Hoverflies are the best known wasp and bee mimics, which do an impressive impersonation of their more dangerous distant cousins.
Many hoverflies have evolved these black and yellow ‘don’t mess with me’ markings, but are just bluffing – they are completely harmless.
While in most other hoverflies, males can be easily identified by their holoptic eyes (eyes touching at the top of their heads) and females by their dichoptic eyes (eyes are further apart), the eyes of Helophilus hoverflies are separated in both sexes, in the male a little less than in the female.
In Helophilus hoverflies, the inner edges of both eyes of the males run more or less parallel, whereas in the females they diverge but you can only see this properly in a frontal face photo.

1st photo: 's Graveland, The Netherlands, 11 August 2012
2nd & 3rd photo: 's Graveland, The Netherlands, 13 August 2012
4th photo: Laarder Wasmeer, Hilversum, The Netherlands, 9 August 2014