Abstract
Various parts of the mating act of Dermacentor variabilis (Say) were isolated and tested to determine which part or parts promoted rapid and complete engorgement in the female. Males unable to transfer spermatophores because of blocked genital apertures were incapable of promoting rapid and complete feeding of females. Males treated with 2.5 kilorads produced no germinal cells beyond primary spermatocytes and mated with females which completed engorgement as soon as those mated by untreated males. Males treated with 7.5 kilorads fed normally, but died immediately after feeding and did not mate. Females exposed to these males did not complete feeding nor did females isolated from all contact with feeding males. The stimulus for female rapid engorgement involves receiving the spermatophore and/or its contents.

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