3. Compound Eyes[1464].—These are the most common kind of eye in hexapod insects, when arrived at their perfect state; in their larva state, as we have seen, their eyes being usually simple[1465]; except, indeed, those whose metamorphosis is semicomplete, which have compound eyes in every state.—In considering compound eyes, I shall advert to their structure, number, situation, figure, clothing, colour, and size.
As to their structure,—when seen under the microscope they appear to consist usually of an infinite number of convex hexagonal pieces. If you examine with a good glass the eye of any fly, you will find it traversed by numberless parallel lines, with others equally numerous cutting them at right angles, so as apparently to form myriads of little squares, with each a lens of the above figure set in it. The same structure, though often not so easily seen, obtains in the eyes of Coleoptera and other insects. When the eye is separated and made clean, these hexagons are as clear as crystal. Reaumur fitted one eye to a lens, and could see through it well, but objects were greatly multiplied[1466]. In Coleopterous insects they are of a hard and horny substance; but in Diptera, &c. more soft and membranous. The number of lenses in an eye varies in different insects. Hooke computed those in the eye of a horse-fly to amount to nearly 7,000[1467]; Leeuwenhoeck found more than 12,000 in that of a dragon-fly[1468]; and 17,325 have been counted in that of a butterfly[1469]. But of all insects they seem to be most numerous in the beetles of Mr. W. S. MacLeay's genus Dynastes. In the eyes of these the lenses are so small as not to be easily discoverable even under a pocket microscope, except the eye has turned white[1470]: it is not, therefore, wonderful, that Fabricius should call these eyes simple[1471]. In some insects, however, as in the Strepsiptera Kirby, the lenses are not numerous: in Xenos they do not exceed fifty, and are distinctly visible to the naked eye[1472]. These lenses vary in magnitude, not only in different, but sometimes in the same eyes. This is the case in those of male horse-flies and flies, those of the upper part of the eye being much larger than those of the lower[1473]. The partitions that separate the lenses, or rather bezels, in which they are set, are very visible in the eyes just mentioned, and those of Xenos; but in many insects they are only discernible at the intersecting lines of separation between the lenses. In hairy eyes, such as those of the hive-bee, the hairs emerge from these septa. Every single lens of a compound eye may be considered as a cornea, or a crystalline humour, it being convex without and concave within, but thicker in the middle than at the margin: it is the only transparent part to be found in these most remarkable eyes. Immediately under the cornea is an opaque varnish, varying according to the species, which produces sometimes in one and the same eye spots or bands of different colours. These spots and bands form a distinguishing ornament of many of the Tabani and other flies. And to this varnish the lace-winged flies (Hemerobius, &c.) are indebted for the beautiful metallic hues that often adorn them. When insects are dead, this varnish frequently loses its colour, and the eye turns white: hence many species are described as having white eyes which when alive had black ones. The consistence of this covering is the same with that of the varnish of the choroid in the eyes of vertebrate animals; but it entirely covers the underside of the lens, without leaving any passage for the light. Below this varnish there are numbers of short white hexagonal prisms[1474], every one of which enters the concavity of one of the lenses of the cornea, and is only separated from it by the varnish just described: this may be considered as the retina of the lens to which it is attached; but at present it has not been clearly explained how the light can act upon a retina of this description through an opaque varnish. Below this multitude of threads (for such the bodies appear), perpendicular to the cornea, is a membrane which serves them all for a base, and which consequently is nearly parallel with that part. It is very thin, of a black colour, not produced by a varnish; and in it may be seen very fine white tracheæ, which send forth branches still finer, that penetrate between the prisms of the cornea: this membrane may be called the choroid. Behind this is a thin expansion of the optic nerve, which is a true nervous membrane, precisely similar to the retina of red-blooded animals. It appears that the white pyramidal threads which form the retina of each lens are sent forth by this general retina, and pierce the choroid by a number of almost imperceptible holes[1475]. From this description it appears that the eyes of insects have nothing corresponding with the uvea or humours of those of vertebrate animals, but are of a type peculiar to themselves.
