| In the | Ant there are | 50 |
| " | Convolvulus Sphinx | 1,300 |
| " | Common House-fly | 4,000 |
| " | Silk-worm Moth | 6,236 |
| " | Goat Moth | 11,300 |
| In the | Dragon-fly | 12,544 |
| " | Butterfly | 17,355 |
| " | Mordella | 25,088 |
According to the calculations of another author there cannot be fewer than 34,650 in the compound eye of a butterfly. Amazing thought! each of this immense host is considered to be a separate eye, receives separate impressions of light, and has a separate structure and organization, both perfect in their kind.
It is not difficult to remove the compound eye of an insect; and in so doing it will be found that each lens is as clear as crystal. The ingenious Réaumur actually succeeded in removing one and adjusting it to a lens; and he found that he could see through the insect's eye very distinctly, only he says the surrounding objects appeared to be greatly multiplied. There is a common optical toy which is ground into a number of facets which communicates this appearance to objects when seen through it, and thus furnishes us with a good illustration of the endless confusion of images which would have perplexed the insect, had not the various beautiful contrivances of which we have spoken been adjusted to prevent it.
But we must guard against a very natural mistake which might arise upon the subject of the eyes of insects from supposing them in any degree comparable to those of higher animals as regards their motions. When man, or an animal, wishes to look at any object, they do so by causing several muscles to be brought into action which move the eyes round so as to receive the rays of light from the particular point where this object is placed; and so admirably arranged is the mechanism by which these movements are effected, that they are as well provided for all the purposes of sight with two eyes as with twenty. In insects no such apparatus exists; the eyes are quite immovable; they are, in fact, set in the head like a gem in a lady's ring, and are altogether removed from the control of the insect. In order to obviate the annoyance and inconvenience which would result from this arrangement, their eyes are formed on the wonderful principles already mentioned, and, instead of insects being furnished with two eyes, they are provided with many thousands! They are thus enabled to enjoy not only the same extent and range of vision with ourselves, but even a much larger.
Insects are also furnished with a contrivance by which they can see objects at a little distance, and objects at a great distance—it may be at the same time; which is more than can be strictly said of ourselves. In men and animals there is a very exquisite apparatus arranged within the eye, by means of which it can accommodate itself to objects close at hand, or again to others at the greatest distance. We can see at one moment a pin at our feet, and at the next the summit of a hill some thirty or forty miles off. Now the laws of light are such, that, to effect this properly, we must have some apparatus in the eye to arrange its focal capacity, so as to receive and concentrate the lines of light proceeding from such different points as the distance of a few inches, and that of many miles. What this apparatus may be is not as yet very satisfactorily determined. But in insects the same result is obtained by a very curious provision.—Some of their eyes are short-sighted, and some long-sighted. The simple eyes are supposed, by Professor Müller, to be the short-sighted eyes, and the compound eyes the long-sighted ones.
The number of compound eyes in insects does not often exceed two, these being made up, it will not be forgotten, by multitudes of single eyes. But in a few, whose habits require that they should be endowed with extraordinary means of vision, there are as many as four. If the reader would betake him to the brook-side, and creep noiselessly along its margin some summer afternoon, until he comes to a quiet glassy pool where the water seems to have forgotten itself and fallen asleep, so still, so silent, and so smooth does it lie, reflecting all the lustre of the deep-blue sky overhead, he will surprise a dancing party of insects busy waltzing at a wonderful rate, now skimming hither, now shooting across the glassy pavement on which they sport, now joining together and wheeling round and round, and again, as the king-fisher comes fluttering down the river as though on some errand of immense importance, breaking up their party and flying into a thousand holes and corners to wait until all is quiet. Let him exercise his activity and patience, and catch one of these giddy insects, which are known to entomologists by the name of the Gyrinus Natator, and he will have a good example of an insect provided with four compound eyes, so that it can see not only before and behind, but upward into the sky, and downward into the clear cool waters on whose surface its happy life is spent. Some insects, like Cyclops of old, are furnished only with one eye; and some, it is said, are quite blind—creatures that never feel the blessed influences of the pleasant sunlight. Like the simple eyes, the compound eyes are sometimes fixed on the end of a little footstalk, so as to give the insect somewhat the appearance of being furnished with a pair of opera glasses, or short telescopes.
In order to ascertain by what means the bee found its way to the hive, whether by seeing it through its compound eyes, or otherwise, Réaumur performed an interesting experiment similar in character to the one before mentioned. He covered with a red varnish, which was quite opaque, the compound eyes of a number of bees taken from the same hive. He then shut them up in a box with several other bees from the same hive which he left untouched. The box was only a few paces distant from the hive from which the bees were taken. He then opened the lid of the box, and those which had not been blinded instantly flew out of it, and entered their habitation. Those whose eyes had been varnished appeared not to care to leave the box at all, and seemed very unwilling to make any attempt to fly; some of them, indeed, flew about from one side to another, but did not go far. Réaumur then threw several of them up into the air, and they immediately began to soar higher and higher, until at length they went out of sight altogether! Réaumur compares the poor insect's manœuvres to those of a crow, whose head and eyes mischievous boys have covered with a paper bonnet: the bird flies upward until its strength is exhausted, when it drops again to the earth. Not only did those bees which he threw up into the air thus soar until they were lost to view, but all the most active of those which were left in the box did so likewise, and Réaumur saw them no more. Not one could find its way to the hive. From this experiment, and from the preceding one, it is evident that both the compound and the simple eyes are necessary to enable the insect to see perfectly; for when either was varnished over, the bees could not find their way to their home again. Réaumur imagines that the cause of the wheeling flight of bees, sometimes observed, now in this direction, now in the opposite, may arise from their eyes becoming perhaps in part obscured by the pollen, or yellow powder of the flowers into which they plunge, the bee thereby becoming partly blind-folded. These experiments are so interesting that they deserve repeating, and it might be tried whether the result would be different if only one eye were blinded. Other variations of the experiments will also suggest themselves.
Although not organs of sufficient size to give their colouring the requisite distinctness and amount of surface which would render it very conspicuous in our estimation, the eyes of insects are often exquisitely beautiful, and vie almost with precious stones in lustre. Their most common colour is black, or brown; but the eyes of many flies glow with fiery colours, some banded with green and purple, some variously figured black and red. Some again glitter like burnished gold shaded with the softest green; and some blaze with a play of colours, like the diamond set in jet. The eye of the dragon-fly, in particular, is a lustrous crystalline object of extreme beauty.