EXPERIMENT STATION BULLETINS. 585 



found that the insecticide was apt to stick especially well around the 

 circle where the margin of the scale was in contact with the plant sur- 

 face. Even under favorable drying conditions, a wet circle of the 

 lime-sulphur solution was likely to be found for two or three hours 

 around the scale-margin — sometimes for a longer period. In many 

 cases the solution had passed u/nder the margin of the scale after an 

 hour or two and had wet the pygidium of the insect as, sometimes, also 

 the sides of the abdomen. None of the insects were found with the 

 dorsal surface of the body wet — i. e., the solution was not able to pene- 

 trate the older portion of the scale. After eight to ten hours or longer, 

 some of the insects would be found with the pygidium stuck fast in the 

 dry wax at the margin of the scale. Also, before the margins of treated 

 scales had become entirely dry, it was found that in some cases soft wax 

 was sticking to the pygidium and as the scale was pulled slowly away 

 from the insect a comparatively broad thread of wax would draw out 

 between them (see plate II, Fig. 4.) This showed that after some time 

 in contact with the scale, the yellow solution was able to soften the 

 more recently secreted wax. Not a single instance was found with this 

 species, where the wax could be dra^vn out into a thread from the 

 edge of untreated scales. 



After scale-insects whose bodies had been wet with the lime-sulphur 

 had died, the portion of the body that had been wet always darkened 

 with decay first (provided the body was kept where it did not dry up) 

 indicating the tissues in that portion had died first. The irregular dark 

 area along the pygidium and abdominal margins of the scale-insect 

 photographed in Plate II, Fig. 3, is a true representation of that condi- 

 tion. 



But the comatose Insects under many of the treated scales (as in 

 former observations) did not seem to have been- wet at all. The insects 

 were so small, however, it was very hard to tell certainly whether some 

 portion of the body might not have been wet slightly by the insecticide. 

 Finally, a means was found to aid in detecting any portion of the body 

 that was wet with lime-sulphur solution. A saturated solution of 

 alcoholic corrosive sublimate was used to give a black precipitate in the 

 presence of any yellow lime-sulphur. If just a little lime-sulphur was 

 placed on the dorsal surface of a white-bodied scale insect, and this 

 was then touched with the corrosive sublimate solution a black spot 

 resulted. 



In contact with a thin walled insect, however, the lime-sulphur seemed 

 to oxidize to the colorless thiosulphate more rapidly than on white 

 Ijaper, and thiosulphate of calcium would not give the precipitate with 

 corrosive sublimate. A fresh saturated solution of silver nitrate could 

 then be used; it would give a black precipitate with calcium thiosul- 

 phate at once. Of course, silver nitrate exposed with organic matter 

 will blacken after a while, but not at once as described with the thiosul- 

 phate. Even with the aid of these two test solutions there were a good 

 many cases of comatose insects (whose scale-coverings had been treated 

 with lime-sulphur) in which no indication of their bodies having been 

 wet with the insecticide could be found. 



This fact added weight to the suspicion that some of the treated in- 

 sects might be comatose from the lack of oxygen. 



Further experiments were started to learn something of the amount 



