424 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



3. That the differentiation into tergal and sternal groups of 

 muscles is carried farthest in the Libellulinae, and that the Aesh- 

 ninae stand intermediate in this respect. 



4. That the greatest complexity both in numbers and in inter- 

 relations is found in the Aeshninae. The Zj^gopterous forms 

 and the Libellulinae are similar in their simpler sets of tergal 

 and sternal muscles. 



5. That the dorso-ventral or "respiratory" muscles are much 

 stronger and more complex in the Anisoptera where the body 

 form becomes more and more flattened. 



These conditions should be contrasted with those of the 

 imagoes : 



The alimentary canal is simplified bj^ the reduction of the 

 larval structures. 



1. The armature of the gizzard is largely lost, but its position 

 has changed little in metamorphosis; in the Anisoptera it is near 

 the second segment, in the Zygoptera, near the fifth and sixth. 



2. The hind gut is greatly simplified, due to the loss of the 

 respiratory function. 



A comparison of the muscles of the imagoes of the three groups 

 will show: 



1. That they are very constant in structure considering the 

 great variations in the external form of the abdomen. 



2. That most of the differences present are quantitative. 



3. That the chief qualitative variation is the presence of a 

 pair of Posterior Dorso-ventral Muscles in each segment of the 

 Anisoptera; this is absent in Zygoptera. 



4. That the Aeshninae and Zygoptera possess Inferior Longi- 

 tudinal Tergo-pleural jMuscles not found in the Libellulinae. 



5. That the sternal muscles of the Zygoptera are much better 

 developed than those of the Anisoptera. 



6. That in both the Zygoptera and Anisoptera there is great 

 disparity between the tergal and sternal muscles. The Inferior 

 Tergals replace in function the atrophied sternals of the Anisop- 

 tera. 



No systematic attempt can be made at this time to homol- 

 ogize the muscles of larva and imago. So many muscles are 

 lost during metamorphosis and the remaining ones are so changed 



