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 ^ gmall convexly thickened clear part of the 

 compound eye of a horse- ..... . , , ^, i_ j n /t^- \ 



fiy, Therioplectes sp. (Photo- chitmized cuticle of the head-waU (Fig. 59) 

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

 Mitchell; greatly magnified.) gp^^j^Hy 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 

 tapering part including or composed of the nervous Fig. 61.— Longitudinal 



or retinal element called rhabdom (Fig. 61). Each f ^tion through a few 



, , ... . , 1 • r I 1 facets and eye-elements 



of these ommatidia perceives that bit 01 the external (ommatidia) of the 



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



light is reflected perpendicularly to its corneal facet. ™°? ' /^'_ "^""vsullfne 



All of these microscopic images, each of a small part cones; p., pigment; r., 



of the external object, form a mosaic of the whole retinalparts;«.K optic 

 ' nerve, (.\fter Exner; 



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