Having explained to you the wonderful and complex structure with which it has pleased the Creator to distinguish the organs of vision of these minute beings, proving, what I have so often asserted, that when animals seem approaching to nonentity, where one would expect them to be most simple, we find them in many cases most complex, I shall now call your attention to the next thing I am to consider—the number of the eyes in question. Most insects have only two; but there are several exceptions to this rule. Those that have occasion to see both above and below the head, the eyes of all being immovable, must have them so placed as to enable them to do this. This end is accomplished in many beetles, for instance Scarabæus L., Helæus Latr., &c., by having these organs fixed in the side of the head, so that part looks upward and part downward; but in others four are given for this purpose. If you examine the common whirlwig (Gyrinus Natator) that I have so often mentioned[1476], which has occasion, at the same time, to observe objects in the air and in the water, you will find it is gifted with this number of eyes. Lamia Tornator (Cerambyx tetrophthalmus Forst.) and some others, of which I make a genus, under the appellation of Tetrops, are also so distinguished. In these insects, one eye is above and the other below the base of the antennæ; in fact, in these the canthus, instead of dividing the eye partially, as in the other Capricorn-beetles, runs quite through it at considerable width[1477]. In Ryssonotus MacLeay (Lucanus nebulosus K.) the eye appears also to be divided in two by the canthus. In the Neuroptera Order there is more than one instance of the same kind. In Ascalaphus there are two considerable eyes on each side of the head, which, though clearly distinct, meet like those of many male flies and the drone. The male, likewise, of more than one species of Ephemera, besides the common lateral eyes and the stemmata on the back of the head, have a pair of compound eyes on the top of a short columnar process[1478]. In the Hemiptera Order, also, an instance occurs of four eyes in the genus Aleyrodes[1479]. Amongst the vertebrate animals, there is an example of eyes with two pupils in Anableps, a genus of fishes[1480], but no vertebrate animal has four of these organs. That many insects should have more than two eyes, will not seem to you so extraordinary as that any should be found that, like the Cyclops of old, have only one. There is, however, an insect, before celebrated for its agility[1481] (Machilis polypoda Latr.), which has a single eye in its forehead; or we may say, its eyes are confluent, without any line of distinction between them except a small notch behind. Now that I am treating of the number of eyes, I must not forget to observe to you, that in some insects no eyes at all have been discovered. In Polydesmus complanatus, on each side of the head there is an eye-shaped portion separated by a suture, in which under a powerful lens I cannot satisfy myself that I can discern any thing like the facets that usually distinguish compound eyes. In Geophilus electricus, another myriapod, they certainly do not exist[1482]. Whence we may conclude, as was before observed[1483], that the faculty of emitting light is rather given it as a means of defence than to guide it in its path.
The situation of compound eyes differs in different tribes. In some, as in the Staphylinidæ, they are planted laterally in the anterior part of the head; in others, the Carabi &c., in the middle; in others again, Locusta Leach &c., in the posterior part. In some, their station is more in the upper surface, either before or behind; so that a very narrow space separates them, or perhaps none at all. Instances of this position of the eyes occur in a minute weevil (Ramphus Clairv.[1484]), and many Diptera, &c. Of those that form an union on the top of the head, some are placed obliquely, so as to leave a diverging space below them, as in many Libellulina[1485], the drone[1486], &c. Others, as Atractocerus, in which the eyes occupy nearly the whole head, and unite anteriorly, have this diverging space above their conflux. In Rhina barbirostris Latr., another kind of weevil, they are confluent below the head, at the base of the rostrum, and a very narrow interval separates them above. In a large number of the Heteromerous beetles, they are set transversely, in the Capricorn ones longitudinally. Their surface, when they are lateral, has usually two aspects, one prone to see below, the other supine to see above. In general the eyes are situated behind the antennæ, so that their position, whether it shall be anterior or posterior, depends upon that of those organs. Often, indeed, as in the last-named beetles, part of the eye is behind and part before the antennæ; but except where there are four eyes, as in Tetrops, they are never placed before or below them.
Though the eyes of insects are generally sessile, yet to give them a wider range they are sometimes, but it rarely occurs, placed, like those of many Crustacea, on a footstalk, but not a moveable one. An instance of this in certain male Ephemeræ has already been mentioned. In the Hemiptera De Geer has figured two species of bugs (Cimicidæ) that are so circumstanced[1487]; as are also all the known Strepsiptera K., though in these the footstalk is very short[1488]: but the most remarkable example of columnar eyes is afforded by that curious Dipterous genus Diopsis, in which both eyes and antennæ stand upon a pair of branches, vastly longer than the head, which diverge at a very obtuse angle from its posterior part[1489].
In their figure eyes vary much. Sometimes they are so prominent as to be nearly spherical: this is the case with some aquatic bugs, as Ranatra, Hydrometra, and several male Ephemeræ[1490]. Very often they are hemispherical, as in the tiger-beetles (Cicindela L.), and the clocks or dors (Carabus L.); but in a large number of insects they are flat, and do not rise above the surface of the head.—With regard to their outline, they are often perfectly round, as in many weevils; oval, as in various bees; ovate, as in other bees (Andrena F.); triangular, as in the water-boatman (Notonecta). They are also often oblong, and occasionally narrow and linear; as in that singular beetle Helæus. In many of the Muscidæ they form nearly a semicircle, or rather, perhaps, the quadrant of a sphere. The eyes of the Capricorn-beetles (Cerambyx L.) have a sinus on their inner side, as it were, taken out of them; so that they more than half surround the antennæ, before which is the longest portion of them. An approach to this shape is more or less observed in the darkling-beetles (Tenebrio L.); but in these the sinus is not so deep. I may under this head observe, that in those Mantidæ that represent dry leaves, and some others, these organs usually terminate in a spine[1491].
