NORTH AMERICAN ODONATA. 181 



venient form used by Plateau, the abdominal movements may be 

 tabulated as follows : 



INSPIRATION. EXPIRATION. 



Slower, with decreasing activity; an 



inspiratory pause. Rapid and short. 



Sterna lowered. Sterna elevated. 



Ventral parts of terga move outwards. Ventral parts of terga move inwards. 



Dorsal carina lowered. Dorsal carina elevated. 



Abdomen descends. Abdomen slightly raised. 



The expiratory movements are accomplished by the abdominal 

 muscles, especially the vertical expiratory. The inspirations are 

 effected by the relaxaticni of the muscles. 



Each pair of abdominal ganglia acts as a respiratory centre for 

 its own segment, independently of the others. Decapitation does not 

 stop the respiratory movements, but diminishes their number and 

 amplitude; in one instance they continued for forty-three hours after 

 removal of the head. 



THE NERVOUS SYSTEM. 



The central nervous system consists of twelve distinct ganglionic 

 masses, of which two are in the head, the supra- and the suboeso- 

 phageal ; three are thoracic and seven are abdominal. 



The supra-cesophageal ganglionic mass consists of the median ly 

 situated brain, or cerebrum, connected with which on either side is a 

 very large optic ganglion. The brain lies between the upper half 

 of the posterior foramen and the partition formed by the chitinous 

 rods (6) as described for the endoskeleton of the head. Its surface 

 is not lobed, but as it supplies nerves to the eyes, ocelli, antennae, 

 frons and labrum, it is to be regarded as compound in its origin, 

 like that of other insects. Blanchard (12) shows the cerebrum of 

 Gomphus foi'cipatus (^ vulgatissimus) to give off one pair of optic 

 nerves, one pair of antennal, one labral pair, one pair uniting in a 

 frontal ganglion and one pair of small posterior tracheal. 



The ojdic ganglia, large in all the Odonata, are esj)ecially so in the 

 Aeschninse and Libellulidse, in correlation with the proportionately 

 large area of the eyes in these groups. Each ganglion is connected 

 with the brain by a distinct stalk — the optic nerve. The histological 

 structure of the optic ganglia has been studied by Berger (14) and 

 Viallanes (16) ; their results agree, and the latter summarises them 

 as follows : 



TRANS. AM. ENT. SOC. XX. AUGUST. 1893. 



