In breeding Glossina palpalis (R.-D.) in captivityin Northern Nigeria, it was found that even though the sexes are segregated within 24 hours of emergence, a few females succeed in producing larvae. Examination of the spermathecae of females thus segregated shows that nearly 4 per cent. are fertilised. This low fertilisation rate is due to the impotence of most males under 24 hours old. When young males of different ages were forced to compete with older males for the females' favours, it was found that young males must be 7 to 8 days old before their virility approaches that of older males.

When virile males are added to mating cages containing virgin females, pairing startsimmediately, but the number of pairs decreases steadily until after 23 hours, pairing has virtually ceased. The cessation of copulation is due to the female becoming unwilling to pair and to her ability to repulse the male; but if she is re-fed her willingness will be restored to a considerable extent. There is no need to leave the sexes together formore than 24 hours, as excellent fertilisation can be achieved in that time.