In  cockroach , various structures like antennae , labium , maxillary palps, labialpalps , compound eyes , simple eyes etc., are sensory in nature .

These structures contain various receptors like chemoreceptors, mechanoreceptors, photoreceptors , thermoreceptors ,etc.

These receptors contain units like ommatidia , sensilla  and scolopodia.

OMMATIDIA:These are the units of photoreceptors like compound eyes.

SENSILLAE:These are the units of cuticular receptors and chemoreceptors.

Each sensillum  consists of a

1.trichogen cell with a movable hair like process ,

2.a tormogen cell that forms the socket around the base of the movable hair and

3.a sensory neuron which starts at the base of the hair and conducts impulses to the central nervous system.

SCOLOPODIA:- These are the units of mechanoreceptors of chordo – tonal organs.

Scolopodium is present below the epidermis. It contains an  1.attachment -cell that attaches the scolopodium to the epidermis ,

 2.scolopale -cell that forms the cilium and a nerve cell.


 1.Mechanoreceptors are cuticular and subcuticular.

Cuticular mechanoreceptors are tactile receptors – sensitive to touch and proprioceptors – precise position of the body parts. These are with units, sensillae.

Chordo-tonal organs contain   subcuticular mechanoreceptors with scolopodia. These are sensitive to ground vibrations .

Tympanal organs , which are sensitive to sound vibrations . These are present in anal cerci.

Jhonston’s organs , which are sensitive to the movement of flagellum with respect to the pedicels and flagella.

Sub-genual organs ,which are sensitive to movement of flagella of antennae.

2. Olfactory sensilla are a kind of  chemoreceptors which are sensitive to smell. These are distributed over the antennae, maxillary paips and labial palps.

3.Gustatory sensilla are also a kind of chemoreceptors , which are sensitive to taste. They are located on theinner surface of the labrum and on the maxillary and labial palps.

4.Thermo receptorsarea kind of  machano receptors and are located on the first, second and third segment of the tarsus of legs.


  A pair of dark kidney shaped compound eyes is present on the dorsolateral sides of the head, one on each side. Each compound eye is composed of about 2000 functional units called ommatidia, which are optically independent. The outer surface of the compound eye is divided into about 2000 hexagonal areas called facets. Each facet is the outermost portion of a corresponding ommatidium.compound-eye-big


                        Each typical ommatidium is  elongated and pyramidal in shape consisting  the following ,

1.Cornea ,

2.Corneagen cells ,

3.Vitrillae ,

4.Crystalline cone ,

5.Retinulae and

6. Rhabdome.


Cornea is the outer most part of  an ommatidium .It corresponds to a hexagonal facet of the compound eye. It is a biconvex  transparent part of the cuticle. It allows light rays to pass through it.Cornea is the refractive region of ommatidium. It is secreted by  specialised cells of epidermis.

2. Caneagen cells or Lenticular cells :-compound eye 1

These are two transparent specialised epidermal cells. They secrete cornea . These cells later become withdrawn to the sides of the ommatidium and form the primary pigment cells.

3. Vitrillae or cone cells ( Semper cells ) :-

These are the four transparent conical cells. They lie below  the corneagen cells. They surround the transparent crystalline cone. Cryastalline cone is secreted by the cone cells.

4.Crystalline cone :-

Crystalline cone is the transparent conical structure that secreted by the vitrellae and is surrounded by them.

Light absorbing dark primary pigment cells called Iris pigmented  sheath  surround the vitrellae.

The region containing  the cornea and crystalline cone constitute the dioptrical  or focussing region of the ommatidium.

Crystalline cone focuses the light on to the next part of the ommatidium.

5. Retinulae and Rhabdome :-ommatidium5

1.Retinulae are innermost and elongated cells of an ommatidium.

2. Retinulae are three in number.These are the photoreceptor cells of ommatidium.

3. They rest on the basement membrane with their inner ends of the long axis.

4. Each retinular cell bears microvilli towards the inner surface of the cell. Microvilli of each retinular  cell collectively form a rhabdomere that contains photoreceptor  pigment.

5. These rhabdomeres  fuse along the axis of the ommatidium to form the rhabdome in the centre.

6. Retinulae are the nerve cells from which sensory nerve fibres leave as optic nerve to reach the protocerebrum.

7.Rhabdome and retinulae  form the retinal or receptor region.

8.Receptor region is surrounded by light absorbing seven secondary pigment cells calle retinular pigmented cells .They serve to isolate each ommatidium.

FUNCTIONING OF OMMATIDIUMsuper -apposition image

The position of retinulae below the vitrillae orcone cells is different in diurnal andnocturnal insects. The manner in which the images of objects formed in both the types ofinsects. The two typesof images formed in ommatidia are

  1. Apposition image,
  2. Superposition image.


  1. Apposition images are formed in diurnal insects like houseflies.
  2. In these insects retinulae  lie immediately below the vitrillae and crystalline cone.
  3. Retinulae are surrounded by secondary pigment sheath , which absorb light rays and prevent them from passing to the adjoining ommatidium.
  4. Vitrillae are surrounded by primary pigment sheath.
  5. Therefore, only light rays entering the cornea of ommatidium converge on the rhabdome. Thus, a small separate image of a part of the object is formed in each ommatidium.
  6. The total image formed in the compound eye is a mosaic of several small images. Such an image is called apposition image because it is formed by the juxtaposition of small discrete images formed in each of the ommatidia.
  7. This type of vision is called mosaic vision.


  1. Super position images are formed in nocturnal insects like cockroach.
  2. In which the retinulae are present deep below the vitrillae and crystalline cone.
  3. Retinal sheath is absent.
  4. Therefore , the rhabdome and retinulae of an ommatidium receive not only light rays that enter through its own cornea but also light rays that enter through the corneas of the adjoining ommatidia.
  5. This results in the overlapping of images. The image formed by overlapping of images is called superposition image. It is a blurred image.

About balarangaiahnarapuram

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