Insect Morphology/2020

Insect Morphology-

Insect morphology is the study and description of the physical form of insects. The terminology used to describe insects is similar to that used for other arthropods due to their shared evolutionary history. Three physical features separate insects from other arthropods: they have a body divided into three regions (head, thorax, and abdomen), have three pairs of legs, and mouthparts located outside of the head capsule. It is this position of the mouthparts which divides them from their closest relatives, the non-insecthexapods,which includes Protura,Diplura, and Collembola.

1- BODY WALL

INTEGUMENT

It is the outermost layer of the insect Ir is divided into 2 parts. They are

1-Epidermis

2-Cuticle

1-Epidermis

The epidermis is the outer layer of the insect. It is one cell thick (unicellular It is commonly called as hypodermis and it lies below the cuticle. It secretes cuticle The epidermal cells have rough endoplasmic reticulum and Golgi complex and pigment granules. The adjacent epidermal cells are held with one another by means of cytoplasmic process, the desmosomes.

All epidermal cells are glandular they secret cuticle and the enzymes concerned in its production and its digestion at the time of moulting. Some of the cells are variously modified to form sense organs (chemo- tacto, hygro and photo receptors)

Each dermal gland is formed by a group of three epidermal cells, the median cell constituting the body of the gland known as Trichogen cell, while the remaining cells constitute the duct of the gland known as tormogen cells.

They also have peristignmatic glands responsible for the hydrofuge properties of the cuticle surrounding the spiracles which prevent the entry of water into the tracheal system. The oenocytes of the blood are produced from epidermal cel which are involved in the intermediary metabolism

II. Cuticle -

It is secreted by epidermis. It covers the whole outside body as well as of foregut, hindgut and trachea.

It is divided into 2 parts

1. Inner region - Procuticle (chitinous)

2. Outer regionEpicuticle (non chitionus)

a- Procuticle

It is thicker layer and contains chitin proteins and other substances. Chitin is the main constituent of the procuticle comprising of 20-50% of its dry weight. It also contains special protein called resilin which is present at the joints of wings, mouth parts and legs for articulation

It has two parts )
Exocuticle Outer part hard and rigid (tanned or sclerotized)

Endocuticle Inner undifferentiated part

Between the two layers there is a region of hardened but not fully darkened layer know as meso cuticle

b. Epicuticle-

It is very thin layer (0.03 to 0.4 microns) and chitin is absent in this layer. During the development of the cuticle special glands in the epidermis produce a waxy material which in dissolved state is carried through the pore canals and deposited over the protein epicuticle

The cuticle consists of 4 layers. They are

Cement layer
Wax layer
Poly phenol layer
Inner curriculum layer

During the process of ecdysis special cells of epidermis secrete cement layer which is deposited over the wax layer in Inner cuticulin layer

Cement layer-

It is a very thin layer outside of the wax layer by the dermal glands or so called Verson's gland in Lepidoptera. It consists of muco polysaccharides which are closely associated with lipids. It absorbs mobile lipid and serves as reservoir of lipids. The main function is to prevent water loss from the body. It is absent in honey bees. It is secreted

ii. Wax layer:

It contains wax molecules and 0.25 microns thick. It consist of long chain hydrocarbons and esters of the both fatty acids and alcohol. It serves as a water proof layer of the integument, as water molecules can not pass through closely packed wax molecules

ii. Poly phenol layer-

This layer is rich in various polyphenols. The poly phenols are transported from the epidermis cells through the pore canals and accumulate on outer surface of the cuticulin layer.

iv. Cuticulin layer-

It consist of lipoproteins and cuticulin. It serves as a
permeability and growth barrier. It also determines the surface properties of the integument

II. Basement membrane-

It is about 0.5 micron thick. The epidermal cells stand on basement membrane, The epidermal cells are anchored to the basement membrane by hemidesmosomes It is formed separate epidermis from the hemocoel, They contain fibrous proteins and glycoproteins. The basement membr forms a continuous sheet beneath the epidermis and at points where mus attached from the degenerate epidermal cells. It

Functions of cuticle-

Cuticle acts as exo skeleton providing support to the body and its organs

2. It Imparts shape to the body

3. Muscles are attached to the cuticle to facilitate various types of body movements

4. Cuticle is invaginated to provide internal support and muscle attachment

5. Wing movements are possible only because of the presence of hard cuticular flight sclerites in thoracic region

6. The epicuticular wax layer provides a mechanism to conserve water in the terrestrial

insects

7 Secretions over cuticular surface in some insects are bactericidal and protect insects from surface infection

8. The intima of fore and hind gut protects epithelium

9. Various parts of cuticle are modified to form sense organs

10. Certain pigments of cuticle imparts colours to insects

Esternal appendages of integument

These are two types

1- Non-cellular outgrowth basic articular characteristies

2- Cellular outgrowth
These are classified as unicellular and multicellular appendages

Setae are formed by a single usually large epidermal cell called, trichogen cell.

The setae is attached by means of a small setal membrane which produced by a second epidermal cell called, tormogen cell.

The setae are of various types

Unicellular outgrowth (macrotrichia). They are great taxonomic importance their distribution differs from species to species These outgrowth of the body are termed a s

# Clothing hairs

These cover the entire body and appendages as found in honey bees

b. Glandular hairs

. These occur around the opening of various epidermal glands and can be seen in few Lepidopteron larvae.

c. Sensory setae:

These constitute various mechano and chemoreceptors and occur on the antennae, mouth parts. logs and various regions of the body

d. Scales-

These structures are highly modified clothing hair and are small, flat and seale like. They cover the wings of all adult Lepidopteran insects.

e- Bristles-

They are stiff and stout and are present in Tachinidae

Multicellular outgrowth These appendages are hollow outgrowth of the integument and are line with epidermal cells. Ex: Spines. These are present on hind tibin of certain Orthoptera or as tubercles that carry a fine branch of trachea and a fine nerve fibre.

Insect Morphology/2020