Receptor Organs of Cockroach

Cockroach bears several types of receptor organs likes other insects to perceive different types stimuli like touch, smell, taste, sound, change in temperature and light. From these all, except the receptors for light—the photoreceptors, i.e., eyes, are found situated in the epidermis of the integument and in fact they are modified epidermal cells called sensillae.

Sensillae:

A sensilla is the fundamental structure of a receptor. It has a modified sensory cell innervated by a nerve fibre, a trichogen cell for the secretion of spines or bristles and some tormogen or hair membrane cells. The receptors of touch, taste and smell have such isolated and simple sensillae, but those of hearing and sight have aggregations of sensillae which form elaborate organs.

The receptors for touch, i.e., thigmoreceptors are located on the antennae and bristles of the legs, body, maxillary palps and cerci. The receptors for smell, i.e., olfactoreceptors are found chiefly on antennae. The receptors for taste, i.e., gustatoreceptors are found on the palps of maxillae and labium.

The receptors for change in environmental temperature, i.e., thermo receptors are found mainly on the pads between the first four tarsal segments on the legs. The receptors for hearing, i.e., auditory or chordotonal receptors are found on the anal cerci which respond to air or earth borne vibrations.

Photoreceptor Organs of Periplaneta:

The photoreceptor organs of insects are simple eyes or ocelli and compound eyes.

But in cockroach, usually compound eyes are found. The compound eyes of cockroach are a pair of large, sessile, black, kidney-shaped structures situated at the dorsolateral sides of the head capsule. It is a complicated structure and covered externally by the cuticle which is transparent. The transparent cuticle covering the compound eyes is divided into a large number of hexagonal compartments (2000 in cockroach) called corneal facets.

These facets possess ectodermal structures beneath them arranged vertically and radially in the compound eyes. One facet and all the elements beneath it constitute an ommatidium which is in fact the visual unit. Thus, a compound eye consists of a large number of visual units or ommatidia and 2000 such visual units are found in a compound eye of cockroach.

Structure of an Ommatidium:

An ommatidium has a bio-convex lens or cornea which is formed by the cuticle becoming thickened and transparent, these form the facets. Below the lens the epidermis forms two clear corneagen cells or lenticular cells which secrete the lens. Below the corneagen cells is a transparent crystalline cone which functions as a second lens, it is surrounded by four vitrellae or cone cells.

The vitrellae secrete the crystalline cone, they taper downwards. All this forms the focusing or dioptrical region. Below the cone and in contact within is a spindle-shaped refractive body, the rhabdome surrounded by seven photoreceptor retinular cells or retinulae which are elongated cells.

The retinular cells secrete the rhabdome which is made up of seven rhabdomeres, one for each retinular cell. The rhabdome and retinulae constitute the receptor region, and below it is a basement membrane of the eye. Each retinular cell joins a nerve fibre at its base, and the fibres enter the optic nerve.

Surrounding each ommatidium and separating it from its neighbours in many insects there are heavily pigmented cells arranged in two groups, an iris pigment sheath around the cone, and a retinal pigment sheath around the rhabdome and retinular cells. The retinal pigment sheath is absent in some insects.

Working of Compound Eye:

Insects have two kinds of ommatidia. In nocturnal insects and many crustaceans the pigment is confined only around the cone cells, their retinulae and rhabdome do not touch the cone. Such eyes are called superposition eyes in which the ommatidia are not separated from each other by pigment.

Their retinulae and rhabdome can be excited by light coming through their own as well as through neighbouring lenses, or light enters an ommatidium through several lenses. Each ommatidium forms a complete image of the entire field of vision, and all the images together form a continuous though partly overlapping superposition image. Superposition images are formed in dim light and are not sharp.

In diurnal insects the ommatidia are separated from each other by two pigment sheaths, their retinulae and rhabdome abut against the cone. Such eyes are known as apposition eyes in which only those rays of light can form an. image which are parallel to the longitudinal axis of an ommatidium and pass directly through the centre of the lens.

Each ommatidium forms a different but adjacent image, and all the ommatidia of a compound eye produce apposition images or mosaic vision composed of as many separate but adjacent images as there are ommatidia. But when the light is dim, the two pigment sheaths retract or move away in diurnal insects (the iris pigment sheath goes up and the retinal pigment sheath moves down).

Then ommatidia are not isolated and light can pass through one ommatidia to another. Each ommatidium forms an image, and the images of all the ommatidia overlap and together form a superposition image. In cockroach, however, the pigment sheaths are not retractable.

But studies made recently do not support the theory of mosaic vision because of the following reasons:

1. Pigment sheaths of many insects are not retractable.

2. The field of vision of each ommatidium is much wider than is supposed, and images formed by adjacent ommatidia overlap each other, so that apposition eyes can also form superposition images contrary to previous ideas, hence, the concept of an ommatidium as a functional unit is wrong.

3. In each ommatidium, not one, but a succession of images are formed by focusing at deeper levels. The power of a compound eye depends on these deeper images produced by groups of ommatidia. The images are not very distinct but they enable the eyes to detect the movements of objects at once. The insect eye can distinguish different colours, though it cannot see all the colours of a spectrum, it can perceive ultraviolet rays.