478 SPECIAL PHYSIOLOGY. 



must be supposed to be reduced to a mere appreciation of light and color, 

 without any distinction of form whatever. 



Amongst Insects, two kinds of eyes are found, the simple and compound. 

 Usually both occur together, but some insects have only simple, and others 

 only compound, eyes. The simple eyes, called stemmata, ocelli, or eye-spots, 

 resemble, in a general manner, the organs of vision in the higher animals, 

 and present the following structures, viz., a minute transparent cornea, close 

 behind which, is a globular lens, resting upon a cup-shaped vitreous humor ; 

 on the posterior convex surface of the latter, a nervous filament spreads out, 

 forming a retina ; and surrounding the whole, is a choroid coat, which, pass- 

 ing on to the anterior surface of the lens, there forms a sort of pupillary 

 aperture ; the color of the choroid coat, and therefore of the ocelli, varies, but 

 is more commonly dark. The compound or facetted eyes present quite a differ- 

 ent structure. In the common cockchafer, for example, the optic nerve, given 

 off by the supra-oesophageal ganglion, swells into a large segment of a sphere ; 

 from this part, an immense number of very short branches are given off, which 

 spread out, behind a layer of pigment called the common choroid, into a mem- 

 branous expansion named the common retina ; from this retinal expansion, a 

 multitude of filaments, corresponding in number to the facets of the cornea, 

 are given off, which, after traversing the common choroid, spread out upon a 

 number of hexahedral transparent prisms, covered in front with minute double 

 convex lenses, the so-called corneal facets (Straus-Durckheim). The compound 

 eye of the Libellula, or dragon-fly, consists of an immense multitude of very 

 minute pyramidal tubes, the blunted apices of which are set closely together, 

 on the bulbous extremity of the optic nerve ; the base of each tube is turned 

 towards the globular surface of the eye, and is invested by a hard facet or cor- 

 neola, which forms a meniscus or concavo-convex lens ; behind this, is a small 

 cavity filled with an aqueous humor. Each tube is invested by a dark-colored 

 layer of pigment, surrounding a clear fluid, which occupies the axis of the 

 tube, and is supposed to correspond with the vitreous humor ; in front, be- 

 tween the cornea and the vitreous humor, this pigmentary membrane presents 

 a small pupillary aperture. The compound eye consists, therefore, of a com- 

 bination of numerous minute independent eyes. The number of single eyes 

 entering into the formation of the compound eyes, is, in some insects, very 

 great ; thus, in the ant there are said to be 50, in the common house-fly, 8000, 

 in the dragon-fly more than 12,500, and in the Mordella beetle upwards of 

 25,000. The compound eyes are usually sessile, and form rounded eminences 

 on the sides of the head, but sometimes they are supported on a pedicle or 

 footstalk. They frequently present a brilliant color, such as a bright green, or 

 green and purple, or even a pure gold color. It has been supposed that the 

 simple ocelli of insects are adapted only for very near vision. The mode in 

 which a visual image is formed by the remarkable compound or aggregate eye, 

 is peculiar. Each minute eye must form an image of a corresponding portion 

 of the visual field, for, owing to its tubular shape, and to the fact that it forms 

 a radius proceeding from a convex surface, it seems probable that only those 

 rays, which fall nearly vertically upon the minute corneal facet, can pass down 

 it, whilst the lateral rays are more or less perfectly excluded. The multitude 

 of separate images, like portions of mosaic, must be combined by the animal, 

 into a single picture, for we cannot conceive that it sees objects multiplied. 

 This act of combination must be a sensorial operation, accomplished in the 

 cephalic nervous apparatus of the insect. The images, separate or combined, 

 formed in such an eye, are not inverted from above downwards, or reversed 

 from right to left, as in the Vertebrate eye, but occupy their normal position ; 

 there is no decussation of the optic nerves, and none of the great motor ner- 

 vous columns. 



The Orthoptera, Hemiptera, Keuroptera, Hymenoptera, and various other 

 insects, have, in the perfect state, from two to three simple eyes, and also com- 

 pound eyes ; in such cases, the simple eyes are usually placed on the top of the 

 head. In those insects in which simple eyes only exist, these are usually situ- 

 ated on the side of the head. When numerous simple eyes are grouped together 

 into one mass, they form the conglomerate or aggregate eye. A few insects are 

 destitute of eyes, such as the neuter Termites, and the Claviger beetle. In the 



