.VORTH AMKRIUAN ODONATA. 179 



The ve)itral pair lie, one just beyond either lateral margin of the 

 abdominal sterna. Each ventral trunk receives a branch from each 

 of the eight abdominal spiracles of its own side of the body. The spira- 

 cles, as already mentioned in describing the abdominal skeleton, lie 

 in the anterior part of the pleural membrane which connects the 

 terga and sterna of the first eight abdominal segments. Each spi- 

 racle is elongated in the direction of the long axis of the body, and 

 in the Aeschnid^ and Libellulidre, at least, is guarded by a chitinous 

 flap, a prolongation of one side of the margin of the opening ; the 

 free edge of this flap and the margin opposed to it are bristly. In 

 the AgrionidiB this flap is apparently absent. Each ventral trunk 

 supplies tracheal branches to the ventral nerve cord, the sternal ab- 

 dominal muscles and the reproductive organs. In the posterior part 

 of the thorax, each ventral trunk, which is there quite small, becomes 

 connected with a branch of the dorsal trunk ; in its course through 

 the abdomen, a number of cross-trachefe connect it with the dorsal 

 trunk ; at about the level of the seventh pair of abdominal ganglia, 

 it becomes connected with the visceral and the dorsal trunk of the 

 same side by means of a large cross branch {Lihellula pulcheUa) . 



Each visceral trunk is attached to its respective side of the alimen- 

 tary canal from near the hind end of the mid-gut to the base of the 

 abdomen {L. pidchella), where it appears to be connected with 

 branches of the dorsal trunks. Each visceral trunk supplies numer- 

 ous branches to the mid-gut; as just mentioned it unites posteriorly 

 with the ventral trunk of the same side and from the point of union, 

 small trachepe proceed to the Malpighian tubules and hind-gut. 



The dorsal trunks are the largest of the three pairs ; in the abdo- 

 men they lie above the alimentary canal, one on either side of the 

 median line, with the dorsal (blood) vessel between them. In the 

 thorax, however, they become ventral (L. jmlchella) and pass into 

 the head, supplying by their numerous branches, the organs in both 

 head and thorax and having two pairs of external orifices — the tho- 

 racic spiracles ; beyond the middle of the abdomen each trunk bi- 

 furcates (L. jmlchella), but the two branches continue backwards 

 side by side to the cross-trunk by which communication is had with 

 the united visceral and ventral trunks, after which they again sepa- 

 rate and supply the region of the hind-gut ; the tergal abdominal 

 muscles are mainly supplied by the dorsal trunks. In the second ab- 

 dominal segment each dorsal trunk has connected with it, by means 

 of tracheal stalks, two or more air-sacs. Ventral and dorsal trunk 

 of each side are connected by cross-trachese. 



TKANS. AM. ENT. SOC. XX. AUGUST. 1893. 



