THE INTERNAL ANATOMY OF INSECTS 135 



Each ommatidiiim 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 retinulas 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 cf 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 n^nnphs and naiads. 



While the development of the compound eyes as a whole is retarded 

 in larvag, a few ommatidia may be developed and function as ocelli 

 during larval life. 



h. 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 orig'.nally 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. 



