Compound eyes are found in most adult insects and hemimetabolous insect larvae. They are constructed from many light-sensing units called ommatidia, each containing photoreceptor cells that form images. Ocelli are simple light-detecting organs that detect movement, with most insects possessing 3 dorsal ocelli on the head. Both compound eyes and ocelli transmit light signals to the brain via photoreceptor cells, interneurons, and neural connections, allowing insects to process visual information.

1. Compound eyes: constructed from many similar
units called ommatidia.
Present in most adult pterygote insects and in the larvae of
hemimetabolous insects.
strongly reduced or absent in wingless parasitic groups,
such as the Phthiraptera and Siphonaptera,
and in female coccids (Hemiptera)
In (Isoptera),- termites compound eyes are greatly reduced or absent in subterranean,
and present in winged reproductive. : the sensory components of the eyes degenerate
during the permanently subterranean reproductive life.
Among Apterygota, compound eyes are lacking in some Thysanura, but Lepismatidae
have 12 ommatidia on each side.
Collembola have up to eight widely spaced ommatidia,
while Protura and Diplura have no compound eyes.

2. Each compound eye may be composed of
several
thousand ommatidia.
• 30 000 in of dragonflies,
• 10 000 in drone honey bees,
• 5500in worker honey bees
• 800 in Drosophila.
• worker ant Ponera punctatissima have only a single ommatidium on each
side of the head.
• Usually the eyes are separate on the two sides of the head, but in some
insects, such as Anisoptera (Odonata) and male Tabanidae and Syrphidae
(Diptera), the eyes are contiguous along the dorsal midline, this being
known as the holoptic condition.

3. Ommatidial structure
• Each ommatidium consists of an optical, light-gathering part and a sensory part, which transforms
light into electrical energy.
• The sensory receptor cells of most diurnal insects end close to the lens, and, because of the
method of image formation, these are called apposition eyes.
• Most night-flying insects, however, have eyes with a clear zone between the lenses and the
sensory components; they are called superposition eyes, and produce brighter images than
apposition eyes.
Apposition eye Superposition eye

4. (b) Ommatidium. (c) Surface view of part of an eye showing the outer surfaces of some corneal lenses
(facets).

5. • The cuticle covering the eye is transparent and colorless and usually
forms a biconvex corneal lens.
• In surface view, the lenses are usually closely packed together,
forming an array of hexagonal facets.
• Each corneal lens is produced by two epidermal cells, the corneagen
cells, which later become withdrawn to the sides of the ommatidium
and form the primary pigment cells.
• Beneath the cornea are four cells, the Semper cells, which, in many
insects, produce a second lens, the crystalline cone. This is usually a
hard, clear, intracellular structure bordered laterally by the primary
pigment cells.

6. sensory elements
• The sensory elements are elongate photoreceptor neurons.
• Generally there are eight receptor cells in each ommatidium
• ( but some species have seven, and others nine).
• Each receptor cell extends basally as an axon, which passes out through the basal lamina
backing the eye and into the lamina of the optic lobe.
• The margin of each receptor cell nearest the ommatidial axis is differentiated into close-packed
microvilli extending toward the central axis of the ommatidium at right angles to the long axis
of the photoreceptor cell.
• The microvilli of each receptor lie parallel with each other and are often aligned with those of
the receptor cell opposite, but are set at an angle to those of adjacent receptor cells
• The microvilli of each receptor cell collectively form a rhabdomere.
• The visual pigment (rhodopsin) is located within the microvillar membrane.
• In many insects such as bees and flies most receptor cells have a twist along their lengths.
• Thus, the orientation of the microvilli of each rhabdomere changes regularly through the
depth of the eye.

7. Compound eyes are typically classified as either apposition eyes, which form multiple inverted images, or superposition eyes,
which form a single erect image.

9. Open and fused rhabdom
• In most insects the rhabdomeres on each other along the axis of the
ommatidium, form a “fused” rhabdom-( the cells are not actually fused).
• but Diptera, Dermaptera, some Heteroptera (Hemiptera) and some
Coleoptera have widely separated rhabdomeres forming an “open”
rhabdom
• Because a fused rhabdom acts as a light guide, all the photoreceptor cells
within one ommatidium have the same field of view.
• In species with open rhabdoms, each receptor cell within an ommatidium
has a separate visual field, shared by individual cells in each of the
adjacent ommatidia.

