PhotorecepHon 391 



through a single ommatidium, the image is often referred to as a mosaic 

 image. In the superposition eye hght which is focused on a single rhabdom 

 (Fig. 106, B) may enter through a number of ommatidia, so that in this 

 case the image is not quite so mosaic. However, in a vertebrate eye, the image 

 that is projected along the optic nerve consists of a record of intensities reg- 

 istered at individual cones or at small groups of rods and/or cones, and thus 

 this image also may be considered as a mosaic of much finer grain. If each 

 retinual cell of an ommatidium is assumed to register different intensities 

 of light and therefore to project a small portion of an image, then the term 

 mosaic takes on a somewhat different meaning, and in the final pathway to 

 the brain this pattern is essentially the same as that in a vertebrate optic 

 nerve. 



Ocelli among the Arthropods. A simple type of arthropod eye, found in 

 the larvae of Lepidoptera, Trichoptera, Collembola, and other insects, 

 consists of a structure comparable to a single ommatidium of a compound 

 eye. These simple eyes are known as stemmata, or lateral ocelli, and may 

 occur singly or be scattered on the sides of the animal in small groups. 

 In the larvae of Tenthredinidae and many Coleoptera there is only one 

 ocellus on each side, but it contains many retinulae, each with several 

 sense cells and a rhabdom, perhaps with pigment cells, all beneath a single 

 lens. This more complicated type of lateral ocellus is capable of distinguish- 

 ing form (Fig. 107) and perhaps color. In the larvae of Cicindela there are 

 six stemmata on each side: two large, two small, and two vestigial. The large 

 ones have 6350 visual cells and are apparently capable of detecting form 

 and, when all of the stemmata are functioning together, of determining 

 distance.'*'* 



Dorsal ocelli are the simple eyes of insects which usually occur in the 

 frontal region of the head between the compound eyes. In general the struc 

 ture is similar to that of the complex stemmata described above. The dorsal 

 ocelli of agrionids and libellids have a tapetum and, when the animal is 

 brought from darkness to bright light, there is a rapid movement of brown 

 pigment across the white fundus within a few seconds. The ocelli have a 

 high light-gathering power, with apertures estimated at f* 1.8 for Eristalis 

 and 1.5 for ¥orm,ica females, whereas the facets of appositional compound 

 eyes have apertures of f 2.5 to 4.5 and therefore a much lower light-gather- 

 ing power. In sphingids and noctuids the superpositional compound eyes 

 have a high light-gathering power, and ocelli are absent. The ocelli are poorly 

 adapted for image formation for at least two reasons: the angular separation 

 of the rhabdoms is great (3—10 degrees), and the image is usually focused 

 far behind the retina (e.g., in Eristalis the retina is 0.11 mm. behind the 

 lens and the focal plane is 0.29 mm.).''*' 



Most spiders have eight ocelli distributed over the dorsal portion of the 

 cephalothorax. Two of these are known as principal or direct eyes, and the 

 others as lateral or indirect eyes. In the direct eyes the sensory portion of 

 each sense cell is in front of the cell body, and in the indirect ocelli the sen- 

 sory portion is behind the cell body. The direct eyes are movable by means of 

 radially distributed muscles and permit the animal to fixate its p/ey with- 

 out shifting its body.^^ 



* Focal length divided by the diameter. 



