Sense Organs in Insects

Background

By Sachin Chorge

Article submitted on January 2008
Article accepted on February 2008

 

Senses are the physiological methods of perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology (or cognitive science), and philosophy of perception.Behaviour is determined, Or at least influenced at every step, by the stimuli to which the sense organs are subjected; and the function of the sense organs and their powers of discrimination are discovered largely by observations on behaviour. It is not possible therefore to separate these two objects completely.  

Definition of "sense"

There is no firm agreement among neurologists as to exactly how many senses there are, because of differing definitions of a sense. In general, one can say that a "sense" is a faculty by which outside stimuli are perceived. School children are routinely taught that there are five senses (sight, hearing, touch, smell, taste; a classification first devised by Aristotle).

Category

Function

Examples

Mechanoreceptors

Detect movements, vibrations, or other mechanical disturbances Tactile receptors,
Proprioceptors,
Sound receptors.

Chemoreceptor

Detect the presence of chemical substances in the air (smell) or on substrates (taste) Taste buds on palps, Antennal, sensilla.

Photoreceptors

Detect the presence and quality of incident light (electromagnetic radiation) Compound eyes, Ocelli
 PHOTORECEPTORS:

Compound eye

Simple Eye

Dermal light sense

Ommatidia

Stemmata, Ocelli

Dermal Receptors


Compound Eyes

A pair of compound eyes is the principle visual organs of most insects; they are found in nearly all adults and in many immatures of ametabolous and hemimetabolous orders.   As the name suggests, compound eyes are composed of many similar, closely-packed facets (called ommatidia) which are the structural and functional units of vision.   The number of ommatidia varies considerably from species to species:   some worker ants have fewer than six while some dragonflies may have more than 25,000.

Externally, each ommatidium is marked by a convex thickening of transparent cuticle, the corneal lens.   Beneath the lens, there is often a crystalline cone secreted by a pair of semper cells.   Together, the lens and the crystalline cone form a dioptric apparatus that refracts incoming light down into a receptor region containing visual pigment. The light-sensitive part of an ommatidium is called the rhabdom.   It is a rod-like structure, secreted by an array of 6-8 specialized neurons ( retinula cells), and centered on the optical axis just below the crystalline cone.   The rhabdom contains an array of closely packed microtubules where light-sensitive pigments (e.g. rhodopsin, etc.) are stored.   These pigments absorb certain wavelengths of incident light and generate nerve impulses through a photochemical process similar to that of vertebrates. The whole ommatidium is surrounded by a curtain of pigmented cell; the “primary iris cells” covering the crystalline cone, and the “secondary iris cells” investing both the primary iris cells and the retinula. The proximal extremities of the ommatidium rest on a fenestrated basement membrane through which the fibres and tracheae run; the nerve fibres pass to the perioptic or outermost tract of the optic lobe of the brain.Most diurnal insects have pigment cells surrounding each ommatidium.   These cells limit a facet's field of view by absorbing light that enters through adjacent corneas.   Each facet points toward a slightly different part of the visual field -- in composite, they render a mosaic-like impression of the environment.   Nocturnal and crepuscular insects have pigment cells that do not completely isolate each facet.   Their ommatidia are stimulated by light from larger fields of view.   This produces a brighter but theoretically less distinct mosaic image.

Retina

The rod like rhabdom is of highly reffractile material; light entering it will therefore be totally reflected from the walls, and thus it forms an ideal structure to receive and conduct light withought loss, even in an eye like that of Simulium in which the rhabdom are curved. This property will also reduce the effect of light reaching the rhabdom obliquely. In the region of the basement membrane there are, in addition, tracheal branches the rhabdom, and the nervous elements are doubly stimulated. The tracheal tapetum is particularly well developed in Noctuids and other nocturnal insects; in these the tracheae divide up at the base of the retina into innumerable fine branches running a parallel course between the retinulae. Although the rhabdom is described as though it were composed of homogenous material it is in fact a complex structure. It is formed by the juxtaposition of the ‘rhabdomeres’ of the retinula cells. The rhobdomere represents the microvilli, or ‘honey-comb border’, which cover the free surface of the retinula cell. The most wide spread photoreceptor substance in the animal kingdom is rhodopsin, formed by the union of ‘retinene’ with protein. Retinene is carotinoid pigment, the aldehyde of vitamin A alcohol. Retinene, partly bound to protein, has been isolated from the head of Apis and other insects: Musca, Locusta, Uropetala(Odonata), Cicindela and Nyctemera(Lepidoptera).