10. The rhabdom
• The rhabdom of apposition eyes usually extends the full length of
the photoreceptor cells between the crystalline cone and the basal
lamina.
• In the ant Camponotus it is 150 mm long
• In Drosophila, with an open rhabdom, each rhabdomere is 80 mm
long.
• It is usually shorter in superposition eyes, and even in apposition
eyes one of the rhabdomeres may be very short.
• There is much variation in the way that the clear zone in superposition
eyes is bridged.

11. • In many Lepidoptera and Coleoptera, the receptor cells extend
to the crystalline cone as a broad column, but the rhabdom is
restricted to the basal region.
• In other Lepidoptera (the Bombycoidea and Hesperioidea), the
receptor cells of each ommatidium form a thin strand, which
may be only 5 mm across, to the lens.
• Beetles with exocone eyes have a similar structure, but it is formed by the Semper
cells and the receptor cells are restricted to a basal position in the ommatidium.
• The sensory parts of each ommatidium are usually surrounded by 12–18
secondary pigment cells so that
• each ommatidium is isolated from its neighbors.

14. Ocelli
• Ocelli (singular Ocellus) are simple photo-receptors
(light detecting organs).
• They consist of a single lens and several sensory cells.
Unlike compound eyes, ocelli do not form a complex
image of the environment but are used to detect
movement.
• Most arthropods possess ocelli. Some species of
arthropod do not possess compound eyes and only
have ocelli.
•

15. Dorsal ocelli
• Dorsal ocelli are found in adult insects and the larvae of
hemimetabolous insects.
• Typically there are three ocelli forming an inverted triangle
antero-dorsally on the head, although in Diptera and
Hymenoptera they occupy a more dorsal position on the vertex.
• The median ocellus shows evidence of a paired origin, as the
root of the ocellar nerve is double and the ocellus itself is
bilobed in Odonata and Bombus (Hymenoptera).
• Frequently, one or all of the ocelli are lost and they are often
absent in wingless forms.

16. Ocelli in insects
• Adult insects typically have three single-lens eyes, called
ocelli.
• Larval holometabolous insects have one or more single-
lens eyes, known as stemmata, on the sides of the
head.
• Some insects also possess epidermal light receptors
• in some cases, light is known to have a direct effect on
cells in the brain.
• Magnetic sensitivity aids orientation in at least some
insects, and has known interactions with light sensitivity.

17. Ocellus structure
• A typical ocellus has a single thickened cuticular lens.
• in some species, such as Schistocerca (Orthoptera) and Lucilia (Diptera), the cuticle
is transparent, but not thickened, and the space beneath it is occupied by transparent
cells.
• Each ocellus contains a large number of photoreceptor cells packed closely together
without any regular arrangement; in the locust ocellus there are 800–1000.
• A rhabdomere is formed on at least one side of each receptor cell,
• the rhabdomeres of (two to seven cells) combine to form rhabdoms.
• The rhabdomeres usually occupy much of the cell boundary
• in the case of Rhodnius (Hemiptera) rhabdomeres are present all around the cells,
form a hexagonal meshwork
• similar in the stemmata of Cicindela (Coleoptera).
• The structure of the rhabdomeres in the dorsal ocelli is the same as that in the
compound eye.
• Pigment cells sometimes invest the whole ocellus,
• but in some species, e.g., cockroaches, they are lacking. A reflecting tapetum,

18. • Each photoreceptor cell gives rise, proximally, to an axon
which passes through the basal lamina of the ocellus and
terminates in a synaptic plexus immediately behind the
ocellus.
• Two anatomical classes of ocellar interneurons originate
here.
• Some have giant axons up to 20 mm in diameter, often
called large (L) fibers, others are of small diameter (S
fibers). About ten large fibers and up to 80 small ones are
associated with each ocellus.
•

20. Neural connections
• In most insects studied the large interneurons end in the brain, but in bees
and flies some extend to the thoracic ganglia.
• Where these descending interneurons are absent, the pathway to the
thoracic motor centers is completed by second-order descending
interneurons.
• The small interneurons connect with several other centers in the brain,
including the optic lobes, mushroom bodies and the central body. The
receptor cell axons synapse repeatedly and reciprocally with each other
and with the interneurons, which also synapse with each other.
• Some of the synapses between interneurons and receptor cells are input
synapses to the receptor cells,
• indicating that the interneurons may modulate the activity of the receptor
cells as well as receiving information from them.

