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Integuments or insect's exoskeleton



An insect's exoskeleton (integument) serves not only as a protective covering over the body, but also acts as a surface for muscle attachment, a water-tight barrier against desiccation, and a sensory interface with the environment.   It is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis, and basement membrane.


Epidermis

Procuticle

Basement membrane

Epicuticle

Exocuticle

Endcuticle

Cuticulin layer

Wax molecule

Cement layer

The epidermis is primarily a secretory tissue formed by a single layer of epithelial cells.   It is responsible for producing at least part of the basement membrane as well as all of the overlying layers of cuticle.   The basement membrane is a supportive bilayer of amorphous mucopolysaccharides (basal lamina) and collagen fibers (reticular layer).   The membrane serves as a backing for the epidermal cells and effectively separates the hemocoel (insect's main body cavity) from the integument.

Differentiation of exocuticle involves a chemical process (called sclerotization) that occurs shortly after each molt. During sclerotization, individual protein molecules are linked together by quinone compounds.   These reactions "solidify" the protein matrix, creating rigid "plates" of exoskeleton known as sclerites.   Quinone cross-linkages do not form in parts of the exoskeleton where resilin (an elastic protein) is present in high concentrations.   These areas are membranes -- they remain soft and flexible because they never develop a well-differentiated exocuticle.

The epicuticle is the outermost part of the cuticle.   Its function is to reduce water loss and block the invasion of foreign matter.   The innermost layer of epicuticle is often called the cuticulin layer, a stratum composed of lipoproteins and chains of fatty acids embedded in a protein-polyphenol complex.   An oriented monolayer of wax molecules lies just above the cuticulin layer; it serves as the chief barrier to movement of water into or out of the insect's body.   In many insects a cement layer covers the wax and protects it from abrasion.



In many insects, certain epidermal cells are specialized as exocrine glands. These large, secretory cells produce compounds (e.g. pheromones, repellants, etc.) that are released on the surface of the exoskeleton through microscopic ducts.

Tiny hair-like projections or surface sculpturing of the cuticle are known as microtrichae or pile (PILL-EE).   These acellular structures consist of a solid core of exocuticle covered by a thin layer of epicuticle.   Larger hairs, bristles, and scales (called setae or macrotrichae) are the product of two specialized epidermal cells:   a trichogen cell (the hair shaft) and a tormogen cell (the socket).   Multicellular projections of the exoskeleton are called spines (or spurs, if movable).   They are lined with epidermis and contain both procuticle and epicuticle.

Skeletal muscles attach to the inner surface of the integument.   Despite small body size, insects have many more muscles than vertebrates because the exoskeleton affords a larger surface area than an endoskeleton (relative to body volume) for muscle attachment.   An insect owes its incredible strength to the geometry of its musculature -- providing optimal leverage for movement of appendages.

The colors found in the integument of insects are produced either by pigment molecules, usually located in the cuticle, or by physical characteristics of the integument that cause scattering, interference, or diffraction of light.   Pigments that are frequently present include the pterines, melanins, carotenoids, and mesobiliverdin.

Color patterns may change over time.   Rapid, temporary changes may occur in response to daily environmental conditions or to the threat of danger.   Slower, more permanent changes are usually related to seasonal changes in the environment or hormonal influences. 

Color of an Insect

Like humans, insects have pigments that give them color. In humans the compound is keratin, but in insects there are many different colors that give different colors. They are as follows:



  1. melanin: yellow, brown, black

  2. carotenoids: red, yellow, orange (hence the name carrot)

  3. pterins: red, yellow, white

  4. ommochromes: red, yellow, brown

  5. anthraquinones: red, orange

  6. aphins: red, yellow, orange and only confined to aphids

  7. chlorophyll derivatives: green. Chlorophyll is the compound that gives leaves their green color.

Since many insects can change color, they can control which pigment is expressed more. But not all do it this way. For example, the Hercules Beetle (Fig 3), Dynastes Hercules, can appear yellow green or black depending on the moisture in the atmosphere. Their epicuticle in their elytra is transparent, beneath it is a spongy yellow layer and beneath that is a black cuticle. During dry conditions, the spongy layer is filled with air and reflects back a yellow color. During humid conditions the spongy layer is filled with water, absorbs light and so the black is reflected. This is believed to be a camouflage adaptation since humid conditions are mostly at night.

