Many people know that only female mosquitoes drink blood. They need the protein-rich blood in order to lay eggs. Probably what fewer people might know is that only male crickets make chirping sounds. They do this to attract female crickets.
Cricket chirps can also be used as a thermometer, too. Simply count the number of chirps a cricket makes in 15 seconds and add 37. The number this results in will be a reasonably accurate temperature in Fahrenheit degrees.
Another bit of gender inequality; only male fireflies can fly.