VI] THE NERVOUS SYSTEM 131 



with the tenuity and length of the abdomen. Thus the very 

 archaic Petalura (fig. 54) has the two cords so closely fused that 

 it is quite impossible to separate them, whereas in the highly- 

 specialized but short- bodied Orthetrum they lie distinctly apart. 



The Thoracic Ganglia. Soon after entering the prothorax, 

 the ventral nerve-cord passes into the fused pair of ganglia called 

 the prothoracic ganglion (tg^). This ganglion gives off nerves to 

 the fore-legs (first crural nerves, tn-^ and to the prothoracic muscles. 

 From this ganglion the two nerve-cords run backwards into the 

 mesothorax, generally diverging slightly from one another, to 

 enter the large mesothoracic ganglion (tg 2 ), a fused structure similar 

 in nature to the prothoracic ganglion. It lies just above the 

 mesosternum, and gives off nerves to the middle legs (second 

 crural nerves, tn), to the muscles of the mesothorax, and to the 

 fore- wings (first alar nerves). The last are best developed in the 

 well-grown larva. In most Dragonflies, this ganglion is con- 

 siderably larger than the prothoracic; in the Libellulidae, the 

 two are approximately equal in size. The metathoracic ganglion 

 lies close behind the mesothoracic (with which it is sometimes 

 fused), just over the metasternum. It gives off nerves corre- 

 sponding with those of the mesothoracic ganglion. 



We have already mentioned that, in the Calopterygidae, the 

 division between meso- and metapleura (first lateral suture, p. 23) 

 remains distinct and complete, whereas in all other Odonata it 

 becomes more or less obliterated. The condition of the meso- 

 and metathoracic ganglia appears to be correlated partly with 

 this fact, and partly with the degree of forward movement of the 

 meso- and metasternum. We may exhibit in a short table three 

 recognizable stages in the evolution of the thorax and its nervous 

 system, arranged in phylogenetic order. 



Stage First Lateral Suture Synthoracic Ganglia Groups 



1 Complete Distinct Calopterygidae 



2 Partly obliterated Distinct Petalurinae, Aeschninae 



* Partlv nhlitpratprf Pnsprl \ Gomphinae, Libellulidae 



I Lestidae, Agrionidae 



The Abdominal Ganglia. In the Dragonfly imago, the abdominal 

 ganglia are arranged as shewn in fig. 54, i.e. one in the posterior 

 half of seg. 1, none in seg. 2, and one in the anterior half of each 



92 



