NO. I INSECT THORAX — SNODGRASS 3I 



are of little use in wingless insects, and that they are in a state of 

 degeneration in the Apterygota. It is, therefore, reasonable to suspect 

 that the pleurites are derivatives of some earlier structure, which has 

 become degenerate in the Apterygota, but which, in the wing-bearing 

 segments of the Pterygota, has undergone a new development by 

 which it has become remodeled into an essential part of the wing 

 mechanism. 



From a comparative study of the appendages of the Arthropoda, 

 Hansen (1893) concluded that the coxa is the second segment of 

 the primitive arthropod limb, and that, in insects, the rudiment of the 

 true basal segment is the trochantin. It was later claimed by Heymons 

 (1899), however, that the trochantin is only a small part of the 

 original basal segment, the major part of which has formed the other 

 pleural sclerites. This assertion Heymons based on a study of the 

 development of the thorax in the Hemiptera. In the embryo, he says, 

 the basal part of the leg divides into a proximal and a distal part, 

 the second becoming the coxa, while the first flattens out and forms 

 those parts of the thorax with which the leg articulates. The basal 

 piece of the limb, or hypothetical basal leg segment, Heymons desig- 

 nated the subcoxa. 



The idea that the arthropod limb in general includes a subcoxal 

 basal segment has been particularly elaborated by Borner. According 

 to Borner's most recently expressed view (1921), the subcoxa is 

 retained as a free basal segment of the limb only in the Pentapoda 

 (Pycnogonida) ; in other arthropods it is either incorporated into the 

 body wall, forming the pleuron in Insecta and in some Crustacea, and 

 in other Crustacea a part of the sternum, or it has entirely disap- 

 peared, as in the Arachnida. 



Crampton and Hasey (191 5) have opposed the theory of the sub- 

 coxal origin of the pleural plates in insects, pointing out that Hey- 

 mons misinterpreted some of the elements in the pleuron of Nepa, 

 and that according to his statement the epimeron is not included in 

 the subcoxa of the Hemiptera, though Heymons says that the sub- 

 coxal region includes both the epi sternum and the epimeron in 

 Blattidcie. The hemipteran species that Heymons studied are, indeed, 

 not good forms on which to base a study of the pleuron, for in 

 Naucoris, one of his examples, the epimeron of the metathorax is 

 rudimentary, and the episternum of the mesothorax is united with the 

 sternum. In each segment, therefore, only a single pleural plate is 

 evident in both nymph and adult, and a failure to recognize these 

 conditions might give the impression that a single pleural plate in 

 Naucoris is the equivalent of the episternum and epimeron in gen- 

 3 



