Insect sense ORGANS (MECHANO-, PHOTO- AND CHEMO- RECEPTORS): IT’S STRUCTURE, FUNCTION, AND MODIFICATION IN DIFFERENT ORDERS OF INSECT.

1. INSECT SENSE ORGANS (MECHANO-, PHOTO-
AND CHEMO- RECEPTORS): IT’S STRUCTURE,
FUNCTION, AND MODIFICATION IN DIFFERENT
ORDERS OF INSECT.

2. CONTENTS
2
What is
Insect
Sense
Organ?
Types of
sense
organs
Function
of sense
organs
Examples
in
different
orders.

3. 3
Sensilla are the organs associated with sensory perception and develop from epidermal cells .
All sense organs (receptors) act as transducers — converting light energy, chemical energy, or
mechanical energy from the environment into electrical energy of nerve impulses in sensory neurons.
Signals generated by insect sensory receptors travel to the brain or ventral nerve cord where they
stimulate appropriate behavioural responses.
finding resources (e.g., food, mate, etc.), avoiding danger, or reacting to changes in the environment.
INSECT SENSE ORGANS

4. 4
• Insect sense organs are grouped into different types of sense organs depending on function.
TYPES OF SENSE ORGANS IN INSECT
Sl . No Types Function Example
1 Mechanoreceptors Detect movements,
vibrations, or other
mechanical
disturbances
 Tactile receptors
 Proprioceptors
 Sound receptors
2 Auditory receptors To detect sound waves.  Delicate tactile hairs.
 Tympanum
3 Chemoreceptors To detect smell and
taste
 gustatory receptor
 olfactory receptor
4 Thermoreceptors detect heat
5 Photo receptors detect light energy  compound eye
 Ocelli
 Stemmata

5. MECHANORECEPTORS
 Mechanoreceptors detect mechanical distortions that can arise from touching an object or from the impact of
vibrations borne through the air, water or the substratum.
 mechanoreceptors are;
 exteroceptors (which respond to external stimuli),
interoceptors (responding to internally generated stimuli)
proprioceptors (which respond to body position and movement).
 Three broad structural categories of mechanoreceptor are present in insects:
i. cuticular structures with bipolar neurons,
ii. subcuticular structures with bipolar neurons, known as chordotonal organs,
iii. internal multipolar neurons which function as stretch or tension receptors
5

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A. Cuticular mechanoreceptors
i. Trichoid sensilla:
 Most commonly, the hairs taper from base to tip. It mainly associated with spur and seta. These cells are
sensitive to touch and are located in antennae and trophi
 All insects have groups of small hairs at some joints in the cuticle , known as hair plates. These act as
proprioceptive organs when stimulated by relative movements of adjacent limb segments.
Source of image:
https://cronodon.com/BioTech/insect_mechanoreceptors.html
Mechanoreceptor hair sensillum, diagram showing the principal
features of mechanosensitive sensillum

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ii. Campaniform sensilla
• Campaniform sensilla are areas of thin cuticle, domed and usually oval in shape .
• The dome of thin cuticle consists of an outer homogeneous layer with the appearance of exocuticle, and an inner
lamellar or fibrous layer.
• Campaniform sensilla are situated in areas of the cuticle that are subject to stress. They occur most Commonly
On the appendages like leg and wing bases.
Source of image:
https://www.scientificbeekeeping.co.uk/Sensillaanat.html
Fig showing the campaniform sensilla and its essential cuticular
features.

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B. Chordotonal organs.
 Chordotonal/ scolopophorous organs are subcuticular receptors that act most commonly as joint
proprioceptors or as hearing organs.
 They consist of single units or groups of similar units of scolopidia, which are attached to the cuticle at one
or both ends.
 In many insects, the scolopidia are clustered into distinct groups called as scoloparia, that are identifiable
morphologically and functionally.
 Each scolopidium consists of three cells
Sensory neuron
cap cell.
 scolopale cell
Source of image: https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/scolopidia
Fig showing basic structure of scolopidium

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1. Johnston’s organ: is a chordotonal organ in the antennal pedicel . It occurs in all adult insects and simplified
form in many larvae. It is generally proprioceptive.
 E.g.: auditory function in male chironomids and culicids.
 Velocity receptors in aquatic insect.
 In honey bee it is used to regulate flight speed.
2. Muller’s organ: A group of numerous scolophores forming a swelling in Acrididae(orthoptera) in the inner
surface of each tympanum and are auditory in function.
3. Subgenual organ: It is located in proximal tibia of each leg, used to detect substrate vibration. Found in most
insect except Coleoptera and Diptera.
4. Pilifer: Is a hearing organ in Hawk moth(lepidoptera)

