182 PHILIP p. CALVERT. 



" From each elementary eye [of the compound eye] proceeds a 

 nerve fibre (post-retinal fibre). All the post-retinal fibres are directed 

 inwards, and traverse in their passage a sort of interposed nervous 

 screen, which I will describe under the name of ganglionic plate. 

 After having traversed this latter, the fibres continuing inwards, in- 

 terlace completely, and so form the external chiasm. The fibres of 

 the external chiasm then sink into a mass of granular substance 

 known under the name of external medullary mass. They reappear 

 to interlace again and form a second chiasm, the internal chiasm, 

 and to accjuire a second mass of granular substance, the internal 

 medullary mass. The nerve fibres, after having traversed this mass 

 reappear to form the optic nerve and proceed directly to the brain. 

 We will add that to each of the masses of granular substance are 

 annexed masses of ganglionic cells, from which they receive prolon- 

 gations." 



The circumoesophageal nerve-ring closely surrounds the alimentary 

 canal, so that the interval between supra- and subcesophageal ganglia 

 is a short one. 



The subcesophageal pair of ganglia lie immediately below and 

 behind the oesophagus as it turns downwards to the mouth. It sup- 

 plies a pair of nerves each to the mandibles, maxillae and labium. 



The three thoracic ganglia are pro-, meso- and metathoracic re- 

 spectively. The prothoracic pair innervates the first pair of legs, 

 the mesothoracic the second legs and the front wings, the metatho- 

 racic the third legs and the hind wings. 



The relative size of these three pairs varies. Thus in Episeschna heros the pro- 

 thoracic ganglia were found to be the smallest of the three, while in Libellula 

 aurijjennis they were fully as large as the other two. In Calopteryx maculata, 

 Aeschna constricta and Episeschna heros the nieso- and metathoracic ganglia are 

 distinct from each other, while in Libellula auripennis, L. pulchella and in Gom- 

 phus vulgatissimus (according to Blancliard's figure), they are united. This union 

 suggests a correlation with the obliteration of a considerable portion of the 

 raeso-metathoracic, or first lateral, suture, but as the same obliteration is also 

 found in the Aeschninje, the union of the ganglia in the Gomphinfe and Libel- 

 luliuEe has probably followed the appearance of the suture — if the correlation 

 really exist throughout these two groups. 



The distribution of the nhdomimd ganglia is, as far as known, the 

 same for all groups. The first pair lies in segment 1, the second in 

 8, the third in 4, the fourth in 5, the fifth in 6, the sixth in 7, the 

 seventh in 8. The abdominal ganglia are smaller than the thoracic, 

 and are proportionately larger in Calopteryx iXvAu in Aeschna. Each 



