Mastery Of The Air

Updated Monday 17th October 2005

David Robinson explains how creatures left the land, and took to the skies

Giant mayflies Copyrighted image Icon Copyright: BBC

At some time during the Carboniferous period, around 300 million years ago, a series of evolutionary steps took place that produced the first flying insects. Probably it is this single development that has led to the enormous diversity of insect species that are found today.

It is appropriate to call this a single development, because the structure of the wings of all orders of flying insects appears to be derived from a common ancestral form.

Fully-developed wings are only found in adults or last larval stages. A typical wing is a thin sheet of cuticle supported and strengthened by a number of tubular veins. The space between the outer layers of the wing where they expand to form a vein carries blood.

Usually, insects (for example butterflies and dragonflies) have two pairs of wings. However, there are many modifications to this basic structure.

The true flies, like the common house fly, have only the anterior pair of wings and the posterior pair are reduced to small balance organs, the halteres. There are only two stable positions of the house fly’s wings, fully up or fully down. Through the elasticity of the thorax, the wings flip from one state to another, with any intermediate position being unstable. Thus, the flight muscles within the thorax only have to contract a small amount to flip the wings from one state to another. This mechanism is called the ‘click’ mechanism and it amplifies the changes in length of the flight muscle up to 600 times in the wing movement.

In other groups of insects the wings have changed during evolution. Grasshoppers and cockroaches have thick, leathery forewings while the hind-wings are larger and much thinner. The forewings of beetles are hardened and form protective covers (the elytra) over the more delicate hind-wings, when the insect is not flying.

Examples of insect wings:

Left forewing of a fly Copyrighted image Icon Copyright: Used with permission
The left forewing of a fly

The left fore and hind wings of a grasshopper Copyrighted image Icon Copyright: Used with permission
The left forewings and hindwings of a grasshopper

The left elytrum of a beetle Copyrighted image Icon Copyright: Used with permission
The left elytrum of a beetle

 
Giant mayflies Copyrighted image Icon Copyright: BBC

In termites and ants, which are social insects, there are wingless forms and winged reproductive forms. The winged forms have large wings that are shed immediately after the nuptial flight. Some insects have become secondarily flightless and have wings that are very reduced. Two groups of insects, the silverfish and the bristletails, diverged from the rest before the evolution of flight and have never had wings, but all the fleas and lice had wings at one time and have now lost them.

Wings are sometimes preserved in the fossil record and there are particularly fine specimens of dragonfly from the Carboniferous coal deposits. The fossil dragonflies appear very similar to present day forms, except that some species were giants. The largest species of the fossil genus Meganeura had a wing span of about one metre, ten times larger than the span of the largest dragonfly that exists now. All the evidence from the fossils suggests that Meganeura was a fast-flying predator like modern dragonflies. Imagine what it would be like to meet a dragonfly of that size – even our present day dragonflies are very impressive!

In gliding flight, the airflow over the outstretched wings is sufficient to provide lift that counteracts the forces of gravity and drag. The leading edge of each wing must meet the airflow with a positive angle, called the angle of attack. This angle can be as much as 50o, which is much more than the maximum of 20o that an aircraft wing can sustain without stalling.

Most insects are too small to make use of gliding flight and fly by beating their wings. The rate at which the wings beat shows large variations between species. For example, large butterflies have a wing beat frequency of around five hertz whereas some midges beat their wings almost 1000 times each second.

Insect Wing beats per second Flight speed/km h-1
Large dragonfly 22 to 28 25.2
Mosquitoes 275 to 575 3.2
House fly 190 to 330 6.4
Honey bees 190 to 250 22.4
Desert locust 18 to 24 16.0

The development of high speed photography helped greatly in the study of insect flight, but getting an insect to fly when a film camera was running at very high speed was very difficult – and think of the cost of the film! Some studies were made using stroboscopic light, where light flashes were synchronised with wing movements to make the wings appear stationary.

But now, high speed video lets us record insect flight more easily and, using disc drives, we can record continuously. So, instead of waiting for the insect to take off and then start the camera (and how can you predict when take-off will happen?), we can leave the camera running until it takes off and then stop the camera afterwards, knowing that we have the complete take-off stored.

There is a fascination in the study of insect flight and the senses associated with it. The interest comes partly from the wide range of subject areas that contribute to the analysis of flight, from biochemistry through to physics, but also the sheer beauty of insect wings and their staggering variety. Flight is an evolutionary development that has led to the unique diversity of insects and their central role in most terrestrial habitats.

 

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