LEE BARKER WALTON 159 



ested in their embryology and morphology have not met 

 with pronounced success. The difficulty is due to the wide 

 diversity of form exhibited by the various orders of in- 

 sects, the pronounced differentiation of the segmental 

 groups mentioned, together with the inability to agree on 

 the ancestral form from which the insects and their allies, 

 the Symphyloidea, Chilopoidea, Pauropoidea, etc., have 

 arisen. 



It is to only one particular phase of the question that I 

 here wish to call attention, namely, the relation of the 

 pleurites and the appendages of the thorax in the insects 

 to the parapodia of the Polychaete Annelids, for it is in 

 the thorax, I am inclined to believe, that one finds the 

 most generalized part of the body so far as the external 

 structure is concerned. 



The pleural or lateral part of each thoracic segment 

 consists of two sclerites, the episternum and epimeron, 

 names given by Audouin (1824) in his study of Dytiscus 

 circumflexus Fabr., the large water beetle. The origin of 

 these pieces has received little attention, although several 

 explanations have been offered. None of these explana- 

 tions seems to me to be adequate. 



Miall and Denny (1886) suggested that the pleurites as 

 represented by the episternum and epimeron belonged to 

 the lateral area of the segment so far as they were present 

 in the insects, but that in the cockroach they could be 

 considered basal parts of the leg adherent to the thorax. 

 Inasmuch as even the constancy of the two pieces was 

 doubted throughout the insects in general, it is evident 

 that their relation to the base of the leg was looked upon 

 as a specialized condition present in the Blattidae alone. 

 Banks (1893) believed that the two pleurites were to be 

 accounted for through the fusion of two primitive seg- 

 ments, a theory which I myself was earlier (1900) in- 

 clined to accept, but which I have long considered unten- 

 able. Crampton (1909) suggested that the two areas 

 were "due to a mechanical stress," apparently forgetful 

 of the heresy thus implied in connection with the inheri- 

 tance of acquired characters. Snodgrass (1909) con- 

 cluded that the apparent division between the areas re- 