21. Neurotransmission
• Illumination produces a sustained depolarization of the photoreceptor cell
which is proportional to light intensity.
• No action potentials are produced in the receptor cells, and graded
receptor potentials are transmitted along the axons to the synapses.
• As at the first synapses behind the compound eye, the signal is amplified
and the sign is reversed.
• ( as in the compound eye, the input signals arising from contrasts in
illumination are of similar amplitude even though the background level of
illumination is different).
• The giant interneurons transmit information to the brain either
electrotonically or by spiking.

22. Image formation
• Because the image produced by the lens is not in focus on the retina in
some species, the function of ocelli was long thought to be only light
detection.
• However, it is now known that in dragonflies and nocturnal wasps, the
median ocelli are focused, and,
• at least in dragonflies, adapted for accurate detection of the horizon.
• Here, the median ocellus can even detect the direction of moving
gratings and is involved in early-stage motion processing and subserving
pitch control.
• Even in species where the lens is under focused there is still potential for
form vision.
• In locust the ocelli are involved in detecting roll, their sensitivity
to rapid changes in light intensity being well suited for the

25. Stemmata
• Stemmata are the only visual organs of larval holometabolous insects.
• (They are sometimes called lateral ocelli, but this term is better avoided as it leads
to confusion with the dorsal ocelli from which they are functionally and often
structurally distinct).
• Extraocular photoreceptor organs in the optic lobes- stemmata.
• In larval Cyclorrhapha (Diptera) they are represented only by internal receptors.
• Some stemmata are simple visual organs, while others are complex camera-type
eyes.

26. Stemmata types
• Stemmata are of two types,
• those with a single rhabdom, and those with many rhabdoms.
• Single rhabdom
• occur in Mecoptera, most Neuroptera, Lepidoptera and Trichoptera.
• Diptera,
• in some species several stemmata are fused together to form a
compound structure with a branching rhabdom.
• In Coleoptera :
• the stemmata of many species have a single rhabdom, but some
species, such as those of larval Adephaga (Coleoptera) have multiple
rhabdoms.
• Stemmata with multiple rhabdoms also occur in larval Symphyta
(Hymenoptera).

27. • In caterpillars each stemma has a cuticular lens
beneath which is a crystalline lens.
• Each lens system has seven photoreceptor cells
associated with it.
• Commonly, three form a distal rhabdom and four form a
proximal rhabdom.
• A thin cellular envelope lies around the outside of the
sense
• cells and is, in turn, shrouded by the extremely enlarged
corneagen cells.
• All the distal cells contain a visual pigment with maximal
absorbance in the green part of the spectrum, while
some proximal cells contain a blue- or ultraviolet-

28. rhabdomeres within the stemmata:
perception
• The rhabdomeres within the stemmata of caterpillars have different visual
fields,
• ( The acceptance angles of the distal rhabdomeres are close to 10 so
they have low spatial resolution.
• The proximal rhabdomeres have much smaller acceptance angles of less
than 2.)
• This, together with the fact that the focal plane of the lens is at the level
of the proximal cells, gives them better spatial resolution.
• The visual fields of adjacent stemmata do not overlap
• so the caterpillar perceives an object as a very coarse mosaic, which is
improved by side-to-side movements of the head, enabling it to
examine a larger field.
• It is known that caterpillars can differentiate shapes and orient

29. single stemma
• The situation is different in larval symphytans (Hymenoptera), which have
only a single stemma.
• These have large numbers of rhabdoms, each formed by eight
photoreceptor cells and each group of cells is isolated by pigment from its
neighbors.
• The lens produces an image on the tips of the rhabdoms
• In Perga, are oriented at about 5 from each other (equivalent to the inter
ommatidial angle incompound eyes).
• Consequently, this type of eye is capable of moderately good form
perception.