Optical mechanism of the compound eye:

As was originally suggested by Johannes Muller(1829) in his so called ‘mosaic theory’, each ommatidium receives the impression of a luminous area corresponding to its projection on the visual field; and it is the Juxtaposition of all these little luminous areas, varying in the intensity and quality of the light composing them, which gives rise to total erect image perceived by the insect. Since insects cannot form a true (i.e. focused) image of the environment, their visual acuity is relatively poor compared to that of vertebrates.   On the other hand, their ability to sense movement, by tracking objects from ommatidium to ommatidium, is superior to most other animals.  Temporal resolution of flicker is as high as 200 images/second in some bees and flies (in humans, still images blur into constant motion at about 30 images/second).   They can detect polarization patterns in sunlight, and discriminate wavelengths in a range from ultraviolet to yellow (but not red). 

The Dioptric apparatus:

The cornea and crystalline cone have rather special optic properties. They have a laminated structure turned inwards and can be pictured as a system made up of series cones with their apices turned inwards superimposed on one another. Examination with microrefrctometer shows that in this system the refractive index is at a maximum at the axis and decreased progressively towards the periphery. From optical stand point the system can be represented by a series of superimposed cylinders whose refringence increases towards the axis, a system called a “lens-cylider”.

Formation of images by Apposition:

In most diurnal insects, Hymenoptera, Diptera, Odonata, many Coleoptera , the cones are surrounded by pigment up to their posterior extremities; they allow the light emerge only at the central point; and the retinulae are short and placed immediately behind the cones. A reversed image of small part of the visual field is formed where the retinula comes in contact with the apex of the cone and has been observed in various insects. But this image has no physiological significance, it merely impresses the retina as a simple luminious point, the apposition of all such points as perceived by the different ommatidia forming the erect image perceived by the compound eye as a whole.

Formation of images by Superposition:

In many nocturnal insects, Lampyridae and other beetles, Noctuidae and other Lepidoptera, the ommatidia are greatly elongated. The retinulae do not lie immediately behind and in contact with the cones but are separated from them by a long interval in contact with the cones, but are separated from them by a long interval occupied by a non-reffractile transparent medium; and the pigment is the iris cells may be concentrated in front between the crystalline cones. The optical system is same as the “lens-cylinder” twice as long as its focal length. Hence the given rhabdom receives rays of light not only through its own facet but also through neighbouring facet. The formation of image in eye is due to a group of bundles of light, each coming through an adjacent facet. So that as focus is lowered the image splits up into number of luminous points. As many as 30 neighbouring ommatidia may unite to concentrate the light in this way upon a single rhabdom. The Image projected will be erect, but there will be no physiological significance. The rhabdom receives only a visual stimulus which is presumably the mean of the components of this image. Since each of these elemental images is formed by the superposition of the light from number of adjacent facets, the compound image as received by the entire retina is termed a “Superposition image”.

Diffraction Images:

In the excised eye of locusta, besides the first (apposition) image, second and third images are formed by the diffraction can be seen in the deeper layers of the retina. These images are of the superposition type; they are formed by the diffraction of light received from the group of adjacent facets. It seems that the rays that contribute to them are not excluded by the investigating pigment of the apposition eye. Indeed there is evidence that Calliphora the wave-lengthof the red end of the spectrum can increase sensitivity. It is exceedingly sensitive to very small movements in the surroundings.

Other type of compound eye:

There are doubtless many intermediate stages between these two types of image formation. In butterflies, for example, which form retinal images of the apposition type, the image formed at the apex of each cone is erect; so that in some eyes of this type the lens and cone may evidently act together as a lens-cylinder of twice its own focal length. This mainly applies to “eucone” and “exocone” eyes with well developed cone. But in certain insects the cones are purely cellular and non refringed e.g. acone eyes of Tipulids, Forficula, Hemiptera, various Coleoptera).Or the cones may be represented by the mass of liquid secreted by the crystalline cells.

Pigment movement in compound eye:

The iris cells contain black pigment which absorbs light, and pale or coloured granules which reflects the light. On account this reflection by the coloured granules which the structure is sometimes termed the ‘iris tapetum’ unlike a true tapetum, however, its function is not to increase the illumination of retina, but to prevent the entry of oblique rays. In butterflies, it shows a central dark spot surrounded by six smaller spots jointed to the first by radial dark lines, and sometimes twelve still smaller peripheral spots in eye. This appearance is termed the ‘pseudopupil’. The central black spot results from the light being absorbed where it falls on the rhabdom or on the black pigment in the retina, while it is reflected from the iris tapetum. In many butterflies, in the middle of the central black spot there is a small luminous spot, due to the light which falls on the rhabdom not being absorbed but reflected from the tracheal tapetum behind. In certain woodland species of butterflies this central spot disappears when the eye is brightly illuminated. This change is due to the movement of pigment in the cells around the basement membrane. It takes place very rapidly and may be complete within 6 seconds. Perhaps it serves to light illumination of the eye approximately constant with rapidly changing light intensities. Pigment movement may also occur also in the apposition eyes of Hemiptera (Notonecta and Corixa).