Though not distinguished by the beauty and animation that give such interest to the eye of vertebrate animals, and exhibiting no trace of iris or pupil, yet from the variety of their colours the compound eyes of insects, though most commonly black or brown, are often very striking. Look at those of one of the lace-winged flies that commit such havoc amongst the Aphides[1492], and it will dazzle you with the splendour of the purest gold, sometimes softened with a lovely green. The lenses of those of Xenos blaze like diamonds set in jet[1493]. You have often noticed the fiery eyes of many horse-flies (Tabanus L.) with vivid bands of purple and green[1494]. Others are spotted[1495]; and Schellenberg has figured one (Thereva hemiptera)[1496], that exhibits the figure of a flower painted in red on a black ground. These colours and markings are all most vivid and brilliant in the living insect, and often impart that fire and animation to the eyes for which those of the higher animals are remarkable. Take one of the large dragon-flies that you see hawking about the hedges in search of prey, examine its eyes under a lens, and you will be astonished at the brilliance and crystalline transparency which its large eyes exhibit, and by the remarkable vision of larger hexagons which appear in motion under the cornea, being reflected by the retina—all which give it the appearance of a living eye. This moving reflexion of the hexagonal lenses in living insects was noticed long since in some bees (Nomada F., Cœlioxys Latr.)[1497]
Compound eyes differ greatly in their size. In some insects, as Atractocerus, the drone-bee, many male Muscidæ, &c., they occupy nearly the whole of the head; while in others, as numerous Staphylinidæ, Locusta Leach, &c., they are so small as to be scarcely larger than some simple eyes of spiders: and they exhibit every intermediate difference of magnitude in different tribes, genera, and species.
Under this head I must say something of the Canthus of the eye; by which I mean an elevated process of the cheek, which in almost all the genera of the Lamellicorn beetles enters the eye more or less, dividing the upper portion from the lower. Though usually only a process of the cheek, yet in the Scarabæidæ the whole of that part forms the canthus[1498]. It only enters the eye in the Rutelidæ, Cetonidæ, &c.; it extends through half of it in Copris; it goes beyond the half in Ateuchus; and in Ryssonotus MacLeay (Lucanus nebulosus K.) it quite divides the eye into two[1499], as I before observed. In Lucanus, Passalus &c. it projects before the eye into an angle; in Lucanus femoralis nearly into a spine; but in Lamprima and Œsalus it does not exist. The part, also, that enters the eye in the Capricorn-beetles may be regarded as a kind of canthus, though it is merely a dilatation of the front.
4. Stemmata[1500].—Having given so full an account of the kinds and structure of the ordinary eyes of insects, you may perhaps expect that I should now dismiss the subject: you would, however, have great cause to blame me, did I not make you acquainted with a kind of auxiliary eyes with which a large portion of them are gifted; I mean those pellucid spots often to be found on the posterior part of the front of these animals, or upon the vertex, frequently arranged in a triangle. These, Linné, from his regarding them as a kind of coronet, called Stemmata. They have been of late denominated Ocelli; but as this latter term is also in general use for the eyelets on the wings of Lepidoptera, I have adhered to that of the illustrious Swede. Neither he nor Fabricius has expressed any opinion as to the use of these organs; but Swammerdam and Reaumur were aware that they were real eyes. The former found that there are nerves that diverge to them though not easily traced, and that they have a cornea, and what he takes for the uvea[1501]; and the latter has supposed that the compound eyes and these simple ones have, the one the power of magnifying objects much, and the other but little, so that the former are for surveying those that are distant, and the latter those that are near[1502]. The same author relates some experiments that he tried with the common hive bee, by which he ascertained that the stemmata, as well as the compound eyes, were organs of vision. He first smeared the latter over with paint, and the animals, instead of making for their hive, rose in the air till he lost sight of them. He next did the same with the former, and placing the bees whose stemmata he had painted within a few paces of their hive, they flew about on all sides among the neighbouring plants, but never far: he did not observe that these ever rose in the air like the others[1503]. From this experiment it seems as if the compound eyes were for horizontal sight, and the stemmata for vertical.