THE INTERNAL ANATOMY OF INSECTS 135 



Each ommatidium of a compound eye has been considered as a 

 separate eye because its nerve-endings constituting the retinula are 

 isolated from the retinulae of other ommatidia by surrounding acces- 

 sory pigment cells ; but a similar isolation of retinulae 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 retina, 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 crystalline-cone-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 whtre they 

 are present in this group they are of a highly developed type and not 

 rudimentary; the compound eyes of Machilis, for example, are as 

 perfect as those of winged insects. 



The absence of compound eyes in larvae. The absence of com- 

 pound eyes in larvae 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 larvae, the develop- 

 ment 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 larvae, 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 larvae 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. 