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 Crickets (orthoptera) and cockroaches(dictyopteran) have Mechanosensory hairs on the cerci .
 Haltere of Diptera have Campaniform sensilla.
 In orthopteran insects have modified mechanosensory hairs on the legs or abdominal tergites can act as
stridulatory pegs.
 Some fire-seeking beetles, (order; coleoptera) (e.g., the buprestid beetle genus Melanophila) and flat bugs
order; Hemiptera have Infrared receptors.
 Orthoptera (Grasshoppers) and some Lepidoptera have Air movement detectors present at the front and
top of the head or on the prothorax for the maintenance of wingbeat, and are sometimes called aerodynamic sense
organs.
EXAMPLES IN DIFFERENT ORDERS

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 In Hemiptera , their ventral surface of the second abdominal segment have hydrofuge hairs which are
dispersed with mechanosensory hairs.
 In Nepa Sp (Hemiptera) have a Mushroom-shaped pressure receptors associated with external openings of the
tracheal system to detect body tilt.
 In the stick insect; order phasmida have hair plates on the trochantin which are stimulated by the coxa are of
principal importance in influencing geotactic behaviour.

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FIG :SHOWING THE INFRARED
RECEPTORS IN MELANOPHILA
FIG : SHOWING HYDROFUGE HAIRS
OF AQUATIC WATER BUG
Source of image:
https://images.newscientist.com/wp-
content/uploads/2012/05/dn21842-1_300.jpg
Source of image:
https://www.sciencephoto.com/media/610617/view/sem
-of-backswimmer-hydrofuge-hair

13. 13
• Delicate tactile hairs: Present in plumose antenna of male mosquito.
• 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.
• Source of image: Chapman, RF.1998 The Insects: Structure and Function
• textbook.
• Fig 7. showing tympanal organ of grasshopper
AUDITORY RECEPTORS

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 scarab beetles (Coleoptera) have Tympanal organs on the neck membrane .
 orthopteran : In prothoracic legs .
 mantids(mantodea) : Between the metathoracic legs.
 aquatic Hemiptera, such as Corixa (Hemiptera) and Plea : on the mesothorax .
 Noctuoidea (lepidoptera ) : on the metathorax
 Acrididae, Cicadidae, Pyralidoidea and Geometroidea (Lepidoptera) and Cicindellidae (Coleoptera): in
the abdomen
 In Chrysopa (Neuroptera) : on the ventral side of the radial vein of the forewing
EXAMPLES IN DIFFERENT ORDERS

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FIG : TYMPANUM ON PROTHORACIC
LEG OF KATYDID
FIG : TYMPANUM ON ABDOMINAL
SEGMENT OF SHG
Source of image:
https://en.wikipedia.org/wiki/Tympanal_organ
Source of image:
https://livingwithinsects.wordpress.com/2
011/12/19/grasshopper-ears/

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 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.
 Multiporous chemoreceptors detect chemicals in vapour form, at distant by smell and are a called as olfactory
receptor, located in trophi and tarsi .Each pore forms a chamber known as pore kettle .
CHEMORECEPTORS
Fig 10. Olfactory sensillum, longitudinal section through
sensillum of single wall type.
Source of image:
https://www.bio.miami.edu/dana/360/360F19_11b.html

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• Some chemoreceptors are;
1. Sensilla trichodea (setae like) : clusters of setae from hair beds, it may be act as tactile receptors, olfactory or
gustatory receptors.
2. Sensilla chaetica (spine like or bristle like): singly located. Mainly gustatory receptors.
3. Sensilla basiconica (as peg, cones, stumpy hairs): thermos , hygroreceptors, gustatory and olfactory in function.
4. Sensilla styloconica ( elevated cones) : gustatory in function.
5. Sensilla squamiformia ( scale like): in Lepidoptera
6. Sensilla placodea ( plate like): olfactory in function mainly seen in antennae.
7. Sensilla ampullacea ( tube like): Antennae of Hymenoptera.

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Sensilla
Chaetica
Sensilla
Ampullacea
Source of image :
https://www.scientificbeekeeping.co.uk/Sensillaanat.html

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 Diptera (mosquito Aedes aegypti) : Olfactory Pores can occur on the antennae .
 Hymenoptera (honey bee antennae ) have hygroreceptive sensilla .
 Blattodea ( Periplaneta americana ) : the sensillum capitulum on the antennae.
 Lepidoptera (silk moth Bombyx mori ): the sensilla styloconica on the tips of the antennal branches .
 Diptera( adult Drosophila ) : The prominent taste hairs present on the labellum contain one chemosensory
neuron.
 Diptera( the sheep blowfly Lucilia cuprina ).: Olfactory sensilla occur on other body regions such as the
genitalia .
 Hymenoptera (parasitic wasps ): Contact chemoreceptors are found on the ovipositor
EXAMPLES IN DIFFERENT ORDERS

