The Structure and Special Physiology of Insects 3 i 



primitive living insects, Campodea and others, have eyes, although only 



simple ones. The larvae of the specialized 

 insects, i.e., those with complete metamor- 

 phosis, also have only simple eyes. The com- 

 pound eyes , are not complex or specialized 

 derivations of the simple ones, but are of in- 

 dependent origin and of obviously distinct 

 structural character. The simple eyes, also 

 called ocelli (Fig. 58), which usually occur to 

 the number of three in a little triangle on 

 top of the head, are small and inconspicuous, 

 FIG. 60. Part of corneal cuti- and consist each of a lens, this being simply 

 cle, showing facets, of the a sma n convexly thickened clear part of the 

 compound eye of a horse- .... , . , , , , , ,, /T ,. 

 fly, Therioplectes s p . (Photo- chitimzed cuticle of the head-wall (Fig. 59) 



micrograph by George O. an d a group of modified skin-cells behind it 

 Mitchell; greatly magnified.) . ,, ., , .,, u u 



specially provided with absorbent pigment and 



capable of acting as a simple light-sensitive or retinal 

 surface. The ocellus is supplied with a special nerve 

 from the brain. The compound eyes are always 

 paired and situated usually on the dorso-lateral parts 

 of the head; they are usually large and conspicu- 

 ous, sometimes, as in the dragon-flies and horse- 

 flies, even forming two-thirds or more of the mass 

 of the head. Externally each compound eye pre- 

 sents a number (which varies all the way from a 

 score to thirty thousand) of facets or microscopic 

 polygonal cuticular windows (Fig. 60). These are 

 the cornea of the eye. Behind each facet is a dis- 

 tinct and independent subcylindrical eye-element or 

 ommatidium composed of a crystalline cone (want- 

 ing in many insects) enveloping pigment (which pre- 

 sumably excludes all light-rays except those which 

 fall perpendicularly or nearly so to the corneal 

 lens of that particular ommatidium), and a slender /IJlp--otl 



tapering part including or composed of the nervous FIG. 6 1. Longitudinal 

 or retinal element called rhabdom (Fig. 61). Each section through a few 

 of these ommatidia perceives that bit of the external {oSmaSdiT^rThe 

 Object which is directly in front of it; i.e., from which compound eye of a 

 light is reflected perpendicularly to its corneal facet. th - ' ^^[f^ 

 All of these microscopic images, each of a small part cones; />.', pigment; r., 



of the external object, form a mosaic of the whole retina l parts; O.K., optic 

 , . . . ... nerve. (After Exner; 



object, and thus give the familiar name mosaic greatly magnified.) 



