Insects Flight

Insects

Insects are the most diverse group of organisms. ...
Most insects have four wings, but their arrangement varies.

In dragonflies, the four wings are all about the same size. The wings can flap out-of-phase: front wings rise as back wings fall.

Many insects, though, flap their wings in unison.

In butterflies and wasps, the larger front wings overlap the hind wings, and at first glance, it appears there are only two wings.

Flies have no hind wings at all, just tiny vibrating clubs that help them sense direction. A beetle’s front wings form protective wing covers called elytra. In flight these provide some lift, but they don’t flap much. The wings can be several times larger than the elytra but fold neatly underneath.
More than 300 million years ago, insects were the first animals to fly. Some people wonder how flight could be the result of gradual evolution. Before any flying insects, there would have to be some insects with wings too small to fly. Wouldn't those small wings merely get in the way, making it difficult for the insect to survive? Flight can evolve only if rudimentary wings have some benefit to the organism. Some insects today do have small wings, and they offer a clue as to how flight could have evolved. The stonefly is an insect that spends its larval stage on the bottom of a creek. Most stoneflies can fly when they become adults. But some stoneflies have small wings which they use as sails, to help them scoot across the surface of the water. Other small-winged stonefly species can flap their wings, so they can skim across the water, even on windless days. These stoneflies show one way flight could have evolved in insects.

Dragonfly Wings

Dragonflies are very manoeuvrable in flight.

The flight muscles can adjust stroke-frequency, amplitude, phase between forewings and hindwings, and angle of attack independently for each of the four wings. For the purposes of description, four main flying modes are recognised:
* counter-stroking, in which forewings and hindwings move up and down about 180° out of phase - this pattern is generally used in cruising flight
* phased-stroking, where the hindwings cycle about 90° before the forewings - this provides more acceleration for manoeuvres
* synchronised-stroking, where the forewings and hindwings move in unison
* gliding, where the wings are held without beating for free gliding, gaining lift in updrafts, or during mating

Dragonflies use a variety of lift-inducing mechanisms, for example classical lift at low angles of attack, supercritical lift at high angles of attack close to the stall, vortex production, especially along the leading edge of the forewings, and vortex-shedding. The angle of attack is the most important variable controlling aerodynamic mechanisms (Thomas et al, 2004)

References

1. Rowe, R.J. http://tolweb.org/notes/?note_id=2471 (accessed 3/11/05).

2. Thomas, A.L.R., Taylor, G.K., Srygley, R.B., Nudds, R.L., and Bomphrey, R.J. (2004) Dragonfly flight: free-flight and tethered flow visualisations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack. Journal of Experimental Biology, 207, 4299-4323.

3. Rüppell, G (2002) Lords of the air. Ch 5 in Silsby, J. Dragonflies of the World. CSIRO, Collingwood
4. Sane, S.P (2003) The aerodynamics of insect flight. J. exp. Biol. 206: 4191-4208
5. Wootton R.J. (1991) The functional morphology of the wings of Odonata. Adv. Odonatol. 5: 153-169.
6. Wakeling, J.M. & Ellington, C.P. (1997). Dragonfly flight I. Gliding flight and steady-state aerodynamic forces. J. exp. Biol. 200, 543-556.
7. Wakeling, J.M. & Ellington, C.P. (1997). Dragonfly flight II. Velocities, accelerations, and kinematics of flapping flight. J. exp. Biol. 200, 557-582.
8. Wakeling, J.M. & Ellington, C.P. (1997). Dragonfly flight III. Lift and power requirements. J. exp. Biol. 200, 583-600.
9. Wootton R.J. (1991) The functional morphology of the wings of Odonata. Adv. Odonatol. 5: 153-169.