Ejaculation by male flies (Drosophila)

The Ejaculatory duct of male flies, when isolated, shows a rapid peristaltic rhythm running from its junction with the vas deferens to the ejaculatory bulb. The frequency of peristalsis is affected by serotonin and by mating.

Layout of male reproductive tract

Serotonin: this increases the frequency of peristalsis (up to a maximum at 0.1 ÁM), while serotonergic antagonists (blockers) reduce the frequency of the rhythm.
Graph showing increased contractions with serotonin
Mating: the frequency of peristalsis is higher in unmated males than in males kept in a vial of mixed-sex flies (where mating is a common occurrence). Males kept in a vial singly, and then allowed to mate also had a lower rate of peristalsis than the unmated mates.

Graph comparing ejaculation rates in mated and unmated male flies

Details of the data and a discussion of the possible relationship between serotonin and mating are found here:
Chris Elliott, Mar 2011