TRACHEATA. 



425 



form in the Distomcsc and in the Ccstoda 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 tliat the characters of 

 the majority of existing lamal forms of Insects have owed tlieir 

 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 larvae 

 amongst the Lepidoptera, Hymenoptera 

 and Coleoptera. Where the larva has 

 not to go in search of its nutriment the 

 grub-like apodous form is assumed. The 

 existence of such an apodous larva is 

 especially striking in the Hymenoptera, 

 in that rudiments of thoracic and abdo- 

 minal appendages are present in the 

 embryo and disappear again in the larva. 

 The case of the larva of Sitaris, already 

 described (p. 42 1 ), affords another very 

 striking proof that the organization of 

 the larva is adapted to its habits. 



It follows from the above that the 

 development of such forms as the Or- 

 thoptera genuina is more primitive than 

 that of the holometabolous forms; a 

 conclusion which tallies with the fact 

 that both palaeontological and anatomical evidence shew the 

 Orthoptera to be a very primitive group of Insects. 



The above argument probably applies with still greater force 

 to the case of the Thysanura ; and it seems to be probable that 

 this group is more nearly related than any other to the primitive 

 wingless ancestors of Insects 1 . The characters of the oral 



FIG. 191. ANTERIOR 

 HALF OF CAMPODEA FRAGI- 

 i.is. (From Gegenbaur; af- 

 ter Palmen.) 



a. antennae ; /. feet ; /. 

 post-thoracic rudimentary 

 feet ; s. stigma. 



1 Brauer and Lubbock (No. 421) have pointed out the primitive characters of these 

 forms, especially of Campodea. 



