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XXII. " On the Tracheal Sf/stem of Siimdium." By 

 Thomas Harold Taylor, M.A., communicated 

 by Prof. Louis Compton Miall, F.R.S. 



[Read October 1st, 1902.] 



The tracheal system of the larva of Simulium (fig. 1) con- 

 sists of a pair of longitudinal trunks running from the 

 thorax to the posterior end of the abdomen. The two 

 trunks are connected by dorsal commissures (fig. 1, d). Of 

 these, there are three in the thorax, one between the pro- 

 and mesothoi'ax, one in the mesothorax, and one in the 

 metathorax. There is also a dorsal commissure in the 

 tracheal system of the head. Anteriorly the trunk ends 

 abruptly between the pro- and mesothorax. Posteriorly 

 it ends in the 8th abdominal segment, where it divides 

 into a dorsal and a ventral branch. From each junction 

 between two segments the ti'acheal trunk passes sharply 

 downwards and then gradually rises to the junction next 

 behind (see fig. 1). Near the lowest point of the curve 

 arises the branch which supplies the segment (fig. 1, v). 

 These segmental branches are present in the metathorax 

 and each of the seven following segments. Each passes 

 ventrally from its point of origin and divides into a 

 descending and an ascending branch. At, or near the 

 point of division there arises a fine branch, the initial 

 thread, which runs towards the skin and ends in a spiracle. 

 The last initial thread arises direct from the longitudinal 

 trunk, and not from the segmental branch (fig. 1, e). The 

 initial threads are solid and devoid of spiral marking. 



Between the pro- and mesothorax, the longitudinal trunk 

 gives off a slender branch which passes dorsally as the 

 anterior dorsal commissure already mentioned. The trunk 

 then bends sharply downwards, and after a short course 

 divides into two branches which supply the head, pro- and 

 mesothorax. At this point arises an initial thread, which 

 is connected with the anterior thoracic spiracle. That part 

 of the descending trachea from which the branches arise, 

 and to Avhich the initial thread is attached, may, from its 

 ultimate fate, be conveniently described as the spirade- 

 chamher (see fig. 2). The corresponding part of the ventral 



TRANS. ENT. SOC. LOND. 1902. — PART IV. (DEC.) 



