Glass-winged Drone Fly

Glass-winged Drone Fly Eristalis similis is an abundant hover fly usually encountered in damp woodland habitat. The trick naturalists have to learn is how to distinguish from its lookalike Eristalis spp cousins, especially the even more abundant (ubiquitous even) Tapered Drone Fly E. pertinax and Common Drone Fly E. tenax.

 

Male Glass-winged Drone Fly, photographed by me in early March this year on vegetation alongside the voie verte (greenway) at Chaumussay.

 

The key differences require taking careful note of the colour (light or dark) of each leg, and certain details of the wing markings. 

• E. similis has dark front and middle feet. So does E. tenax, but E. pertinax has pale front and middle feet.
• E. similis has bicoloured hind tibiae. E. tenax has dark hind tibiae but E. pertinax has bicoloured hind tibae. This feature is surprisingly easy to spot, so worth bearing in mind.
• E. similis has a long lightish brown pterostigma (a rectangular mark near the middle of the leading edge of the wing). E. tenax and E. pertinax have small squarish dark brown pterostigma.
• E. similis never has a wing shade (a dark smudge on the transparent wings). E. tenax and E. pertinax often have wing shades.
• E. similis has black hind femora, bands of hairs on the eyes, a dark abdomen with triangular orange marks on the second segment, plumose (feathery) arista (a bristle that projects off the antenna) and a black stripe down the middle of the face. Other Eristalis spp have some or other of these characters too, but not all of them together in combination. 
• E. similis and its lookalikes are about 15 mm long. The species is on the wing from March to September, whilst the lookalikes can be seen in any month. Apart from these three there are a number of other Eristalis spp present in France, and they do all look rather alike. To know if you have a fly in the Eristalini tribe (Drone flies and their relatives) look for the loop in the median vein on the wing. Other flies do not have this.
  

Drone flies are all mimicking honey bees, and they are frequently mistaken for bees by the public. The news media will often publish a stock image of a drone fly to illustrate an article about bees, much to the exasperation of entomologists. Drone flies are terrific and very active pollinators. They spend much of their time as adults visiting flowers, but their life cycle is very different to bees. Drone fly larvae develop in wet pockets that form in rot holes in mature tree trunks, and sometimes they can be found in gardens in containers which have accumulated a mixture of water and rotting vegetation. The larvae are known as rat-tailed maggots, on account of their long thin breathing tubes, which they use like a snorkel while under water and consuming decaying plant material in a bucket, a ditch or a rot hole.

 

For comparison, a male Common Drone Fly photographed by me in early May 2023 at la Cabane, a Zone d'Interêt Ecologique, Floristique et Faunistique (ZNIEFF) that I monitor.

 

For comparison, a male Tapered Drone Fly that I photographed in April 2004 in London, England.

This is how a biodiversity surveyor spends their time -- making sure they can accurately identify creatures that others overlook and disregard.