OP INSECTS. 1 6\ 



elude the attempt to describe it in every particular case; 

 but the normal or prevailing arrangement is nearly 

 the following : All the minute internal ramifications 

 converge into several trunks, which gradually become 

 wider till they debut on a spiracle. Tracing the course 

 of these trunks upwards from the spiracle, they are 

 generally found to divide into two principal branches, 

 one of them becoming ramified ad infinitum, the 

 other of a more uniform diameter and running across 

 to unite with a similar one from the opposite side. 

 A more complete union, however, is established 

 among all the principal branches, by two longitudinal 

 trunks extending the whole length of the body, one 

 on each side, not far from the margin and parallel 

 with it. These are nearly of the same diameter 

 throughout, and intersect the transverse branches not 

 far from their origin. This continual anastomosis 

 provides for the utmost freedom of conveyance in 

 every direction, and places the stigmata on opposite 

 sides of the body in direct communication with each 

 other. In addition to these there is sometimes a 

 longitudinal tube in the centre, and in certain instances 

 there are not fewer than five or six running length- 

 wise. Some of the principal longitudinal tubes give 

 off branches to the legs, and a considerable number 

 enter the cavity of the head.* Although the tracheae 



* To illustrate the distribution of the air-vessels, we have 

 given a figure of those of Nepa cinerea after M. Leon Dufour. 

 See PI. III. fig. 9, in which a, a, a, represent the lateral spi- 

 racles, in this instance obsolete, because the air is admitted 

 by terminal tubes ; 6, 6, ventral spiracles, at the base of the 



