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21

effectiveness over the two previous dosages. In the first series of experiment I all chemicals effected a 100 per cent mortality. These results are shown in figure 16. Nine days after the initial treatment, the second series was begun (figure 17) j this produced a 100 per cent kill with all chemicals, but dieldrin distinguished itself from the others by effecting this complete mortality in a shorter period of time than the other three chemicals.

A noticeable parallel was found between the order of action of chemicals in experiments II and III; and a presentation of the results of both of these experiments follows. Comparison of figures 18 and 20 reveals that dieldrin again exceeded all other chemicals by showing a 100 per cent mortality in both cases in the shortest period of time. Aldrin followed with a 100 and 81 per cent kill, respectively, in the second and third experiments. In the second series of these two experiments, which were initiated 22 days after the first treatment (figures 19 and 21), dieldrin maintained a 100 per cent kill in experiment II, but its effect was diminished to a 53 per cent mortality in experiment III, which was, however, still far higher than that obtained by any other chemical in this experiment.

Chlordane and heptachlor were equally effective in the first series of experiment II by producing a 100 per cent kill, as shown in figure 18. Chlordane exceeded heptachlor to a small degree in the first series of experiment III with a 79 per cent kill as compared with 64 per cent for heptachlor as given in figure 20. The situation differed somewhat in the second series of experiment II (figure 19) inasmuch as chlordane surpassed heptachlor by showing a 67 per cent kill as compared with 30 per cent for heptachlor. In the final series of