LIBRARY

THE ANATOMY OF INSECTS 31
tive during t.he day are said to be diurnal; those active in the evening and twilight,
crepuscular; and those active at night, nocturnal. Most insects have two
kinds of eyes: compound eyes and ocelli. Compound eyes -large and conspicuous
eyes composed of a few or a large number of facets, ommatidia, or sensilli£
which are tube-like and arranged in a series pointing towards a common inner
center where the light falls upon the retina and is transformed into stimuli
transmitted to t.he brain. A stationary object appearing in an eye-field that
consists of hundreds or thousands of facets is thought to create a mosaic image.
while a moving object passing across such a series of separate lenses is detected
by setting up stimuli in each series of facets as it advances. The eyes are
sometimes provided with a lateral tubercle (in most aphids) or may be divided
transversely (in whirligig beetles) and rarely partly stalked (in a few mayflies
(Cloeon]) and stalked (in a few DIPTERA). The facets may also vary in
size in different paris of the same eye; and the whole may be greatly reduced in
size and in numbers of facets in many insects. Compound eyes function to determine
motion and react. to color patterns and to light stimuli.
Ocelli (sing. ocellus) - simple, single lenses that are thought to be chiefly
"stimulatory organs which accelerate phototaxis by increasing the sensitivity
of the brain to light-stimuli received through the compound eyes" (Eltringham).
They are thought t.o respond to light and may faintly discern objects in
motion near at hand. Those of the adults are often classified as primary,
dorsal, or primitive ocelli when three in number and are arranged in a triangle
on the vertex and frons. The median ocellus is sometimes double in dragonflies
and ants, indicating a primitive condition of two pairs. It occurs in nymphs and
adult PTERYGOTA. Adaptive or lateral ocelii, frequently referred to as ocellance
(sing. ocellana) and stemmata (sing. stemma), occur on the lateral sides of
the head of larVal. They vary in number from one in TENTHREDINIDlE
to a maximum of six or seven on a side in NEUROPTERA, COLEOPTERA,
and LEPIDOPTERA. They are particularly well developed in the tiger beetles
and enable these active larVal to capture living prey.
Organs of smell- exceedingly variable. The best developed ones appear to
be the sensoria on the antennre of aphids and other HOMOPTERA and the
sensilla on the antennre of most insects. These consist of innervated oval or
circular pits covered by a fiat or convex membrane. In aphids they are most
numerous on the antennre of winged individuals and are especially abundant in
males, whether alate or apterous. A few occur on the wingless parthenogenetic
females. On the hind tibire of the sexual female aphids are sensoria-like organs
which may function as organs of smell. The sensillre of the higher orders may
occur in very great numbers on the antennre of certain beetles, moths, bees, and
other insects. Organs of smell also occur on the palpi which are furnished with
sensilla. Sensilla with setre or cones may arise from the antennre, palpi, or the
body cuticle.
Organs of taste - occur in connection with the mouth parts, pharynx, antennre,
tarsi, and other parts of the body and are structures similar to those of
smell.