Mayflies have two penises

(9 March 2018) Mayflies emerge in… May. At the Northern hemisphere that it. Here on the mountain they are emerge in Summer. They live most of their lives in water, and only a few days flying around. That’s why sightings are not that common. Mayflies make the most primitive order of living, winged insects; they can be found on every continent.
The eggs are laid into water, where the young mayflies emerge as ‘nymphs’, staying in the water. Mayflies go, like many insects, through different ‘instar’-stages when they grow: each time they become too big for their skin (called exoskeleton) they crawl out of it to grow even more. When they are ready to emerge from the water, they have reached the last stage of molting: they are now called subimago’s.
The primitive character of the mayfly becomes clear at this stage: in all insects this stage is the adult stage, but not in mayflies. They will molt one more time to become fully-fledged adults called the imago’s.
During their imago-stage, lasting from only a few minutes to a couple of days, mayflies are only concerned with reproduction, they do not even have a functional mouth and the intestines are filled with air. Uniquely among insects, mayflies possess paired genitalia, with the male having two aedeagi (penis-like organs) and the female two gonopores (sexual openings). After mating the females lay the eggs while flying over the water… and then they die as well.

Adult mayflies, called imago’s, don’t have a mouth. They don’t eat while mature, only reproduce.

Recognising a mayfly is not hard: they have three ‘tails’. The outside two tails are called cerci and are remnants of what once were legs; they are sensory organs, acting as tactile or touch receptors. Many insects have these cerci: ear-worms use them as weapons, spiders to make webs (they are then called spinnerets).
Between the two cerci is a third caudal filament, giving the mayfly its famous ‘three tails’.
The best chance of seeing a mayfly is during the day over creeks or other places with running water. At night they can be found resting on leaves.
But NOT in May!