Sexual selection: Darwinian selection for traits - including behavioral traits -  that promote success in competition for mates or the best mates (operating through discrimination of mates or direct competition).  For males - the sex typically subject to the most intense sexual selection -  the competition is ultimately for fertilizations.

The mating behaviour of scorpionflies illustrates sexual selection and sexual conflict.  The male in the photo on the left is curling his abdomen and emitting a pheromone from his abdominal tip.  Thus he calls a receptive female.  In this case the male would probably be rejected outright because his prey is too small. If he obtains a copulation, he would be unlikely to fully inseminate his mate because she would break off the copulation early.  In contrast, the male in the right photo (left insect) has handed over a large prey which will guarantee him a full copulation (An example of material benefits mate choice).  In this case sexual conflict over mating frequency  would be likely to arise.  This would be exhibited behaviorally after a full sperm complement has been passed (approx 20 minutes) and the male pulls the prey from her, while she struggles to retain it.  Here's the conflict: there would be sufficient resources left in the prey for copulations with additional females by the male, whereas the current female would gain (food) from continuing the meal beyond 20 minutes.  This scorpionfly is a bittacid (Harpobittacus).  The other system I described involved panorpids (Panorpa) in which male obtain matings three ways: using a dead bug or a salivary gland secretion as a nuptial gift or - when no food is available - to force copulate with a female (another case of sexual conflict - see below).

Gwynne, D. T. (1984). “Nuptial feeding behaviour and female choice of mates in Harpobittacus similis (Mecoptera: Bittacidae).” Journal of the Australian Entomological Society 23: 271-276.

Thornhill, R. (1980). “Sexual selection in the black-tipped hangingfly.” Scientific American 242: 162-172.

Thornhill, R. (1981). “Panorpa (Mecoptera: Panorpidae) scorpionflies: systems for understanding resource-defense polygyny and alternative male reproductive effort.” Annual Review of Ecology and Systematics 12: 355-386.