The female then discharges and eats his offering which is thought to contribute to her ovarian development. In several other species, such as Empis tessellata and Empis opaca , for instance, the offering must come first. Males of E. opaca and tessellata have to present a female with a dead insect if they are to get anywhere. Moreover, the length of the ensuing copulation appears to be directly related to the size and quality of the dead insect that has been exchanged and which females probe and examine carefully. In this style of mating, however, the female does not necessarily get the chance to eat the entire gift.
Genuine gifts are energetically costly to procure.
No surprise then that some males of these species cheat. Several times Preston-Mafham watched E. opaca males offer females large balls of willow fluff instead of insects.