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.