Integuments (part 1)
The integuments of insects like that of other animals are an outer dermo-skeletal covering of the body and are derived from the embryonic ectoderm. It is functionally a composite structure that serves as a skin, skeleton, food reservoir, and switches on post-embryonic development by undergoing moulting repeatedly.
A. Structure and chemistry:
It is basically composed of three layers; the inner basement membrane, middle epidermis, and the outer cuticle. The epidermis is commonly called as the hypodermis as it lies below the cuticle. It is thin layer of integument that is cellular, while the inner basement membrane as well as the outer cuticle, both is a cellular structure. The epidermis secretes the cuticle and the cuticle is modified; later on into various skeletal sclerities, appendages, and sensory organs and internal linings of the fore a hind gut, the tracheal system, some organs of reproduction and the exocrine glands:
1. The basement membrane: it is formed from the degenerated epidermal cells, and appears as a non living, amorphous, granular, inner lining of the integument. It separates the epidermis from the hemocoel. It is about 0.5 micron thick. The histochemical studies reveal that it is composed primarily of the neutral mucopolysaccharides. On inner surface of the basement membrane, are attached the muscles, hemocytes and oenocytes. Sometimes, the stellate tracheal cells, collagen fibrils and connective tissue fibrils are imbedded in the basement membrane.
2. The epidermis: it is a unicellular layer formed from the polygonal cells. The polygonal cells are polyploidy, possessing a large number of nucleoli. The cytoplasm is characterized by containing various types of pigment granules. The adjacent epidermal cells are held with one another by means of cytoplasmic process, the desmosomes. Each epidermal cell produces a large number of cytoplasmic processes apically, the pore canals traversing the cuticle and opening above a cuticulin layer. The epidermal cells differentiate in some regions of the integument and constitute various types of mechano and chemoreceptor organs, the dermal glands and particularly in dipteran larvae, the peristigmatic glands around the spiracles. Each dermal gland is formed by a group of three cells; the medial cell constituting the body of gland is formed by a group of three cells, the medial cell constituting the body of the gland and is termed as tormogen cells. During moulting, some epidermal cells modify in to the so called moulting glands which secretes the moulting fluid that digests the old endocuticle to ensure further growth of the new cuticle.
Sometimes, the myofibrils penetrate the epidermis from the site of muscle attachment.
3. The cuticle: the cuticle is secretory product of epidermis. It forms an outermost thick layer of the integument and determines surface pattern and physio-chemical properties of integuments.
It is differentiated into three major regions; outer epicuticle, middle exocuticle and inner endocuticle. The epicuticle is non-chitinous while the exo and the endocuticle are chitinous regions. The exo- and the endocuticles are differentiated from the initially secreted procuticle due to sclerotization or tanning that takes place on the upper region. The exocuticle, therefore represents the sclerotized or tanned and the endocuticle nonsclerotized or untanned, undifferentiated regions of the cuticle.
Besides these three static regions, two more non-static regions develop at the time of moulting. I.e. between the exo- and endocuticles as a transitional semi hard and little darkened region commonly known as the mesocuticle. The other region arises in between the endocuticle and the epidermis in the form of a membrane and is called as the ecdysial membrane or the subcuticle. These structures can be seen only during the larval and pupal moults and are completely absent from the cuticle of the adult insects. The functional significance of these regions is still obscure.
a. The epicuticle: the epicuticle is a very thin outermost layer and varies in thickness from 0.03 to 0.04 micron. It is composed of three or four superimposed layers; outer cement layer, second wax layer, third polyphenol layer and inner one cuticulin. The polyphenol layer is reported only in some insects.
1. The cement layer: it is secreted by the dermal glands or so called verson’s gland in Lepidoptera. It is composed of the lipoprotein complex resembling the natural product, shellac secreted by the lac insect, laccifer lacca. It also contains the carbohydrates like al laccose. It functions as a varnish and provides protective external surface to the integuments. It absorbs the mobile lipids that are used for sealing over the surface abrations in order to prevent water loss from the body. It serves as reservoirs for the lipids too.
2. Wax layer: prominent layer about 0.35 micron thick layer containing partially oriented wax molecules.
Composition:
Hydrocarbons: 48 t0 58 %
Fatty acids: 25 to 18 %
Esters: 9 to 11 %
Cholesterol: 2 to 3 %
Polymers: 12 to 15 %
Note: unlike the plant wax the insect wax lacks the alcohol.
It is a mono layer in solid or liquid phase and hydrophilic groups of molecules are adsorbed on cuticulin layer. It serves as a water proof layer of integument.
3. the polyphenol layer:
It is most commonly described in the blood sucking bugs. Often appears in liquid state containing various types of polyhydric phenols-homocatechol, protocatechol, dopacatechol etc...
The polyphenols are transported above the cuticulin layer by the pore canals, Secreted by the epidermal cells. At the time of sclerotization the phenols are converted into the quinone in the presence of the phenol oxidase. Quinone gets tanned first. The protein of cuticulin layer and protein of outer procuticle together forms the exocuticle.
4. The cuticulin layer: refractile amber coloured layer formed of a lipoprotein-cuticulin. It is a highly resistant to mineral acids of and most of the organic solvents. It also serves as a growth barrier and determines the surface properties of the integuments.
b. THE CHITINOUS CUTICLE:
It is composed of the exo- and the endocuticle. It is a stratified structure differentiated as an exocuticle and endocuticle. The exocuticle is darkly pigmented, hard, and sclerotized. It provides strong mechanical support to the body size due to its toughness and inelastic properties.
I. The lamellar organisation: the chitin microfibrills lie parallel to each other to form a group of layers about 20 to 25 Å thick. The orientation of the micro fibrils differs from one lamella to the other; hence they can be easily distinguished and counted in the sections. Observations of the locusts have shown that lamellar cuticle is formed during the night and the non lamellar cuticle is formed during the day time. Thus a fully formed cuticle will have alternate bands of the lamellar and non lamellar cuticle. The lamellar regions are elastic in nature hence facilitate the elasticity to the endocuticle for flexibility and stretching properties
II. The pore canals: they run throughout the cuticle. They are about 0.15 to 1.0 micron in diameter. They encase the cytoplasmic fin processes of epidermal cells. In newly synthesised cuticle the pore canals are arranged in a spiral course and contain he cytoplasmic filaments. But in the mature cuticle they become straight and contain cubicula substances. After penetrating the cuticulin layer the pore canals divide. Sometimes the site of division is separated from the pore canal by a plate called the pore plate. The pore plate separates the wax fluid-filled region from the chitin filled region. There are about 15000 canals per square mm in endocuticle of sacrophaga. And about 200000 per cuticle per square mm in periplaneta. Functions: the pore canals transport the secretions of the epidermal cells to the upper surface of the procuticle to facilitate its growth and sclerotization.
(To be continued.................)
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