Popular Science Monthly/Volume 17/May 1880/How Insects Direct their Flight
By M. J. DE BELLESME.
THE works of M. Marey have nicely determined the difference between the manner in which birds and insects fly. The bird can change at will the angle of vibration of his wings, and therefore these organs serve to steer his flight. The insect is deprived of this power, because the angle of vibration, as a rule, is invariable in each species, the flying-muscles not being in the wings, but in that part of the thorax which supports the wings.
Knowing these facts, I concluded that if the wing of the insect be merely a motor apparatus, the steering function must be sought for elsewhere; and, from numerous experiments made upon insects of every order, I am convinced that the steering power depends upon the position of the head and thorax, this, in its turn, depending upon the respective positions of the center of gravity and the axis of suspension (l’axe de sustention). Both these elements are sometimes movable, but more often it is the center of gravity which changes.
There are a few insects in which the motor and steering functions are united, the flying-muscles being attached to the wings. These insects fly gracefully, like birds; still, the abdomen is very long, flexible, and greatly augments the ease and variety of motion; this is specially apparent in the Agrions—the dragon-fly, for example. It is probable that the Lepidoptera (butterflies, moths) should be ranged in the same category, for the movement of their wings is something like that of birds; the anatomy of their thoracic muscles, however, has not yet been completely analyzed.
In the Hymenoptera (bees, wasps, etc.) are found the first indications of separate functions of translation and direction. The wings, having acquired a very perfect automatism by which the axis of suspension has become permanently fixed, are solely devoted to the function of movement. The abdomen has become pediculated and exceedingly mobile. As it bends up or straightens out, the center of gravity is carried forward or backward. The Cynips (gall-flies, etc.) and Ichneumonides (insects that prey upon the eggs of other insects) offer extreme examples. If the abdomen be prevented from moving, the animal can still fly, but can not direct its course. Moreover, the posterior legs of these insects are long, as in the Polistes and Megachiles, and this also aids in displacing the center of gravity.
In the Orthoptera (crickets, grasshoppers, etc.) the abdomen is but slightly movable, and the steering power is almost wholly in the hindfeet; but, as these are already differentiated for an equally important function, that of jumping, they lend themselves with a bad grace to the former function, and so the hopping insects fly very badly.
Thus far we have seen the two pairs of wings devoted to the motor function. We come now to a class of insects in which the functional adaptation is not secured through organs performing other functions and lending themselves to extra duties, but where the adaptation is secured through its own proper organs—one of the pairs of wings, indeed, which, diverted from the motor function, has become a steering apparatus.
In the group of Coleopters (beetles), only one pair of wings, the posterior, serve to sustain the insect; the surface of support, therefore, is considerably diminished, and the flight is heavy or clumsy. But this disadvantage is compensated by the greater extent of surface afforded by the non-modified pair. This surface, indeed, is so great that the insect is obliged to fold it up during rest. The abdomen of the Coleoptera is stiffly fixed to the thorax, and consequently is but slightly movable; but this quality is unnecessary, since a special organ, the elytrum (wing-case), has power to displace the center of gravity. Raised up over the thorax during flight, the elytrum forms a little swaying mass above the center of gravity, and the slightest motion of this mass affects the balance of the insect. Remove the wing-cases, and the insect still flies, but has no power to direct its motion, which is upward, downward, or horizontal, according to the position of the center of gravity at the moment of the experiment. M. Plateau has clearly demonstrated that the normal position of this point varies with each species.
One very small group, the Cetoniadæ, fly with the wing-cases down—an interesting fact, for in this instance they act upon the axis of suspension, and effect a step toward the state of complete differentiation which we find in the following group.
In the Diptera (mosquitoes, flies, etc.) the steering faculty reaches its highest development. The second pair of wings is transformed into organs having the special function of steering, the balancers or poisers; and these insects have accordingly a remarkable perfection of movement. A single pair of wings does all the flying, and, as they are not large, the diminution of the supporting surface is compensated by greater rapidity of vibration. I have proved by experiment that the balancers act by displacing the axis of suspension. Suppress the balancers, and the flight becomes fatally downward, because the normal and invariable position of the center of gravity is in front of the axis of suspension; the animal, therefore, can not modify his movement in any way, the abdomen being but slightly movable, and the balancers cut off. If, now, we come to his relief and attach a tiny weight to his abdomen, just sufficient to carry the center of gravity back to its normal place, we restore to the insect the power to perform all his aërial evolutions.