No. 634] PHYLOGENY OF THE ARTHR OPODA 



409 



Insecta 



I quite agree with Crampton that Handlirsch has pre- 

 sented little or no evidence that the Insecta were derived 

 directly from the trilobites. His chief point was that the 

 most ancient known insects, the Palaeodictyoptera, were 

 amphibious, and that their larvae, which lived in water, 

 were very like the adult. His second was that the wings 

 of the Palaeodictyoptera probably worked up and down 

 only, and that the two main wings were homlogous with 

 rudimentary winglike outgrowths on each segment of the 

 body. These outgrowths resemble the pleural lobes of 

 trilobites, and were considered to have been derived from 

 them. Comstock, who has recently reviewed the ques- 

 tion, does not see any evidence that the Palaeodictyoptera 

 were amphibious, and I do not think any entomologist 

 or paleontologist has accepted the idea of a direct trans- 

 formation of pleural extensions of segments of trilobites 

 into wings. The "para-notal" theory certainly does not 

 involve any such conception. That the insects are de- 

 rived indirectly from the trilobites is, however, entirely 

 possible, and Professor Crampton has marshalled the 

 data for one such possible line of derivation through the 

 Crustacea in the article to which allusion was made in 

 the opening sentences of this essay. Another theory is 

 that advanced by Tothill, who suggested that the Insecta 

 arose through some chilopod-like tracheate, rather than 

 directly from a marine organism. Tothill 5 has pointed 

 out that in the germ band, spiracles appear as early as 

 the limb-buds, and may thus indicate a tracheate ancestor 

 for the insects. This is, of course, discounting the pos- 

 sible effect of acceleration on the embryo, but the whole 

 anatomy of the insects indicates long separation from 

 the marine ancestor. The germ band of the chilopod is 

 somewhat more primitive than that of the insect, for in 

 some species both antennules and antennae are present, 

 and the maxillae and first maxillipeds are biramous. The 



a Am. Jour. Sri., Vol. 42, 1916, p. 373. 