Migration of pigments are much more striking and important in the superposition eyes if nocturnal forms. Most of be active also in the day-time, and their eyes poises an arrangement comparable with the mammalian iris by which they can be adapted to different light intensities.


Simple eyes

Ocelli :

Two types of "simple eyes" can be found in the class Insecta:   dorsal Ocelli and lateral Ocelli (=stemmata).   Although both types of Ocelli are similar in structure, they are believed to have separate phylogenetic and embryological origins.

Dorsal Ocelli are commonly found in adults and in the immature stages (nymphs) of many hemimetabolous species.   They are not independent visual organs and never occur in species that lack compound eyes.   Whenever present, dorsal Ocelli appear as two or three small, convex swellings on the dorsal or facial regions of the head.   They differ from compound eyes in having only a single corneal lens covering an array of several dozen rhabdom-like sensory rods.   These simple eyes do not form an image or perceive objects in the environment, but they are sensitive to a wide range of wavelengths, react to the polarization of light, and respond quickly to changes in light intensity.   No exact function has been clearly established, but many physiologists believe they act as an "iris mechanism" -- adjusting the sensitivity of the compound eyes to different levels of light intensity.

Stemmata :  

The eyes of larval and pupal forms are termed as “Stemmata” or “Lateral Ocelli”. These are very variable in structure. Lateral ocelli (stemmata) are the sole visual organs of holometabolous larvae and certain adults (e.g. Collembola, Thysanura, Siphonaptera, and Strepsiptera). In larvae of Lepidoptera, Trichoptera, Sialis, Myrmeleon  they form a group, each member of  which has a structure something like single ommatidium of a compound eye consisting of a cornea and crystalline lens and 7 retinal cells. The eyes of Collembola are of sane type; there are about eight on either side, each with the structure of an ommatidium of eucone type. On the other hand, in larvae of Tenthredinidae, many Coleoptera, each eye, of which there is usually only one on either side, consist of a single transparent lens-like thickening of the cuticle (the cornea) with underlying epidermis/ and below this a number of retinulae each composed of two or there innervated visual cells grouped round a rhabdom. The eyes are of this type in Pediculus. The visual cells may themselves contain pigment; or there may be pigment cells of various distributions. In some larvae the eyes are rudimentary; for example, in Ceratopogon they consist only of a pair visual cell and two overlying pigment cells.Stemmata always occur laterally on the head, and vary in number from one to six on each side.   Structurally, they are similar to dorsal ocelli but often have a crystalline cone under the cornea and fewer sensory rods.   Larvae use these simple eyes to sense light intensity, detect outlines of nearby objects, and even track the movements of predators or prey.   Covering several Ocelli on each side of the head seems to impair form vision, so the brain must be able to construct a coarse mosaic of nearby objects from the visual fields of adjacent Ocelli.

From these considerations of structure it has been concluded that the Ocelli are adapted to the immediate perception of small changes in light intensity. But although they must be stimulated by light the insect often shows no outward response to such stimulation. Ants with their Ocelli alone uncovered behave as though blind; bees and Drosophila after blackening the compound eyes no longer show any reactions to light. In cicada Cryptotympana, on the other hand, the Ocelli as well as the compound eyes play an obvious part in light perception, and asymmetrical covering of the Ocelli leads to circus flight. In some insects, although the Ocelli by themselves are incapable of evoking reflex movements in response to flight. Drosophila and Apis the insect with Ocelli are uncovered responds more rapidly to change in the light intensity.


Extra-ocular Photoreception

Some (perhaps most) insects respond to changes in light intensity even when all known photoreceptive structures are rendered inoperative.   This dermal light sense has been attributed to the response of individual neurons in the brain and/or ventral nerve cord.   There is also convincing evidence that some insects can perceive infrared radiation (heat), although specific receptors for this ability have not been described.

 Blind cavernicolous beetles of the genus Anophthalmus respond to the light of a candle. Blinded Blattella and Perplaneta still settle for preference in the dark after complete blinding. Caterpillars will go towards a source in the dark after complete blinding. A dermal light sense is present in all the tergites of the larvae of Acilius and Dytiscus but is particularly well developed in the region of the spiracles at the end of the abdomen. The antennae of apis fabae appear to be the site of a dermal light sense which is source of kinetic stimulation. In all these cases it is important to be sure that it is the light itself which is the effective stimulus, and not the heat into which it degenerates after absorption.

Summary

Vision is the perception of light. Insects perceive light through three classes of sensory organs. Most adult insects have a single pair of compound eyes. Larvae of hemimetabolous insects and most adults have usually three simple eyes called ocelli. These are typically located on the dorsum of the head capsule, and they are sometimes called dorsal ocelli. Larvae of holometabolous insects do not have compound eyes. These insects perceive forms, to a limited extent through stemmata located on either side of the head. It is not possible to separate Behaviour and sight (observations) completely


http://www.biology-online.org/articles/sense-organs-insects.html