginal discs of the Muscidse are derivatives of the embryonic 

 epiblast. The above considerations by no means explain the whole of 

 Weismann's interesting observations, but an explanation is I believe to be 

 found by following up these lines. 



More or less parallel phenomena to those in Insects are found in the 

 development of the Platyelminthes and Echiuoderms. The four disc-like 

 invaginations of the skin in many larval Nemertmes (vide p. 163), which 

 give rise to the permanent body wall of the Nemertine, may be compared 

 to the imaginal discs. The subsequent throwing off of the skin of Pilidium 

 or larva of Desor is a phenomenon like the absorption of part of the larval 

 skin of Musca. The formation of an independent skin within the first 

 larval form in the Distomese and in the Cestoda may be compared to the 

 apparently independent formation of the imaginal discs in Musca. 



The fact that in a majority of instances it is possible to trace an 

 intimate connection between the surroundings of a larva and its 

 organization proves in the clearest way that the characters of the 

 majority of existing larval forms of Insects have owed their origin to 

 secondary adaptations. A few instances will illustrate this point. 



In the simplest types of metamorphosis, e.g. those of the Orthoptera 

 genuina, the larva has precisely the same habits as the adult. We 



find that a caterpillar form is assumed by 

 phytophagous larvas amongst the Lepidop- 

 tera, Hymenoptera and Coleoptera. Where 

 the larva has not to go in search of its nu- 

 triment the grub-like apodous form is as- 

 sumed. The existence of such an apodous 

 larva is especially striking in the Hymen- 

 optera, in that rudiments of thoracic and 

 abdominal appendages are present in the 

 embryo and disappear again in the larva, 

 The case of the larva of Sitaris, already de- 

 scribed (p. 349), affords another very striking 

 proof that the organization of the larva is 

 adapted to its habits. 



It follows from the above that the de- 

 velopment of such forms as the Orthoptera 

 genuina is more primitive than that of the 

 holometabolous forms; a conclusion which 

 ANTEEIOE tallies with the fact that both palseonto- 

 logical and anatomical evidence shew the 

 Orthoptera to be a very primitive group of 

 Insects. 



The above argument probably applies 



FIG. 192. 



HALF OF CAMPODEA FRAGI- 

 LIS. (From Gegenbaur; af- 

 ter Palmer.) 



a. antennas; p. feet; p'. 

 post-thoracic rudimentary 

 feet; s. stigma. 



with still greater force to the case of the 



