The large wings of an adult Malaysian Moon Moth (Actias maenas). Moon moths are a type of Silkmoth.
Apart from the vertebrates, insects are the only other group of animals to have successfully evolved flight. The wings are composed of two membranes of the cuticle pressed together and supported by a series of veins. The pattern of veins, the venation, is not haphazard but very regular, though it does show modifications. These are very useful in identification. The wings are more or less triangular in form and certain regions may be recognised:-
Regions of the wing
Illustration of the main regions of the insect wing.
Illustration of the main regions of the insect fore wing.
Illustration of the main regions of the insect hind wing.
The basic pattern of longitudinal veins, the venation, have been shown in a previous article. This pattern may be modified in a variety of ways, such as fusion of veins, loss of veins or even the development of additional ones. Primitively these main veins are connected by a series of cross veins which are seen clearly in fossil forms as well as present day Dragonflies & Damselflies and Mayflies.
Dragonfly Wing showing the series of cross-veins
Illustration of the cross veins on a Dragonfly wing.
In most insect orders these cross veins are reduced or absent. Primitively the pairs of wings beat independently of one another. This is rather inefficient, the effectiveness of the wing beat may be increased by the wings acting together. A number of different methods of coupling the wings to achieve this have been developed. The jugum (also called the jugal area) of the fore wing is involved in some of these.
Insects have one of two different arrangements of muscles used to flap their wings:-
Direct flight muscles
Direct flight muscles are found in insects such as dragonflies and cockroaches. The wings pivot up and down around a single pivot point. The wings are raised by a contraction of muscles attached to the base of the wing inside (toward the middle of the insect) the pivot point. The wings are then brought down by a contraction of muscles that attach to the wing outside of the pivot point.
Illustration of the operation of an insect's wings using direct flight muscles. The darker muscles are those in the process of contracting.
Indirect flight muscles
Indirect flight muscles are found in more advanced insects such as true flies. Indirect flight muscles are connected to the upper (tergum) and lower (sternum) surfaces of the insect thorax. A second set of muscles attach to the front and back of the thorax. The wings are raised by the muscles attached to the upper and lower surface of the thorax contracting. This brings the top surface of the thorax down and, along with it, the base of the wings. As a result the wing tips pivot upwards. The wings are then lowered by a contraction of the muscles attached to the front and rear of the thorax. This forces the upper surface of the thorax to raise and the wings pivot downwards.
Illustration of the operation of an insect's wings using indirect flight muscles. The darker muscles are those in the process of contracting.
Synchronous and asynchronous muscle
Insects that beat their wings less than one hundred times a second use synchronous muscle. Synchronous muscle is a type of muscle that contracts once for every nerve impulse.
Insects that beat their wings more rapidly use asynchronous muscle. This is a type of muscle that contracts more than once per nerve impulse. This is achieved by the muscle being stimulated to contract again by a release in tension in the muscle. This can happen more rapidly than through simple nerve stimulation alone.