GENERAL REMARKS. 281 



whenever the Thysanuriform and secondary adaptive 

 forms are present m the growth of the same individ- 

 ual. No one can compare the swollen, soft, round- 

 bodied grubs with the active Thysanuriform larva, 

 especially when occurring in the growth of the same 

 beetle, without realizing that the former is due to 

 specialization by reduction. That their structures, 

 although degraded by this process, are suitable to the 

 conditions under which they live has been pointed 

 out by many writers ; notably, Graber, Riley, Lubbock, 

 and Packard. This reduction becomes still more ap- 

 parent when we regard the larvae of Diptera and the 

 grubs of the weevils among Coleoptera, the latter being 

 generally without legs, and the former also deficient in 

 these organs and in large part without a differentiated 

 head. If these or the caterpillars or other secondary 

 larval forms similar to them were isolated, and their 

 subsequent development into pupae and adults un- 

 known, naturalists would not admit that they pos- 

 sessed close affinities with the adult insects of the 

 same groups, and they would be considered as more 

 rudimentary or simpler in structure than any Thy- 

 sanuran or Thysanuriform lan-a. In the most 

 speciahzed forms of Coleoptera, the weevils, the early 

 development of a footless grub, a reduced form 

 similar to the maggot of the Diptera, replaces both 

 the Thysanuriform larva and also the active six- 

 footed gmb of the normal groups of beetles. The 

 Insecta furnish such apparently isolated examples, 

 and, on account of the absence of intermediate forms, 

 it has been supposed that these could be put in evi- 

 dence against the derivation of the orders of which 



