Simple eye

THE ORGANS OF PHOTORECEPTION
A. THE GENERAL FEATURES
The two types of eyes of insects.—insects possess two types of eyes, the ocelli or simple eyes and the compound or facetted eyes. Typically both types of eyes are present in the same insect, but either may be absent. Thus many adult insects lack ocelli, while the larva of insects with a complete metamorphosis lack compound eyes. When all are present there are two compound eyes and, typically two pairs of ocelli; but almost invariably the members of one pair of ocelli are united and form a single median ocellus The median ocellus is absent in many insects that possess the other two ocelli. The distinction between ocelli and compound eyes.—The most obvious distinction between ocelli and compound eyes is the fact that in an ocellus there is a single cornea while in a compound eye there are many. Each ommatidium of a compound eye has been considered as a separate eye because its nerve-endings constituting the retinula are isolated from the retinube of other ommatidia by surrounding accessory pigment cells; but a similar isolation of retinui exist in some ocelli. It has also been held that in compound eyes there is a layer of cells between the corneal hypodermis and the retinas. the crystalline-cone- cells, which is absent in ocelli; but in the ocelli of adult Ephemerida there is a layer of cells between the lens and the retina, which, at least, is in a position analogous to that of the crystallinecone-cells; the two may have had a different origin, but regarding this, we have, as yet, no conclusive data.

The absence of compound eyes in most of the Apterygota.—
Typically insects possess both ocelli and compound eyes; when either kind of eyes is wanting it is evidently due to a; loss of these organs and
not to a generalized condition. Although compound eyes are almost universally absent in the Apterygota-in the few cases where they are present in this group they are of a highly developed type and not rudimentary; the compound eyes of Machills, for example, are as perfect as those of winged insects.

The absence of compound eyes in 1arve.—
The absence. of compound eyes in larva is evidently a secondary adaptation to their particular mode of life, like the internal development of wings in the same forms. In the case of the compound eyes of larva, the development of the organs is retarded, taking place in the pupal stage instead of in an embryonic stage, as is the case with nymphs and naiads. While, the development of the compound eyes as a whole is retarded in larva, a few ommatidia may be developed and function as ocelli during larval life.

B. THE OCELLI: There are two classes of ocelli found in insects: first, the ocelli of adult insects and of nymphs and naiads, which may be termed the primary ocelli; and second, the ocelli of most larva possessing ocelli, which may be termed adaptive ocelli.

The primary ocelli.—The ocelli of adult insects and of nymphs and naiads having been originally developed as ocelli are termed the primary ocelli. Of these there are typically two pairs; but usually when they are present there are only three of them, and in many cases only a single pair. When there are three ocelli, the double nature of the median ocellus is shown by the fact that the root of the nerve is double, while that of each of the other two is single. In certain generalized insects, as some Plecoptera, (Fig. 150) all of the ocelli are situated in the front; but in most insects, the paired ocelli have either migrated into the suture between the front and the vertex (Fig. xi), or have proceeded farther and are situated in the vertex.