Vol. 1.] Woodworth. — Wing Veins of Insects. 17 



The theory has been suggested by Hagen ('82) and others, 

 but I believe in most cases only tentatively, that a thoracic 

 segment, like the head, is a composite region of the body, and 

 owes its complexity, in part at least, to its possessing traces of 

 the primitive segments. The wing in this case might even be 

 considered a modified leg, as it is in birds. It need scarcely 

 be said that, in this form at least, the theory is certainly unten- 

 able. Very recently the idea of a double origin of the thoracic 

 segment has lieen seriously revived in a somewhat new form 

 by Walton ('00). As the result of a study of the basal portion 

 of the leg, he conceives that the peculiar structures found there 

 are the result of the fusion of two successive legs, the posterior 

 one being rudimentary, Init C()ntril)uting a large portion of the 

 basal segment. The two pleural pieces, according to this view, 

 would represent the sides of the two primitive segments. Wal- 

 ton does not discuss the bearing of this theory on the problems 

 of the wing and of the notal structures. His idea in the matter, 

 however, seems to be that the wing belongs to the second prim- 

 itive segment, and the corresponding organ in the anterior 

 segment is represented in the mesothorax l)y the patagia in 

 Lepidoptera and the tegula in Hymenoptera. The dorsal divi- 

 sion between the two primitive segments would evidently be 

 that between scutum and scutellum. All the facts that have 

 been cited in favor of this theory can be exjjlained fully as 

 satisfactorily without supposing the segment to have a double 

 origin. The greater part of what follows regarding the special- 

 ization of the thoracic somite would not l)e seriously affected 

 even if the double origin were demonstrable. We shall assume, 

 however, that each segment of the thorax is developed from a 

 single articular somite. 



The key to the solution of the prol)lem of the specialization 

 of the thoracic segment appears to be its skeletal function. 

 The skin of the insect is the sole place of attachment of the 

 muscles of the organs of locomotion. The thorax is specialized 

 for this function and correspondingly relieved of other duties. 

 The head contains the mouth and most of the sense organs, 

 and the abdomen, at least in the adult insect, performs most of 

 the vegetative and reproductive functions, and neither of these 

 regions shows skin structures that can be definitely identified 

 with the parts of the thoracic segment, except in a most 

 general wav. Thus, the peculiarities of the thorax both in 

 2— V 



