182 AN INTRODUCTION TO ENTOMOLOGY 



legs as do naiads of insects with an incomplete matamorphosis. The 

 larvae of Lepidoptera have short legs which correspond to only a part 

 of the legs of the adult. While the larvae of Diptera have no external 

 indications of legs. 



The development of prole gs in some larvce. A striking feature of 

 many larvae is the presence of abdominal organs of locomotion ; these 

 have been termed prolegs; the prolegs of caterpillars are the most 

 familiar examples of these organs. 



The prolegs were so named because they were believed to be merely adaptive 

 cuticular formations and not true legs ; this belief arose from the fact that they are 

 shed with the last larval skin. Some recent writers, however, regard the prolegs 

 as true legs. It is now known that abdominal appendages are common in the 

 embryos of insects; and these writers believe that the prolegs are developed 

 from these embryonic appendages, and that, therefore, they must be regarded as 

 true legs. 



If this is true, there has taken place a remarkable reversal in the course of 

 development. The abdominal legs, except those that were modified into append- 

 ages of the reproductive organs, the gonapophyses, were lost early in the phylogeny 

 of the Hexapoda. The origin of complete metamorphosis must have taken place 

 at a much later period; when, according to this belief, the abdominal appendages, 

 which had been latent for a long time, were redeveloped into functional organs. 



The development of tracheal gills. A striking feature of many larvae 

 is the possession of tracheal gills. This is obviously an adaptive 

 characteristic the development of which was correlated with the 

 assumption of aquatic life by forms that were primarily aerial; and 

 it is also obvious that the development of tracheal gills has arisen 

 independently many times ; for they exist in widely separated families 

 belonging to different orders of insects that are chiefly aerial. They 

 are possessed by a few lepidopterous larvae, and by the representatives 

 of several families of Neuroptera, Coleoptera and Diptera. On the 

 other hand, in the Trichoptera the possession of tracheal gills by the 

 larvss is characteristic of nearly all members of the order. 



The internal development of wings. This is perhaps the most re- 

 markable of the sidewise developments of larvae. Although larvae 

 exhibit no external indications of wings, it has been found that the 

 rudiments of these organs arise at as early a period in insects with a 

 complete metamorphosis as they do in those with an incomplete 

 metamorphosis ; and that during larval life the wing rudiments attain 

 an advanced stage in their development. But as these rudiments are 

 invaginated there are no external indications of their presence during 

 larval life. The details of the internal development of wings are dis- 

 cussed later. 



