The eyes are of two kinds, the compound, which are always present in mature insects, and the ocelli or simple eyes, which may or may not be present. When present there are usually three, which if we join by lines, we will describe a triangle, in the vertices of whose angles are the ocelli. Rarely there are but two ocelli, and very rarely but one. The simple eyes (Fig. 4, _f f f_) are circular, and possess a cornea, lens and retina, which receives the nerve of sight. From the experiments of Réaumur and Swammerdam, which consisted in covering the eyes with varnish, they concluded that vision with these simple eyes is very indistinct, though by them the insect can distinguish light. Some have thought that these simple eyes were for vision at slight distances. Larvæ, like spiders and myriapods, have only simple eyes. The compound eyes (Fig. 2, _e_) are simply a cluster of simple eyes, are situated one on either side of the head, and vary much in form and size. Between or below these are inserted the antennæ. Sometimes these last are inserted in a notch of the eyes, and in a few cases actually divide each eye into two eyes. The eyes may meet above as in drones (Fig. 4), most two-wing flies and dragon-flies, or they may be considerably separated, as in the worker-bees (Fig. 5). The separate facets or simple eyes, of each compound eye, are hexagonal, or six-sided, and in the microscope look not unlike a section of honey-comb. The number of these is prodigious--Leeuwenhoek actually counted 12,000 in the eye of a dragon-fly--while some butterflies have, over 17,000. The compound eyes are motionless, but from their size and sub-spherical shape, they give quite a range of vision. It is not likely that they are capable of adjustment to accord with different distances, and it has been supposed, from the direct darting flight of bees to their hives, and the awkward work they make in finding a hive when moved only for a short distance, that their eyes are best suited to long vision. Sir John Lubbock has proved, by some interesting experiments with strips of colored paper, that bees can distinguish colors. Honey was placed on a blue strip, beside several others of various colors. In the absence of the bees he changed the position of this strip, and upon their return the bees went to the blue strip rather than to the old position. Our practical apiarists have long been aware of this fact, and have conformed their practice to the knowledge, in giving a variety of colors to their hives. Apiarists have frequently noted that bees have a rare faculty of marking positions, but, for slight distances, their sense of color will correct mistakes which would occur if position alone was guide. APPENDAGES OF THE THORAX.