Reproductive Isolating Mechanisms in Two Sibling Species of Mites (Acarus siro, a. farris) (Acari: Astigmata) Commonly Found Together in Stored Grain

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Abstract
Single male Acarus siro and Acarus farris were crossed in the laboratory with different proportions of A. farris and A. siro females. Males of both species mated continuously and rarely restricted mating to conspecific females. Overall sexual activity of males in two-female cultures varied according to species, and was age-dependent. No differences between species in mating activity was noted in three-female cultures. Male A. siro mated at the same rate in two- and three-female cultures but the rate of male A. farris declined slightly in the presence of a third female. In two-female cultures, female A. farris contained more spermatophores than female A. siro regardless of species of male. Differences between species were less pronounced and partially dependent upon female species ratio in three-female matings. Heterogeneity values for differences in spermatophore numbers between species were significant in all experiments, indicating that most matings were conspecific in some cultures but hybrid matings more frequent in others. Male A. siro and A. farris differed with respect to mating preferences, male siro appearing to mate at random with respect to the two species although they usually mated more frequently with A. farris females. Males of A. farris preferred to mate conspecifically but mated freely with siro females. Only 6–14% of cross-mated females developed eggs, regardless of species of male, culture age, or proportion of females. Post-mating mechanisms are responsible for reproductive isolation between A. siro and A. farris.

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