Male fireflies give a nuptial gift -- sperm wrapped in a nutritious, high-protein package -- to females during mating

A variety of explanations have been put forward as to the purpose of bioluminescence. Some fireflies emit light in a particular flash pattern that uniquely identifies its species. Females respond to males of their own species with a corresponding flash pattern.

Most fireflies stop eating once they become adults. flightless females in some species devote all their energy to churning out eggs and so gain an advantage over their winged cousins, but only receive sperms (without the nuptial gift) from partners . 

 
Read more at: https://phys.org/news/2011-04-fireflies-flightless-females-gifts-males.html#jCp