Mechanoreceptors in insects include: 1. Cuticular structures like hair-like projections and dome-like campaniform sensilla with bipolar neurons. These detect mechanical stimuli like bending and vibration. 2. Chordotonal organs, which are subcuticular receptors that act as proprioceptors at joints or for hearing. They contain bundles of neurons attached to the cuticle. 3. Internal multipolar neurons that function as stretch or tension receptors in structures like muscles or connective tissue. These detect changes in tension or stretch.

1. Mechanoreceptors
THE SENSE ORGANS

2. Insect mechanoreceptors
• Insect mechanoreceptors can be found almost anywhere on the surface of
an insect’s body.
• They may act as tactile receptors, detecting movement of objects in the
environment, or
• they may provide proprioceptive cues (sensory input about the position
or orientation of the body and its appendages).
• These receptors are innervated by one or more sensory neurons that fire
in response to stretching, bending, compression, vibration, or other
mechanical disturbance.
• Some mechanoreceptors produce a phasic response when stimulated
— that is, they fire once when activated and again when deactivated.
• Other receptors generate a tonic response, firing repeatedly as long as a
stimulus persists.
• Neural processing centers in the brain or segmental ganglia interpret the
combinations of tonic and phasic signals sent from nearby receptors.

3. Mechanoreceptors in insects:
1. cuticular structures with bipolar neurons.
• A. hair-like projections from the cuticle with a basal socket
• B. dome-like campaniform sensilla.
• Both types have similar arrangements of neurons and sheath cells called a porous sensilla
• Mechanosensory hairs
• The wall of the hair consists of exocuticle with an outer layer of epicuticle.
• the hairs taper from base to tip, and they are often known as trichoid sensilla
• Shorter, peg-like
• multimodal (chemo- and mechanosensory) hairs with a single pore at the tip are referred to as basiconic
• Mechanosensory hairs on the cerci of crickets and cockroaches, for example, are very long relative to their
diameter and do not taper; they are called trichobothria.
• Some insects possess club-shaped hairs that can signal body orientation relative to the gravitational field.

4. Hair sensillum. Diagram showing the principal features of a
mechanosensitive sensillum.

5. Externally a thin cuticular joint membrane is
continuous with the general body cuticle and that of
the hair.
Beneath this, bridging a gap between the body cuticle
and the hair, are rings of suspension fibers.
inside, is the fibrous socket septum.
Socket septum runs across the outer part of
the receptor lymph cavity to the dendrite
sheath.
suspensory structures restore the hair to
its original position following imposed
movement.

6. Campaniform sensillum

7. campaniform sensillum: areas of thin cuticle, domed; usually oval in shape, with a diameter of 5–30 micron.
The dome of thin cuticle consists of
an outer homogeneous layer like exocuticle
a middle layer of transparent spongy cuticle
an inner lamellar or fibrous layer
• dome is connected to the surrounding cuticle by a
joint membrane.
• The dendrite sheath, enclosing a single dendrite,
is inserted into the center of the dome,
• and a molting pore may be visible on the outside.
• Campaniform sensilla are situated in areas of the
• cuticle that are subject to stress commonly
• they are usually found close to the joints.
• Orthopteroid insects have a single campaniform
sensillum at the base of each of the tibial spines.
Campaniform sensilla also occur on the
mouthparts, on the basal segments of the
antennae, on veins close to the wing, base and
on the ovipositor.
• large numbers are present on the haltere of
Diptera.

8. SENSORY CELLS
• Air movement detectors.: Some insects have hairs that are specialized for the detection of air
movements, sometimes including sounds.
• Grasshoppers and some Lepidoptera have groups of small trichoid mechanoreceptors on
specific regions at the front and top of the head
• Pressure receivers. Most aquatic insects are buoyant because of the air they carry beneath
the water surface when they submerge, but Aphelocheirus (Hemiptera: Heteroptera), living on
the bottom of streams and using a plastron for gas exchange, is not buoyant. The plastron
only functions efficiently in water with a high oxygen content, such as normally occurs in
relatively shallow water, so some mechanism of depth perception is an advantage to
Aphelocheirus
• On the ventral surface of the second abdominal segment of Aphelocheirus is a shallow
depression

9. containing hydrofuge hairs.
• These hairs are inclined at an angle of about 30 to the surface of the cuticle and mechanosensory
hairs are dispersed among them. The volume of air trapped by these groups of hairs changes with
pressure as the insect moves up and down in the water column, bending the sensilla. The insect
responds to an increase in pressure by swimming up, but there is no behavioral response to a
decrease in pressure.
• Mushroom-shaped pressure receptors associated with external openings of the tracheal system in
• Nepa (Hemiptera) detect body tilt.

