Carpenthu and Poi-lakd — Lateral Spiracles in ILjpoclerma Larva. 77 



places somewhat distinct (fig. 11, et. 2 a) as a very clear stratum with the 

 lamination parallel to the surface showing distinctly. I'.eneath it is the 

 epidermis (tig 11, ep.) consisting as usual of a sheet of flattened epithelial 

 cells. 



The Tracheal System of the Fourth-stage Larva of Hypoderma. 



The tracheal system of the fourth-stage larva of Hypoderma bovis consists 

 of a pair of longitudinal trunks dorso-lateral in position and running the 

 whole length of the body, and communicating with the exterior by a pair of 

 thick-lipped, strongly chitinized posterior spiracles, situated on the apparently 

 hindmost segment of the body, towards its dorsal aspect (figs. 1, 2, 3, p. s.). A 

 pair of anterior spiracles (figs. 1, 2, 3, a. s.) and six pairs of lateral spiracles 

 (figs. 1, 2, 3, /. s.) are also present, but the tail-spiracles alone are functional, 

 the air-tubes connected with the anterior and lateral spiracles being plugged 

 with cores of chitin. The longitudinal trunks are connected posteriorly by 

 one, and anteriorly by three, transverse tracheae (fig. 3, p. c., a. c). The 

 posterior connexion is so close to the hinder body-wall and spiracles as not 

 easily to be seen. Of the three anterior connectives, the foremost is the 

 largest, the two hinder being comparatively fine. The longitudinal trunks give 

 off a number of branches, the chief of which are as follows : — Beginning from 

 the posterior end, each trunk gives off from its outer side a tube which at 

 once divides into two, and each branch runs downwards and forwards for 

 some distance until it splits up and is lost in the fat-body ; this tube 

 apparently belongs to the eighth abdominal segment. In front of this 

 each trunk gives off twelve tubes arranged in an outer and an inner series 

 of six, belonging to the abdominal segments from the second to the 

 seventh inclusive. The inner tube (figs. 2, 3, d. int. tr.) keeps near the dorsal 

 body-wall, and soon divides into many fine branches. The outer tube 

 runs ventralwards and gives off three principal branches ; of these, that 

 nearest to the trunk goes to the digestive tract (fig. 3, dig. tr.), coming off at 

 an angle from the middle tube (fig. 3, /. tr.), which is distributed to the 

 ventral and lateral body-wall, muscles, &c, and which gives off near its base 

 the outermost branch (fig 3, sp. tr.), an exceedingly fine tube — as described 

 below, plugged up for nearly all its length and connected with the vestigial 

 lateral spiracle. The intestinal trachea of the fourth abdominal segment on 

 either side (figs. 3, 20, dvj. tr. i) is strongly dilated, forming a pyriform sac 

 whose chitinous lining is exceedingly thin, and wants the usual spiral 

 strengthening. This dilated trachea in the ripe warble-maggot was seen 

 and figured by Joly ('46, pi. viii, fig. 13) seventy years ago, and a series of 



