Although the resemblance may seem distant the fever fly (Dilophus febrilis) is related to gnats, midges and crane flies. Close examination does show some similarities with other species within this group but one thing is indisputable, it is a fly!
The fever fly is related to the St Mark's fly which also appears in spring and is similar in appearance in many ways but the fever fly is some what smaller being only about 10mm long. The fever fly is certainly most common in late April and throughout May but they can be seen after that well in to August and beyond. Early in their season they are often seen in swarms in woodlands and along hedgerows and this is a good indicator of the species.
The larvae live in soil feeding on leaf litter and rotting vegetation.
The big question is why are they called "fever flies"? Although related to mosquito they do not bite and do not spread diseases and so do not cause fever and nor do they feast on corpses of people who have died from a fever! I can find no explanation in any of my books or on the Internet.