The Structure and Special Physiology of Insects 3 1 
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, 
and consist each of a lens, this being simply 
a small convexly thickened clear part of the 
chitinized cuticle of the head-wall (Fig. 59) 
and a group of modified skin-cells behind it 
specially provided with absorbent pigment and 
Fig. 60. —Part of corneal cuti¬ 
cle, showing facets, of the 
compound eye of a horse¬ 
fly, Therioplectes sp. (Photo¬ 
micrograph by George O. 
Mitchell; greatly magnified.) 
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 section through a few 
e . • , • . . /" , 1 , facets and eye-elements 
of these ommatidia perceives that bit of 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. ™° th * C °cr y e s f a mne 
All of these microscopic images, each of a small part cones; p., pigment; r., 
of the external object, form a mosaic of the whole retina l parts; o.n., optic 
J nerve. (After Exner; 
object, and thus give the familiar name mosaic greatly magnified.) 
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