1. Lecture no. 5
Structure and functions of insect cuticle
and molting
R K PANSE
Assistant Professor
Entomology
2. Structure of insect cuticle
The body wall or integument of insects forms an
exoskeletal covering of the insect body. It forms a
composite structure which forms the skeleton of the insect
body. Integument consists of 3 layers:
1. Inner basement membrane
2. Middle epidermis (or) hypodermis
3. Cuticle (exoskeleton)
3.
4. Basement membrane: It is the basal part of the body wall formed
from degenerated epidermal cells and appear as non-living cells.
Epidermis: It is an unicellular layer formed from polygonal cells
which modifies in to columnar during the process of moulting.
These cells consists of well developed nucleus and other
cytoplasmic contents. All the epidermal cells are glandular and
secrete cuticle and the enzymes involved in production and
digestion of old cuticle during moulting.
5. The epidermal cells get differentiated in to following types
based on the function they perform and may modify in to
a) dermal glands producing cement layer
b) trichogen cell producing hair like seta or trichome.
c) moulting glands secreting moulting fluid which digests
the old cuticle
d) peristigmatic glands around the spiracles in case of
Dipteran larvae
6. Cuticle: It is outermost thick layer of integument secreted by
epidermis. It is divided in to two regions
A) Upper epicuticle B) Inner procuticle
A. Epicuticle: It is a thin outermost layer varying in thickness
from 1-4μ. Chitin is absent in epicuticle. It consists of the
following 4 layers.
7. 1. Cement layer: Secreted by dermal glands and is composed of
lipoprotein. It protects the body from external damage.
2. Wax layer: It is consisting of long chain hydrocarbons, esters
of fatty acids and alcohols. It serves as water proof layer
preventing water loss from the body
3. Cuticulin (outer epicuticle) layer: It serves the purpose of
permeability and also acts as growth barrier.
4. Polyphenol (inner epicuticle) layer: It is a non-static layer
containing various types of phenols which are mainly
used in resistant to acids and organic solvents
8. B. Procuticle: It is differentiated in to exo and endocuticle after
sclerotization process.
Exocuticle is darkly pigmented, hard and sclerotized. It offers
rigidity to the cuticle and consists mainly chitin and a hard protein
called sclerotin.
Endocuticle is soft, light coloured and unsclerotized. It contains
more chitin but lacks hard protein sclerotin
Pore canals: These are numerous fine vertical channels from
epicuticle through out the length of the cuticle. They are useful in
transportation of cuticular material and enzymes to the outer pro
and epicuticle parts.
9.
10. Two major components of insect cuticle are
i) Chitin ii) Proteins
i). Chitin : It is a nitrogenous polysaccharide. (C8H13O6N)x.
It is named by Odier in 1834. It consists of high molecular
weight polymer of anhydro-N-acetyl glucosamine.
ii). Proteins: Cuticle has 3 types of proteins
(a) Arthropodin: It is soft water soluble protein present in
endocuticle. The conversion of arthopodin in to sclerotin is
known as sclerotization or tanning.
(b) Sclerotin: It is also called tanned protein which is present
only in exocuticle.
(c) Resilin : It is a rubber like elastic protein which is
colourless and present in joints such as wing, leg joints,
clypeolabral joints or suture.
11. • Outer layer or “skin”
• Functions:
It is the outer layer of insect.
Protection of soft parts
Muscle attachment
Support
Site for sensory organs
Helps prevent desiccation
Reduces pathogen entry
It provides shape and rigidity to the animal body
Functions of insect cuticle
12. Molting
Moulting : Series of changes, which developmental stages undergo
for the formation of new cuticle and structures
associated with it. Periodically outgrow and replace its
exoskeleton.
Ecdysis : The process of shedding the cuticle.
Apolysis : Separation of epidermis from the cuticle giving narrow
space in between.
Sclerotization: The process by which the cuticle is hardened by
substances other than chitin.
Tanning: The process by which the cuticle is darkened.
Exuvia :The cuticular parts discarded during moulting
Stadium: The time interval between the two subsequent moulting
Instar: the form assumed by the insect in any stadium.
13. Steps of molting
Step 1
Apolysis -- separation of old exoskeleton from epidermis
Step 2
Secretion of inactive molting fluid by epidermis
Step 3
Production of cuticulin layer for new exoskeleton
Step 4
Activation of molting fluid
Step 5
Digestion and absorption of old endocuticle
Step 6
Epidermis secretes new procuticle
Step 7
Ecdysis -- shedding the old exo- and epicuticle
Step 8
Expansion of new integument
Step 9
Tanning -- sclerotization of new exocuticle
