20 THE IMAGO [CH. 



in shape. Posteriorly they are closely connected with the sides 

 of the prothorax. Two others (3 mi, 4 mi) are very small, and lie 

 freely in the neck membrane [157]. 



The orifice of the neck, which is the posterior continuation of 

 the posterior foramen of the head, is so minute that it is difficult 

 to believe that the ventral nerve-cord, oesophagus, salivary ducts 

 and tracheal trunks could possibly all pass through it. Yet such is 

 actually the case. It is interesting to note that, if the head of 

 a Dragonfly be removed, either accidentally when striking with the 

 net, or purposely with fine scissors, the body will continue to live 

 for as much as two days longer, while the wings vibrate vigorously 

 whenever the thorax is touched. 



The movements of the head on the neck are brought about by 

 a number of pairs of small muscles arising from the prothorax 

 and anterior part of the mesothorax. They are placed dorsally, 

 ventrally, and laterally, so that the head can turn freely in all 

 directions. 



THE THORAX (figs. 1, 7). 



The thorax of the Dragonfly is very highly specialized, in 

 accordance with the requirements of the very powerful wings 

 and the altered functions of the legs. On the one hand, the 

 prothorax, though remaining a distinct segment, becomes greatly 

 reduced in size. On the other, the meso- and metathorax are 

 large and powerful, and become fused together into one solid 

 mass, which may conveniently be termed the synthomx. 



In general, three regions can be distinguished in each of the 

 three thoracic segments. These are the tergum or true dorsal 

 region, the. sternum or true ventral region, and the pleura or lateral 

 portions. By the addition of the prefix pro-, meso-, or meta-, we 

 indicate to which segment the part in question belongs. The 

 same prefixes may be attached to the name of each particular 

 sclerite. 



The Prothorax (fig. 7). This is a small segment supporting the 

 first pair of legs, and the neck or microthorax. It varies greatly 

 in shape. As a general rule, it shews the more normal proportion 

 of parts in the most archaic groups, while it becomes highly reduced 

 or otherwise altered in the most advanced forms. 