Molting Process of Insects
Molting & its Importance:
In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer layer or covering), either at specific times of the year, or at specific points in its life cycle.
Moulting can involve shedding the epidermis (skin), pelage (hair, feathers, fur, wool), or other external layer. In some groups, other body parts may be shed, for example, wings in some insects or the entire exoskeleton in arthropods.
Molting is the process by which insects grow. Generally accomplished through the early years of the insect's existence, molting allows the body of the insect to expand under controlled and protected conditions. In comparison, our (human) bodies expand with the growing of our bones and muscles as we age.
Insects, on the other hand, utilize what is called an exoskeleton. This exoskeleton is basically their underlying bone structure that is located on the outside of their bodies with corresponding organs and muscles located underneath this hard shell. So in order for the insect to grow - i.e. increase in size - the insect must shed its current skin in favor of the new skin underneath. This process is known in the insects’ molting.
Depending on the species, molting can actually occur about 5 to 60 times in the life span of an insect and is generally regarded as one of the most vulnerable processes that an insect can go through. Yet this process is naturally required by insects to continue to grow into a full adult stage.
Not surprisingly, molting is not limited to insect species alone as even spiders and snakes undergo the procedure as needed.

Steps & Mechanisms of Insects molting:


To undergo the process of molting, an insect must begin to take in air or water by either swallowing it in naturally or raising its internal blood pressure. This instigates the process of molting that begins. The result is a soft, expandable exoskeleton suitable for further growth. This process is repeated several times during the life span of an insect depending on the species. The new exoskeleton will eventually harden and retain the original coloring of the insect as it matures and is exposed to the elements and everyday wear-and-tear.
Ecdysis is the moulting of the cuticle in many invertebrates. This process of moulting is the defining feature of the clade (আকর্ষণীয়) Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, tardigrades, and Cephalorhyncha. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnants of the old, empty exoskeleton are called exuviae.
After moulting, an arthropod is described as teneral, a callow; it is "fresh", pale and soft-bodied. Within one or two hours, the cuticle hardens and darkens following a tanning process analogous to the production of leather. During this short phase the animal expands, since growth is otherwise constrained by the rigidity of the exoskeleton. Growth of the limbs and other parts normally covered by hard exoskeleton is achieved by transfer of body fluids from soft parts before the new skin hardens. A spider with a small abdomen may be undernourished but more probably has recently undergone ecdysis. Some arthropods, especially large insects with tracheal respiration, expand their new exoskeleton by swallowing or otherwise taking in air. The maturation of the structure and colouration of the new exoskeleton might take days or weeks in a long-lived insect; this can make it difficult to identify an individual if it has recently undergone ecdysis.
Ecdysis allows damaged tissue and missing limbs to be regenerated or substantially re-formed. Complete regeneration may require a series of moults, the stump becoming a little larger with each moult until it is a normal, or near normal, size.

Process of Molting:


In preparation for ecdysis, the arthropod becomes inactive for a period of time, undergoing apolysis or separation of the old exoskeleton from the underlying epidermal cells. For most organisms, the resting period is a stage of preparation during which the secretion of fluid from the moulting glands of the epidermal layer and the loosening of the under part of the cuticle occur. Once the old cuticle has separated from the epidermis, a digesting fluid is secreted into the space between them. However, this fluid remains inactive until the upper part of the new cuticle has been formed. Then, by crawling movements, the organism pushes forward in the old integumentary shell, which splits down the back allowing the animal to emerge. Often, this initial crack is caused by a combination of movement and increase in blood pressure within the body, forcing an expansion across its exoskeleton, leading to an eventual crack that allows for certain organisms such as spiders to extricate themselves. While the old cuticle is being digested, the new layer is secreted. All cuticular structures are shed at ecdysis, including the inner parts of the exoskeleton, which includes terminal linings of the alimentary tract and of the tracheae if they are present.

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