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FIG . SHOWING OLFACTORY
RECEPTORS IN ANTENNAE OF
MOSQUITO
FIG . SHOWING CHEMORECEPTORS
IN WASP
Source of image :
https://www.researchgate.net/figure/Schematic-
representation-of-insect-olfaction-A-shows-the-
antennae-of-tsetse-with_fig1_263897249
Source of image:
https://www.slideshare.net/JawwadMirza1/gen
omics-of-insect-chemoreception-60959318

21. 21
THERMORECEPTORS
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.
PHOTORECEPTORS
 Mainly insects have photoreceptors like;
 adult insects and larval hemimetabolous insects have a pair of compound eyes.
 three single-lens eyes, called ocelli .
 Larval holometabolous insects have one or more single-lens eyes, known as stemmata,

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A. Compound eyes
 many individual units called ommatidia.
 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.
 Sensory part contains six to ten visual cells called retinular cells covered by secondary pigment cells, a light
sensitive rod Rhabdom contains light sensitive pigments called rhodopsin.
Fig 13. showing vertical cross section of
ommatidium.
Source of image:
https://desperatelyseekingstrepsiptera.
wordpress.com/

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1. Apposition type: The sensory receptor cells of most diurnal insects, The mosaic image formed .
2. Superposition type: Most nocturnal , produce brighter images than apposition eyes.
TYPES OF OMMATIDIA
Fig : Showing the apposition and superposition type ommatidia
Source of image:
https://www.sciencedirect.com/science/article/
abs/pii/B9780124104662000042

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B. Dorsal ocelli
 Found in adult insects and the larvae of hemimetabolous insects
 Typically, there are three ocelli forming an inverted triangle antero-dorsally on the head.
 It varies from 0-3 in numbers. It contains a single corneal lens with many visual
cells individually secreting the rhabdomere.
 It maintains the diurnal rhythm and not involved in image perception.
C. Lateral ocelli (stemmata)
• visual organs of larval holometabolous insects.
• Larval stemmata occur laterally on the head and vary in number from one on each side in
tenthredinid larvae (Hymenoptera: Symphyta) to (most typically)
six on each side in larval Lepidoptera.
• It helps to detect form, color, and movement, also to scan the environment.
Fig . showing the frontal view of ocellus of grasshopper
Source of image: Chapman, RF.1998 The Insects: Structure and
Function Textbook.

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 Phthiraptera and Siphonaptera, and in female coccids (Hemiptera).: Compound eyes are strongly reduced
or absent.
 Anisoptera (Odonata) and male Tabanidae and Syrphidae (Diptera), the eyes are contiguous along the dorsal
midline, this being known as the holoptic condition.
 In most insects have fused rhabdom but Diptera, Dermaptera, some Heteroptera (Hemiptera) and some
Coleoptera have widely separated rhabdomeres forming an “open” rhabdom.
 In many Lepidoptera, closely packed tracheae form a layer at the back of the eye. This layer, which reflects light
back into the eye, is called a tapetum.
 Odonata and Bombus (Hymenoptera ): The ocellus itself is bilobed .
EXAMPLES IN DIFFERENT ORDERS

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 Mecoptera, most Neuroptera, Lepidoptera and Trichoptera, Diptera, In Coleoptera the stemmata of many
species have a single rhabdom.
 larval Adephaga (Coleoptera), larval Symphyta (Hymenoptera) have Stemmata with multiple rhabdoms.
Fig : Showing compound
eyes of an insect. Source
of image:
https://indianapublicmedi
a.org/amomentofscience/
insect-
Fig : Showing the 3 dorsal
ocelli of an insect. Source
of image:
https://en.wikipedia.org/
wiki/Simple_eye_in_inver
tebrates
Fig . Showing the lateral ocelli in larva of
holometabolas insect. Source of image:
https://www.wikiwand.com/en/Simple_eye_i
n_invertebrates

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• Bullock & Horridge, (1965) found that Johnson’s organs which were located in the second segment of the antenna of
some insects and act as receptors for movement of the antenna, as gravity detectors and as modified hearing organs with
directional sensitivities for sound localization.
• Simpson et al., (1991) found that Changes in chemoreceptor sensitivity in relation to dietary selection by adult Locusta
migratoria. During somatic growth in adult Locusta migratoria L. there were variations in the responsiveness of chemo
sensilla on the maxillary palps.
• REFERENCE
 Chapman RF. (1998). The Insects: Structure and Function, 5: 740-791, ed. S. J. Simpson and A. E. Douglas.
Cambridge Univ. Press, Cambridge,2013.
 https://www.researchgate.net/publication/351978868_Insect_Sensory_System ( Accession date on 31/01/2023)
 K. Phani Kumar and C.P.Viji (2014) Entomology refresher,2,18-21,
 http://ecoursesonline.iasri.res.in/mod/page/view.php?id=101206(Accession date on 31/01/2023)
 K.N. Ragumoorthi, V. Balasubramani, M.R. Srinivasan, N. Natrajan ( 2003) Insecta an introduction.,2.96-100 .,
A.E. Publications, coimbatore
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