In most water strider species, the male forcibly inserts his genitals into the female after mounting her.
Female red-backed water striders have evolved a ‘genital shield’ to block this forced entry.
They even have enlarged spines above the shield to make forced entry even more difficult.
Males can only reach the vagina opening if she extends it for them.
The image on the left is a close up of the female water strider’s genital shield.
Picture B through G show how the female 'opens' and 'closes' the shield.
However, male water striders have evolved a very unusual counter-strategy.
After mounting the female, the male rhythmically taps the water with his legs, creating ripples.
The ripples attract underwater predators, forcing the female to extend her genital opening to quickly finish mating so she can escape.
Since she is on the bottom, she is the most at risk of being eaten.
Here is a video if you want to see this in action!
References and Further Reading