One thing that people sometimes struggle with is telling the difference between different types of insect. This shouldn’t be a surprise – quite a lot of insects mimic other species, especially those that have a sting or some other form of defence. There are a number of hoverflies that are bee, wasp or bumble-bee mimics, and some of them are active as early as March. So, how do you tell them apart?
Bees have four wings, and flies have two wings. The name of the order Diptera that flies belong to means ‘two wings’. Flies have two small knob-like protrusions called halteres where the hind wings would be, which are used for control in flight. However, it is usually difficult to see two pairs of wings on a bee like this one.
What you can see are the long antennae. Bees have antennae with 12 or 13 segments, while flies have shorter antennae, and hoverflies have three segments and a bristle (called an arista) protruding from each antenna. (The aristae contain sensors for temperature and humidity.) You can see the short antennae and aristae on this photo of a Tapered drone fly (Eristalis pertinax) in the garden on 27th March 2020.
Hoverfly or Bee?
So, if you see an insect like the one below. How do you know it’s a hoverfly and how do you go about identifying it?
Two wings (maybe difficult to tell), short antennae, not ‘wasp-waisted’ – flies don’t have the same narrow waist between the thorax and the abdomen that bees, wasps and ants have. (See this Wikipedia article on insect morphology for what some of the terms like thorax mean.) Behaviour is also important. Hoverflies do tend to hover. The one in the photo had been hovering around the bird bath for a while before it decided to settle on the wall in the sun and clean itself. One feature that is specific to hoverflies (all apart from one rare species in the UK) is the presence of a ‘vena spuria’ in the wing, a line that cuts across the veins in the wing, and which can be seen in the photo below. But that isn’t easy to see unless you have captured the fly or taken a good enough macro photograph.
Identifying the species of hoverfly
The other features highlighted in the photo help to identify this particular species of hoverfly. Other points are described below.
The first thing is to look at its head and see whether there is a clear distinction between the head and the thorax. In this case there is. You can see it in this photo where the fly has twisted its head round to clean its mouthparts with its foreleg.
Then the loop in the vein R4+5 identifies this as belonging to one of two groups of hoverflies – Eristalini and Merodontini. (R4+5 means that it is a Radial vein, and there used to be two, numbered 4 and 5 and still separate in some flies, but which have merged in hoverflies.) The petiole is a short length of vein between where vein R1 meets vein R2+3 and the edge of the wing, and its presence again is important to identify this particular hoverfly. Finally, one of the distinguishing features of ths particular hoverfly is that the tarsi (the end parts of its legs) are yellow on the forelegs and middle legs. This is clearly visible in the photo of its head where it is helpfully holding up its forelegs for inspection! Another feature that we can’t see in these photos is the pattern of yellow and black on the face, which would help to identify it.
This hoverfly is a Tapered drone fly (Eristalis pertinax). It is one of a family of which ten species are found in Britain. This one is a male, which we can tell because its eyes meet at the top of its head. (In some species males have eyes that don’t meet at the top, so in those species this isn’t a way to differentiate between the sexes.) Between the eyes in a triangular area, you can see the three ocelli, which are light-sensitive organs. Eristalis pertinax is described by Ball and Morris as the commonest of hoverflies that emerge in March.
If you want to find out more about hoverflies, the book I use is the one in the WILDGuides series: Britain’s Hoverflies – a field guide, 2nd Edition, Stuart Ball and Roger Morris, 2015, Princeton University Press. You can get this from suppliers like the Natural History Book Service.