There are many, many species world wide, over 50 in Denmark alone. That really has no significance for the fly fisher, but ants do. The most important time is high/late summer, when the individuals (males and females) that migrate to form or join new colonies fly off. They are the winged individuals. They hatch in numbers and it doesn’t take much wind for them to blow on to the surface of rivers and lakes. Especially in Sweden and Norway this is an event that fly fishers look forward to and look for - closely.
I’ve never really experienced out as a hatch, so to speak, here in Denmark, but ant imitations are still important to have in the box.
As soon as the spring heat sets in, the ants come out of winter hibernation and start roaming around and the will drop into the water every now and then. Before the migration an ant imitation is often a good get-out-of-jail-fly that a difficult fish will often pick off.
I carry three versions - a red/orange one on a #12 hook, with and without wings and a #16 black one. I certainly don’t carry boxes of them, but a room in the dry fly box always has a small selection of ant imitations. They are simple and fast to tie.
They fish well high and dry with a little floating, but also damp in, and even just under, the surface.
Have a great weekend!
PS. This FP is a rare instance, where parts of it recycled from a blog entry I've written elsewhere. If you've read that, I apoligise. I was busy this morning.