ARTHUR DEWITT WHEDOX 423 



f 



Such lines of modification would naturally affect the structure 

 of certain systems more than others: the digestive tract, the 

 tracheal system, and the muscles. The reproductive organs in 

 their growth through larval and adult stages are apparently as 

 unaffected as though they had not existed in an animal possessed 

 of a hemimetabolic life cycle. So also it may be said of the nerve 

 chain, that it is changed in but very minor ways during growth 

 and metamorphosis; considerable stretching out of the con- 

 nectives, and some consolidation of the pairs of ganglia. The 

 nerves and their branches must of course be modified to meet 

 the simpler muscular and other structures of the adult. 



The most profound changes then are to be seen in the alimen- 

 tary canal, the muscles, and the tracheae. The last we cannot 

 discuss here. 



In the alimentary canal of the nymphs there are but two 

 marked variations: 



1. The nature and position of the gizzard. In structure it 

 is always possessed of an armature, but this varies with the group 

 (See Higgins, 1901). Its normal position is near the second or 

 third abdominal segment, but a great deal of variation is found 

 and is probably due to functional condition. The anterior end 

 of the rectum and the beginning of the oesophagus are held firmly 

 to their places but the digestive tube between is quite free to 

 move forward or backward between the thorax and the sixth 

 segment. This is even more true of the Zygoptera than of the 

 Anisoptera. 



2. The modification of the rectum to function as a respiratoiy 

 organ in the Anisoptera; The remarkable enlargement of the 

 rectum and its development of a highly complex tracheal supply 

 make this organ one unique in the anatomy of the alimentary 

 canal. As the Zj^goptera do not possess this condition there is 

 a great contrast between the two groups. 



A comparison of the figures of the muscles of the three groups 

 of larvae will show: 



1. That all Odonate larvae have this system strongly devel- 

 oped. 



2. That in the Zygoptera the muscles are arranged over nearly 

 the whole inner wall of the tubular body. The longitudinal 

 muscles greatly predominate. 



TRANS. AM. ENT. SOC, XLIV. 



