Fundamentals of Entomology

Sense organs

• Sensilla are the organs associated with sensory perception and develop from epidermal cells. The different types of sense organs are:

  1. Mechanoreceptors
  2. Auditory receptors
  3. Chemoreceptors
  4. Thermo receptors and
  5. Photo receptors.

1. Mechano receptors (detect mechanical forces)
i. Trichoid sensilla: Hair like little sense organ. Sense cell associated with spur and seta. These cells are sensitive to touch and are located in antenna and trophi (mouth parts).
ii. Campaniform sensilla (Dome sensilla): Terminal end of these sensilla is rod like and inserted into dome shaped cuticula. These cells are sensitive to pressure and located in leg joints and wing bases.
iii. Chordotonal organ: The specialized sensory organs that receive vibrations are subcuticular mechano receptors called chordotonal organ. An organ consists of one to many scolopidia, each of which consists of cap cell, scolopale cell and dendrite. These organs are interoceptors attached to both ends of body wall.

Functions :
i. Proprioception (positioning of their body parts in relation to the gravity).
ii. Sensitive to sound waves, vibration of substratum and pressure changes.
iii. Johnston's organ: All adults insects and many larvae have a complex chordotonal organ called Johnston's organ lying within the second antennal segment (Pedicel). These organs sense movements of antennal flagellum. It also functions in hearing in some insects like male mosquitoes and midges.
iv. Subgenual organ: Chordotonal organ located in the proximal tibia of each leg, used to detect substrate vibration. Subgenual organs are found in most insects, except the Coleoptera and Diptera

2. Auditory receptors (detect sound waves)
i. Delicate tactile hairs: Present in plumose antenna of male mosquito.
ii. Tympanum: This is a membrane stretched across tympanic cavity responds to sounds produced at some distance, transmitted by airborne vibration. Tympanal membranes are linked to chordotonal organs that enhance sound reception. Tympanal organs are located
* Between the metathoracic legs of mantids.
* The metathorax of many nectuid moths.
* The prothoracic legs of many orthopterans.
* The abdomen of short horned grasshopper, cicada.
* The wings of certain moths and lacewings.

3. Chemoreceptors (detect smell and taste)
1. Detect chemical energy. Insect chemoreceptors are sensilla with one pore (uniporous) or more pores (multiporous). Uniporous chemorceptors mostly detect chemicals of solid and liquid form by contact and are called as gustatory receptor. Many sensor neurons located in antenna are of this type.
2. Multiporous chemoreceptors detect chemicals in vapour form, at distant by smell and are acalled as olfactory receptor. Few sensory neurons located in trophi and tarsi are of this type. Each pore forms a chamber known as pore kettle with more number of pore tubules that run inwards to meet multibranched dendrites.

4. Thermoreceptors (detect heat)
Present in poikilothermic insects and sensitive to temperature changes. In bed bug it is useful to locate the host utilizing the temperature gradient of the host.

5. Photoreceptors (detect light energy) a. Compound eyes:
The compound eye is based on many individual units called ommatidia. Each ommatidium is marked externally by a hexagonal area called facet. Compound eye is made up of two parts called optic part and sensory part. Optic part contains a cuticular lens called corneal lens secreted by corneagenous cells and crystalline cone covered by primary pigment cells. Function of the optic part is to gather light. Sensory part contains six to ten visual cells called retinular cells covered by secondary pigment cells which collectively secrete a light sensitive rod at the centre called rhabdom. Rhabdom contains light sensitive pigments called rhodopsin. Each ommatidium is covered by a ring of light absorbing pigmented cells, which isolates an ommatidium from other. Nerve cells are clustered around the longitudinal axis of each ommatidium.