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Dragonflies

February 2018 | by Andy McIntosh

Dragonflies and damselflies are some of the most impressive fliers in the insect world and, with four wings individually controlled, fly rather like helicopters.

Dragonflies are agile, fast fliers, and can reach speeds of 10-15 metres per second, or 25-35 mph. Damselflies, though also very agile, have a different mode of flying and will more often move from one hovering motion to another, somewhat like humming birds.

Life cycle

Dragonflies and damselflies (both from the insect order, Odonata) have a life span of more than a year. There are three stages in the life cycle. They begin their lives as eggs, which hatch in water and then quickly become nymphs (most of the life cycle of a dragonfly is lived in the nymph stage and one rarely sees this). Finally, they become the adult dragonflies or damselflies.

Nymphs

Once the eggs hatch, the larva becomes a nymph in water. Its wings are present, but not grown, and are within the casing of what looks like a crusty hump hanging onto its back.

This part of the life cycle can take up to four years to complete under water. The nymphs live in ponds, or the marshy areas of streams or calmer backwaters of rivers, and are voracious feeders, using a unique lower lip that they project to hook their prey.

They may eat creatures as big as tadpoles or small fish and have been observed to eat smaller dragonfly nymphs. They can even become cannibals and eat nymphs of their own species!

Metamorphosis

When the nymph reaches adult size, it comes out of the water, never to return, and sheds its last moult. Remarkably, the nymph, which was breathing underwater using gills, now changes to breathing in air through spiracles, as all land insects do.

A spiracle is more than simply a hole in the thorax of the dragonfly or damselfly; it has a special valve so that air is sucked into the trachea behind the opening. To make sure that this valve only begins to operate once the dragonfly or damselfly nymph has left the water, a set of thin, white threads is woven through these tubes and trachea in the new thorax so that only when they are finally coming out of their old casing, are these threads pulled out of the spiracles.

The new dragonfly inhales initially with its mouth, but full respiration is achieved once the spiracles are operational. At the same time, blood is pumped through the veins of its powerful wings, that then become stiffened aerodynamic surfaces. The whole process is remarkable, since not only is it effectively a new creature from the old, but it is particularly astonishing since the breathing which used gills in water is now changed skilfully to breathing in air.

King of the flying insect world

The fast flight of dragonflies, with a top speed of 15 metres per second (35 mph), is achieved primarily because of the individual muscle arrangement for controlling the four wings.

As a consequence, there is a power stroke in both directions that makes for very strong flight. The flight muscles of dragonflies can adjust the wing beat frequency, amplitude (extent of wing beat), phase difference between forewings and hindwings, and also angle of attack of each of the four wings independently.

This means that dragonflies can out manoeuvre most other insects and are king of hunters in the insect world. They will catch smaller insects at speed on the wing, and sometimes smaller dragonflies themselves become the meal for larger species of dragonflies.

However, dragonflies themselves are not without predators. There are birds such as kites, hobbies, wagtails and swifts that, with keen eyesight and agile flight, can actually catch this top insect sky hunter in mid-flight.

Natural resonance and pterostigma

Typically, dragonfly wings beat in the range of 30 to 50 cycles per sec. But, at the end of each wing and on its leading edge, there are dark spots called pterostigma, which are heavier than the rest of the wing.

During maximum forward flight speed, if the wings were lighter (without pterostigma), they would start to flutter uncontrollably. This would be due to the natural vibration frequency of the wing being excited by aerodynamic forces that depend on the speed of the dragonfly. This is called the ‘resonant frequency’, which depends on the mass distribution of the wing.

With pterostigma the wings stay balanced, because the mass distribution of the wing is greater at the tip; and the threshold speed — where the natural frequency for uncontrolled vibration occurs — is raised outside the range of dragonflies. So, the pterostigma are precisely placed stabilisers!

Wing twist and four-wing interaction

Each wing has the capacity to twist upwards (supination) and downwards (pronation). This is also a very important feature for gaining extra lift for each wing beat. It is similar to the humming bird wing motion and enables the dragonfly or damselfly to manipulate the vortices (spinning air) shed from each wing.

When the pair of forward wings are out of phase with the rear pair, the hind wings pick up some of the vortices shed from the forewings, so that on the downstroke of the hind wing, it senses the updraft from these vortices. This greatly reduces the induced drag (that is drag due to lift) of the individual wing, and therefore the wings together are more efficient than acting separately.

There is a significant issue that these three features (muscles, pterostigma and wing interaction) raise. It is, actually, the detailed knowledge of aerodynamic engineering which uncovers the intricate design features in nature’s flyers.

Precision engineering requires design, and no engineer looking at Odonata wings could come to another conclusion. Wings do not make themselves. The design of muscles to operate the wings; the design of pterostigma all in exactly the right place; the design of the four wings with a different shape for the fore wings to the hindwings: all these illustrate the principle of irreducible complexity.

By irreducible complexity is meant that only when the whole system is operating together is the design apparent. A wing without the right muscle arrangement is useless and a dragonfly without pterostigma is also useless. Intricate design speaks of God, the Lord of creation, who loves detail.

Professor Andy McIntosh

A version of this article has also appeared in the book Wonders of Creation — design in a fallen world, co-authored with Professor Stuart Burgess and Brian Edwards, published by Day One, 2017 (ISBN: 9781846255656). The dragonfly image is of a Scarlet Marsh Hawk Dragonfly, with its striking, iridescent, scarlet red abdomen.