NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS ']\ 



The compound eyes are well developed in nearly all adult insects ; 

 their absence is to be regarded as due to a degeneration of the organs 

 and not as representing a primitive eyeless condition. In the Col- 

 lembola, and in Lepisma of the Thysanura, the compound eye does 

 not have the typical form. In the Collembola there are from i 

 to 12 small eyes on each side of the head in some species, and in 

 ■Lepisma a group of 12 similar eyes on each side. These eyes have 

 an external resemblance to the scattered lateral ocelli of caterpil- 

 lars and some other larvae of the higher insects, and in internal^ struc- 

 ture they are in some ways also similar to these eyes ; but they have 

 other characters that are distinctive of compound eyes. Their struc- 

 ture in general is that of a primitive compound eye with aberrant 

 features, and they are, therefore, probably to be regarded as de- 

 generate ommatidia of compound eyes. This conclusion is strength- 

 ened by the fact that in Machilis, another member of the Thysanura, 

 the lateral eyes are distinctly of the typical compound eye type (fig. 

 32 A), and retain the primitive and crustacean character of having 

 the corneagenous cells beneath the lens. 



In the compound eye of most other insects the corneagenous cells, 

 as already noted (fig. 32 B, CorCl), withdraw from beneath the 

 cornea in the mature stage and take positions at the sides of the cone, 

 where they acquire a deposit of pigment, and become the cells known 

 as the primary pigment cells, or corneal pigment cells. 



The four cone cells (fig. 32 C, Cn), when they form a typical cone, 

 fuse completely ; their protoplasm becomes converted into a clear hya- 

 line substance, and their nuclei remain in the outer or basal part of 

 the cone (A, B). Eyes with a cone of this type are designated 

 cucone eyes. In some insects, however, especially in the Diptera, 

 the cone cells secrete a transparent substance which forms a conical 

 mass beneath the cornea held between the surrounding pigment 

 cells, but the cone cells remain distinct with their nuclei beneath 

 the vitreous mass. Eyes with an imperfect cone of this kind are 

 distinguished as pscudocone eyes. In some of the Diptera, again, the 

 eyes are of an acone type, the cone cells remaining distinct without 

 forming a vitreous body of any sort. 



The retinulre cells (fig. yz A, B, Ret) extend from the cone, some- 

 times embracing its apex, to the basement membrane. In eyes of 

 simpler construction they form one layer of cells, but their arrange- 

 ment is subject to much variation in the various modifications of 

 the retinular structure in dififerent insects. The nuclei of one or more 

 of the cells may migrate toward the base of the ommatidium, and 

 often the cells themselves become arranged in two layers, some- 



