Are Hoverflies the Masters of the Air?

Hoverfly morphology: the wings

The function of any wing be it an insect or a bird is to give it maneuverability to help catch its prey, avoid a predator, and maybe even assist in mating. 
 
Ever seen a hoverfly just hovering around flower petals, leaves or a stem infested with aphids?
 
It’s probably a female and what it is trying to do is lay an egg on the plant without the aphids sending out chemical alarm calls. The egg of that hoverfly if laid successfully will hatch and mature into a hoverfly larva that will eat the aphids whilst it develops into an adult several weeks later. It goes without saying that this is why hoverflies are a gardener’s best friend, as they offer free pest control, if they are encouraged into your garden.
 
I am no physicist, and the purpose of the blog is not to even try to explain the process of flight. I hope however to ask the question of whether hoverflies are masters of the air, being able to frankly hover.The wings of a hoverfly are a thin transparent membrane designed such that it gives it strength and form through a series of struts known as veins. These veins help prevent the wings from corrugating or collapsing during flight.
 
All flies are classified as Diptera meaning 2 wings (Greek: Di=two, Ptera=wing). Hoverflies have 2 wings, compared with Hymenoptera (Bees, Wasps & Ants), Odonata (Dragon and Damselflies), Neuroptera (Lacewings), and Megaloptera (Alderfly) etc which have 4. Some are easier to spot than others, but all the insects below have 2 pairs of wings whereas hoverflies have just 1 pair. Hoverflies belong to the order Diptera (true flies) along with houseflies, horseflies, robber flies and mosquitoes.

In hoverflies the 2^nd pair of wings have devolved into halteres, club-like structures found under the wing. The halteres’ purpose being to aid balance, acting like a gyroscope and giving the hoverfly details about how much it is pitching and rolling in the air. Each wing of the hoverfly has 6 longitudinal veins and several cross veins; these give it strength but the ones which are parallel and closest to the hind edge of the wing help limit the number of tears when flying. These hind edge veins can also be diagnostic as they slightly vary from species to species. The Eristalini and Merodontini tribes even have a loop in one of the central outer longitudinal veins which again is a distinguishing feature. But the one vein that all hoverflies possess is the false vein called the Vena spuria, found in the centre of the wing unconnected to all others. False because it is not actually a vein but a chitinised fold and receives no haemolymph nor connection to the thorax. This is not something easily identified in the field and depending on the size of the hoverfly and available lighting can even be missed.

The picture above shows the key features of the hoverfly wing for identification purposes.

 

Vena Spuria – unique to hoverflies (Syrphidae) a chitinised longitudinal fold that looks like a vein but is unconnected. Exception Psilota anthracina which has no vena spuria.
 
False Margin – outer-cross veins close to the wing margin; hoverflies have 2 most other Diptera have 1 or none, see wing designs below.
 
R-M cross vein – always present in hoverflies in the middle of the wing. The orientation of this connecting vein can vary from species to species.
 
Stigma – coloured area of the wing near the upper edge. This can vary from species to species being an open cell, dark square or rectangle, a fleck and even a splodge that extends across the middle of the wing.
 
Squama – lobes at the base of the hind margin of the wing near the halteres. The presence of absence of hairs on the squama can be important in identification, not a characteristic easy to see in the field.
 
Alula – a hinged flap located at the base of the wing; this allow the wings to fold back and be held against the body when at rest. In flight it can be used to fine-tune direction.
 
The wings below show other true flies closely related to hoverflies for comparison. Hopefully, you can see that the wings look similar but on closer examination you should notice there are slight differences, particularly in the venation and especially those near the edge. The housefly is the closet species to a hoverfly in terms of vein wing structure, but it does not have the vena spuria fold.

A hoverfly beats its wings at 120-150 beats per second which propel it forward at speeds of 9mph. In comparison a hummingbird can only beat its wings at 50bps yet reach a maximum speed of over 60mph, but its typical speed is around 28mph. A dragonfly which like hummingbirds can hover and even fly backwards typically beats it wings at 30bps and can reach speeds typically of 25mph.

So, in conclusion, hoverflies are not the fastest nor the quickest but unlike a housefly are much easier to capture into a pot! I read somewhere that it takes six beats of a hoverfly wing for it to take off. Their venation allows them to hover and even fly backwards, not unlike the abilities of a dragonfly or hummingbird. The false margins give it protection when in flight, see below how tatty the bees’ wings have become. But despite all those features what makes hoverflies unique is the vena spuria and its not even a vein but a spurious vein. 

     Photo: J.Huddleston

So, they might not be masters of the air, but they are special little critters that should be appreciated.

Further reading:

 
The Natural History of Hoverflies by Graham E.Rotheray and Francis.S.Gilbert https://www.nhbs.com/the-natural-history-of-hoverflies-book
 
The Comstock–Needham system is a naming system for insect wing veins, devised by JohnComstock and GeorgeNeedham in 1898. It was an important step in showing the homology of all insect wings.
 
A website dedicated to the wing structure of insects although mainly non-UK examples Diptera | DrawWing

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