﻿46 BULLETIN 1147, U. S. DEPAETMENT OF AGRICULTURE. 



(6) Sample 45 is least soluble and sample 10 is most soluble in the 

 bodies of insects. Samples 10 and 25 were totally water soluble 

 before they were eaten, but after being eaten only about three- 

 fourths of the arsenic was obtained as soluble arsenic. 



(c) While no general deductions can be made as to the average 

 percentages of arsenic found soluble in the bodies of insects, when 

 the figures under "control results" are subtracted from these aver- 

 ages, as a general rule, the higher the percentages of arsenic made 

 soluble by the juices of the insects, the higher are the rates of toxicity. 

 Using lead arsenate (sample 39) as a standard, the last statement is 

 strongly supported by the results obtained with the first seven 

 arsemcals (samples 39, 28, 57, 27, 71, 45, and 64), but it is not so 

 strongly supported by the following five samples (64C, 62, 90, 23, 

 and 74). 



(d) The percentages of water-soluble arsenic in the original samples 

 of arsenicals bear no relation to the toxicity of those arsenicals, 

 except in the case of those which are totally water soluble. 



(e) As a general rule, the larger the average amount of arsenic 

 in the insects analyzed, the higher is the rate of toxicity of that 

 arsenical. Using average weights of the un dried insects fed on all 

 14 of the arsenicals and average amounts of arsenic per insect, a 

 bee weighing 98 milligrams contained 0.0119 milligram of arsenic, 

 a silkworm weighing 1,370 milligrams contained 0.1063 milligram, 

 and a Ceratomia weighing 1,620 milligrams contained 0.0219 milligram 

 of arsenic. Thus, although a silkworm is 14 times as large and a 

 Ceratomia is 16 times as large as a bee, the silkworm contained 9 

 times as much arsenic as did the bee and 5 times as much as did 

 the Ceratomia. This difference in amount of arsenic probably may 

 be explained by the fact that for bees and silkworms the spray mix- 

 tures were used five times the usual strength, while for the Cera- 

 tomia the usual strength (1 pound to 50 gallons of water) was suffi- 

 cient to kill the insects within 24 hours. 



(/) None of the water extracts of the bodies of the insects fed 

 on the various arsenicals showed an alkaline reaction, and the 

 highest acid reaction was 5.8 (pH value). As an average pH value 

 for the 14 arsenicals, the bees gave a value of 6; the silkworms, 5.7; 

 and the Ceratomia, 6.1; and as an average pH value for any arsen- 

 ical against all three insects, the only figures obtained are 5.8, 5.9, 

 and 6. Again it is shown that the pH value has nothing to do with 

 the percentage of arsenic rendered soluble by the insect juices. 



Experiments like those performed on the three foregoing species 

 of insects were also performed on another large but easily killed 

 caterpillar {Datana integerrima G. & R.). As the number of these 

 caterpillars was limited, only samples 39, 57, and 64 were used against 

 this species, so that the results obtained could not be easily incor- 

 porated in Table 21. They are, however, similar in all respects 

 to those already discussed. 



MINIMUM DOSAGE OP LEAD ARSENATE REQUIRED TO KILL SILKWORMS. 



By means of a needle-pointed pipette, an acid lead arsenate (sample 

 39) was dropped upon fresh mulberry leaves. Upon evaporation 

 of the water from these drops, the portions of leaves bearing the 

 white spots were fed to large hungry silkworms in the last instar. 



