50 GNATS OK MOSQUITOES — CHAPTER III 



ment, which divide up into branches that pass to the con- 

 tained muscles and viscera. Between these lateral thoracic 

 branches I have not been able to make out any longitudinal 

 communication ; but like the lateral abdominal branches they 

 extend as pervious tubes nearly to the skin of the larva, and 

 are continued to the latter as an impervious chitinous cord, 

 surrounded by true dermic tissue, to actually blend with the 

 external chitinous covering of the larva. In the abdominal 

 region the arrangement is essentially the same, only here 

 there is a continuous longitudinal anastomosis between the 

 lateral branches, one of which issues from the main trunk 

 of all but the last segment, and after a short course divides 

 into three main branches — an anterior to anastomose with 

 the posterior branch of the lateral trachea of the segment 

 in front of it, a posterior to communicate with that behind 

 it, and a continuation, which after distributing branches 

 of air supply is continued to the skin, first as a pervious 

 tube and then as an impervious cord, exactly as in the 

 thorax. These cords represent the rudiments of the future 

 segmental stigmatic trunks of the imago, and it is by 

 their agency that the discarded lining of the portion of the 

 tracheal system belonging to the segment is withdrawn 

 from the body whenever the larva undergoes its periodical 

 change of skin. From these three principal segmental 

 branches there issue branchlets which carry air to the 

 various muscles and viscera of the segment. These lateral 

 stigmatic cords can best be studied in the abdominal region ; 

 and if a cast skin, such as will be found floating in abund- 

 ance on the surface of any vessel in which larvae are kept, 

 be carefully examined, it will be seen that the main longi- 

 tudinal air sac has broken up into segmental lengths, and 

 that each piece is attached by its cord to the skin of the 

 <;orresponding segment ; for it must be remembered that 

 not only the outer skin but the whole of the lining of the 

 tracheal system is thrown off at each ecdysis. The struc- 

 ture of the peculiar dorsal horn of the eighth segment 

 remains to be described. Each of the two main stems 

 passes up through it separately, to end in a pair of stigmata 

 at its extremity, whose openings are considerably smaller 



