This species shows strong sexual dimorphism, meaning that males and females look very different.

Males are around 12mm long with very large eyes and clear wings, whereas females are around 14mm long with much smaller eyes and heads and smoky brown wings. Males use their large eyes to help them spot and catch females in the mating swarm, and to see and chase off rival males. 

Adult St Mark’s flies only live for about a week so they spend most of their lives in the larval stage.  After mating females lay their eggs in the soil and die shortly afterwards.

The larvae feed and grow over the autumn and winter before emerging as adults the following spring.