THE POLYCHAETE ANCESTRY OF THE INSECTS. 

 I. THE EXTERNAL STRUCTURE' ='- 



Two general hypotheses exist relative to the evolution 

 of insects. The one is the Crustacean theory of Miiller 

 (1864), Haeckel (1866), Wood-Mason (1879) and oth- 

 ers who have maintained that the insects have evolved 

 from Crustacean forms resembling the immature "Zoaea" 

 stage where there were three pairs of mouth parts and 

 three pairs of walking appendages corresponding to those 

 found in insects. Modifications of the Crustacean theory 

 have been presented by Handlirsch (1903, etc.), who 

 holds that insects originated from Trilobite ancestors, 

 extinct forms placed as an aberrant group of Crusta- 

 ceoidea by most systematists, and by Crampton (1918, 

 etc.), who believes that the insects have arisen from forms 

 similar to the Anaspidacea or Mysidacea, small Crustaceans 

 of primitive structure. 



The other interpretation is the Campodean theory of 

 Brauer (1868, etc.), which derives the insects from the 

 "Myriapoda" — the Chilopods in the strict sense — and 

 the Onychophoroidea by way of campodeoid forms. This 

 is on the basis of the resemblance of the larvae of many 

 orders to the existing Campodea, a genus of apparently 

 primitive insects belonging to the Thysanura. It is the 

 latter theory that has received the approval of the greater 

 number of systematists who have taken an interest in 

 problems of phylogeny. 



That there is quite another interpretation of the origin 

 of insects indicating their descent neither from the Chilo- 

 pods and the Onychophorans by way of Campodea, from 

 the Trilobites, or from the generalized Crustacea, but di- 

 rectly from the Polychaete Annelids, is my decided opin- 

 ion. This I would term the Polychaete theory. 



^Contributions from the Samuel Mather Science Hall. Biology (Ser. 31). No. 1. 



'■Reprinted from The American Naturalist, vol. 61 (1927), pp. 226-2J0. 



