TRACES AT A. 353 



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 appendages in this group, 

 the simplicity of their metamorphosis, and the presence of ab- 

 dominal appendages (fig. 102), all tell in favour of this view, while 

 the resemblance of the adult to the larvre of the Pseudoneuroptera, 

 etc., points in the same direction. The Thysanura and Collembola 

 are not however to be regarded as belonging to the true stock of the 

 ancestors of Insects, but as degenerated relations of this stock ; much as 

 Amphioxus and the Ascidians are degenerate relations of the ancestral 

 stock of Vertebrates, and Peripatus of that of the Tracheata. It is 

 probable that all these forms have succeeded in retaining their 

 primitive characters from their degenerate habits, which prevented 

 them from entering into competition in the struggle for existence with 

 their more highly endowed relatives. While in a general way it is 

 clear that the larval forms of Insects cannot be expected to throw 

 much light on the nature of Insect ancestors, it does nevertheless 

 appear to me probable that such forms as the caterpillars of the 

 Lepidoptera are not without a meaning in this respect. It is easy to 

 conceive that even a secondary larval form may have been produced 

 by the prolongation of one of the embryonic stages ; and the general 

 similarity of a caterpillar to Peripatus, and the retention by it of post- 

 thoracic appendages, are facts which appear to favour this view of the 

 origin of the caterpillar form. 



The two most obscure points which still remain to be dealt with 

 in the metamorphosis of Insects are (1) the origin of the quiescent 

 pupa stage ; (2) the frequent dissimilarity between the masticatory 

 apparatus of the larva and adult. 



These two points may be conveniently dealt with together, and 

 some valuable remarks about them will be found in Lubbock 

 (No. 420). 



On grounds already indicated it may be considered certain that 

 the groups of Insects without a pupa stage, and with a larva very 

 similarly organised to the adult, preceded the existing holometabolic 

 groups. The starting-point in the metamorphosis of the latter groups 

 was therefore something like that of the Orthoptera. Suppose it became 

 an advantage to a species that the larva and adult should feed in a some- 

 what different way, a difference in the character of their mouth parts 

 would soon make itself manifest ; and, since an intermediate type of 

 mouth parts would probably be disadvantageous, there would be a 

 tendency to concentrate into a single moult the transition from the 

 larval to the adult form of mouth parts. At each ordinary moult 

 there is a short period of quiescence, and this period of quiescence 

 would naturally become longer in the important moult at which the 

 change in the mouth parts was effected. In this way a rudimentary 



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

 forms, especially of Campodea. 



B. E. 23 



