1916] How Gases Enter Insects 225 



RESULTS. 



White grubs or the larvee of wood-borers, showed a black- 

 ened area about the spiracles only. Too long an exposure to 

 the vapor causes a uniform blackening of the chitin. Mealy 

 bugs exposed for two to four hours showed a black area around 

 each spiracle, and in addition a black area around the anus, 

 extending up the alimentary canal. The anal lobes which con- 

 tain many wax glands were also blackened, showing that the gas 

 had entered the wax pores. The wax of the mealy bug was not 

 blackened by the treatment. This is of particular interest, as 

 the mealy bug contains only two pairs of spiracles located near 

 the anterior end of the body. Contrasted with the mealy bug, 

 one finds that the immature stages of the white fly, which is 

 well-protected by a wax coat, only shows a penetration of the 

 gas through the spiracles. The white fly has four pairs of 

 spiracles evenly distributed, one pair being located near the anus. 



Spraying plants infested with mealy bugs with a weak soap 

 solution to remove the wax greatly increases the ease with 

 which these insects can be killed by hydrocyanic gas fumigation, 

 while little effect is noticed in its efficiency for white fly. 



A species of soft scale, Coccus sp., showed penetration 

 through the spiracles, the anus and also the wax glands scattered 

 about the body. Aphids show the spiracles marked, while 

 some osmic acid seems to enter at the nectaries. 



CONCLUSION. 



These few experiments show that gases can penetrate 

 through other parts of the body than the spiracles. It seems, 

 however, that very thin chitin is impermeable to osmic acid. 

 Even the vapor of osmic acid, however, is not a good penetrating 

 agent. Oxygen and probably other gases, particularly hydro- 

 cyanic acid, which is extremely soluble in water has greater 

 penetration and may even penetrate through thin layers of 

 chitin. 



