Dec, 1916.] Relationships of Apterygotan Insects. 267 



carina, rather sparsely punctate, sides pieces more coarsely and densely punc- 

 tate ; abdomen moderately punctate. Length 9 mm. 



Southwestern Texas (Chapman Grant). 



A narrower and smaller insect than our other species of Plas- 

 toccnis from all of which it differs in the carinate prosternum. The 

 antennal rami are shorter, less densely ciliate and with shorter hairs 

 than in schanmii or mcgalops and are nearly as in fratcr. 



THE ORDERS AND RELATIONSHIPS OF APTERY- 

 GOTAN INSECTS.^ 



By G. C. Crampton, 

 Amherst, Mass. 



The ancestors of the Arthropoda were, in all probability, very 

 similar to Annelidan worms, and, although the Annelida, like all 

 recent forms, have developed many characters peculiar to themselves, 

 certain members of the group have preserved some exceedingly 

 primitive features, which enable us to infer what the ancestors of 

 Arthropods must have been like. 



Although the discussion of the probable lines of descent leading 

 up to the development of the Insectan type of Arthropod is beyond 

 the province of the present paper, it may be remarked that the 

 " Apodidse " have departed but little from the condition which was 

 doubtless characteristic of the ancestral Crustacea, and such Crusta- 

 ceans as Apus and Branchippiis (which are not far removed from 

 such Trilobites as Triarthrus, Neolcnus and Nathorstia) have de- 

 parted but little from the probable ancestral condition of Arthropods 

 in general. These Crustacea and Trilobita, then, present as nearly 

 as any Arthropods now known, the characters present in the earlier 

 forms, and enable us to gain some idea of what the common ancestors 

 of the Arachnida, Merostoma, Trilobita, Crustacea, '' Myriapoda " 

 {sensti lato), Hexapoda, etc., were like. 



1 Contribution from the Entomological Laboratory of the Massachusetts 

 Agricultural College, Amherst, Mass. 