10. Plastron respiration
• Plastron respiration Some insects have specialized structures holding a permanent
thin film of air on the outside of the body in such a way that an extensive air–water
interface is always present for gaseous exchange. This film of gas is called a plastron
and the tracheae open into it so that oxygen can pass directly to the tissues.
• In adult insects the plastron is held by a very close hair pile in which the hairs resist
wetting because of their hydrofuge properties and their orientation.
• In the adult Aphelocheirus, the plastron covers the ventral and part of the dorsal
surface of the body.
• The hairs which hold the air are 5–6 mm high and about 0.2 mm in diameter.
• They are packed very close together. They are able to withstand a pressure of about
400 kN m2 before they collapse. Hence, this is an extremely stable plastron that will
only be displaced by water at excessive depths.
• The spiracles open into a series of radiating canals in the cuticle and these connect
with the plastron via small pores.
• These canals are lined with hairs so that the entry of water into the tracheal system is
prevented.

11. Proprioceptors Trichoid sensilla. Groups or rows of small hairs often occur at joints between leg segments (see
Fig. 8.7a), on the basal antennal segments, on the cervical sclerites, at the wing base and elsewhere. The hairs
are positioned so that movements of one part of the cuticle with respect to another cause the hairs to bend (Fig.
23.6).
Individual hairs may have preferred directions of response.
Often the hairs in a hair plate are graded in size,

12. subcuticular structures with bipolar neurons,
known as chordotonal organs
• internal multipolar neurons which function as stretch or tension
receptors.
3.

13. CHORDOTONAL ORGANS
• Chordotonal, or scolopophorous, organs are
• subcuticular receptors that act most commonly as
• joint proprioceptors or as hearing organs. They consist
• of single units or, more commonly, groups of similar
• units, called scolopidia, which are attached to the
• cuticle at one or both ends. In many insects, the
• scolopidia are clustered into distinct groups, called
• scoloparia, that are identifiable morphologically and
• functionally (Fig. 23.7). Each scolopidium consists of
• three cells: a sensory neuron, an enveloping or
• scolopale cell and an attachment (cap) cell

14. CHORDOTONAL ORGAN:SCOLOPIDIUM

15. SUBGENUAL ORGAN IN ANT

16. Johnston’s organ
• Johnston’s organ is a chordotonal organ in the
• antennal pedicel with its distal insertion in the
• articulation between the pedicel and flagellum.
• It occurs in all adult insects and, in a simplified form,
• in many larvae. It consists of a single mass or several
• groups of scolopidia which respond to movements of
• the flagellum with respect to the pedicel.

17. Tympanal organ of a grasshopper
. (a) Diagrammatic horizontal section
through the metathorax and first
abdominal segments showing the
positions of the tympanal
membranes. Notice that the air sacs
form a continuum across the body.
(b) Diagram showing the attachment
of the scolopidia to the inside of the
tympanum. The cuticle of the air sac
which normally covers Muller’s
organ and the tympanum has been
removed. The orientations of
scolopidia in different parts of the
chordotonal organ are indicated by
arrows. Letters show the positions of
cells, some of whose sensitivities
are shown in Fig. 23.16a. The
styliform body, elevated process and
folded body are cuticular structures

18. Tibial chordotonal organ of bush cricket: The complex of
chordotonal organs exposed when the
dorsal (anterior) cuticle of the tibia is
removed (Decticus)

19. • Stretch and tension receptors
• Stretch and tension receptors differ from other insect
• mechanoreceptors in consisting of a multipolar
• neuron with free nerve endings, while all the others
• contain one or more bipolar neurons. They have a
• variety of forms. Sometimes they are an integral part
• of an oriented structure such as a muscle fiber or a
• strand of connective tissue, but in other cases they
• have no specific orientation or associated structures.
• The term “stretch receptor” is sometimes used rather
• loosely to include all multipolar receptors, but can be
• misleading as some respond to isometric tension
• rather than degree of stretch.

21. • 2.subcuticular structures with bipolar neurons, known as
chordotonal organs
• 3. internal multipolar neurons which function as stretch or
tension receptors.