Brighter female glow-worms lay more eggs than their dim rivals and are more attractive to potential nocturnal mates.
Juhani Hopkins at the University of Oulu in Finland and his colleagues allowed 26 female glow-worms (Lampyris noctiluca) to mate in the lab. The glowing lanterns of the insects varied in size from 7 square millimetres to 19 square millimetres - larger lanterns produce a brighter glow. Each glow-worm laid between 25 and 195 eggs, with those perceived by the researchers to be brightest laying the most. Male glow-worms presented with fake females also preferred those with brighter lights.
The lanterns of female glow-worms may provide clues about fitness to males, who are unable to assess size - also an indicator of fecundity - in the dark.