30. • 1. The larvae of the tiger beetle, Cicindela, have six Stemmata
• (like caterpillars, but with a large number of photoreceptor cells in each stemma, as in the
Hymenoptera).
• The largest of the stemmata has about 5000 receptor cells
• - each of which forms a rhabdomere on all sides so that the rhabdoms are in the form of a
lattice.
• It is possible that spatial resolution in these eyes is limited because of optical pooling and
perhaps electrical coupling.
• 2. In the larvae of the visual-oriented, predatory sunburst diving beetles Thermonectus
marmoratus, several of the 12 stemmata have multiple retinae so that, together with two
lensless eye-patches, this species has 28 retinae.
• Four of the stemmata are long and tubular, with horizontally extended but vertically very
narrow retinae. When the larva approaches a potential prey the whole head and body
move up and down in the sagittal plane so that the four horizonal retinae scan vertically
across the target before a strike is made. The remaining stemmata probably act as
movement detectors, allowing the animal to orient the tubular stemmata toward potential
prey.

31. • The optic lobes of larval insects consist of a lamina and medulla
comparable with those associated with compound eyes of adults and, at
least in
• caterpillars, each stemma connects with its individual cartridge in the
lamina. In all these types of stemmata,
• the photoreceptor cells contain screening pigment granules
• in addition to the visual pigment. Movement of the granules – away from
the rhabdomeres in the dark and
• toward them in the light – provides sensitivity adjustment. Caterpillars have
three visual pigments and
• the neural capacity to distinguish colors.
• The larvae of several holometabolous species have been
shown,experimentally, to respond to the plane of polarization of incident
light. In neither case is the behavioral importance of these abilities
understood.

32. Other visual receptors: Dermal light sense
• A number of insects, such as Tenebrio larvae,
• respond to light when all the known visual receptors are occluded.
• The epidermal cells are apparently sensitive to light.
• This is also suggested by the pigment movements which occur in isolated
epidermal cells of some insects, i.e.,
• Several families of butterflies are known to have photoreceptors on the
genitalia of both sexes. In Papilio there are two receptors on each side,
each consisting of a single neuron lying on a nerve a short distance below
the epidermis. The cuticle above the neuron is transparent. The cells are
called phaosomes
• and they probably monitor the positions of the genitalia during copulation.

33. Sensitivity of the brain
• In several insect species, light affects neural activity directly by acting on
the brain, not via the compound eyes or ocelli.
• This commonly occurs in the entrainment of diurnal rhythms.
• In some species day length – regulating diapause – is registered directly
by the brain.
• Typically, such rhythmicity is mediated by cerebral cell clusters containing
rhabdomeric structures and sometimes screening pigments, sometimes
called adult stemmata.
• One such structure, the Hofbauer-Buchner eyelet in Drosophila, has been
identified as an essential circadian photoreceptor.
• Similar structures have been found in several species of beetles, bees
and hawk moths, where they contain two classes of opsin, sensitive to
ultraviolet and blue.
• Other extraretinal cells in the optic lobes express green-sensitive opsin.
• Together, these various spectral inputs potentially facilitate the detection of
changes in ambient light to control various photoperiodic rhythms

34. Magnetic sensitivity and photoreception
• A number of insect species have been shown to respond to changes in magnetic field,
• and it is possible that they use the Earth’s magnetic field in navigation.
• Two mechanisms have been proposed to account for this response.
• Some insects are known to contain particles of magnetite, an iron oxide, which might be affected by the magnetic
field.
• In worker honey bees- the magnetite is contained in innervated trophocytes in the abdomen.
• 2.
• In many species, however, geomagnetic orientation has been demonstrated to depend on the spectral composition
of ambient light, suggesting a role of photoreceptor proteins.
• The blue-light photoreceptor cryptochrome protein (CRY), involved in setting the circadian clock in insects, is required
for magnetoreception
• in Drosophila:
• This protein is expressed in various parts of the body,
• but also in the eyes, which, at least in birds, seem to mediate sensitivity to the Earth’s magnetic field by
generating a neural map of this field.
• In insects, CRY is also expressed in the eyes, but the precise location and physiology of magnetoreception,
as well as its complex interaction with light sensitivity, remains to be determined.