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Lecture No. 5 Structure and Functions of Insect Cuticle and Molting

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Lecture No. 5 Structure and Functions of Insect Cuticle and Molting Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP Lecture no. 5 Structure and functions of insect cuticle and molting (definition & importance) Structure of insect cuticle The insect exoskeleton is usually called a cuticle. The cuticle is the characteristic feature of arthropods and is to a large extent responsible for the success of insects as terrestrial animals. The body wall or integument is the outer layer of insect and bends inwards at various points to form supporting ridges or braces. The body wall of insect is composed of three principal layers: the cuticle, epidermis and basement membrane. General structure of the insect cuticle Cuticle: The cuticle is an outer layer which contains a characteristic chemical compounds called chitin, proteins and pigments. The cuticle is made up of three principal layers: epicuticle, exocuticle and endocuticle. The exo- and endo- cuticle together referred as procuticle. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP A. Epicuticle: Epicuticle is the outer, thin, delicate layer without any chitin and may have four sub- layers I. Cement layer: The cement layer is very thin layer outside most of wax layer and consists of mucopolysaccharide. It protects underlying wax and not produced by all insects. It is secreted by dermal glands and protects the body from external damage. II. Wax layer: The wax layer contains many different compounds but wax comprises over 90%. 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. III. Polyphenol layer or Inner epicuticle: it is the thickest layer of 0.5 to 2.0 µm immediately outside the procuticle and chemically consists of tanned lipoproteins. It is a non-static layer containing various types of phenols which are mainly used in resistant to acids and organic solvents. IV. Cuticulin layer or outer epicuticle: This is very thin trilaminar layer only of about 15 nm. It is the first formed layer of new cuticle product at each molt protecting the new procuticle from the molting enzymes. The material forming the outer epicuticle is highly polymerized lipid is often referred to as cuticulin. It serves the purpose of permeability and also acts as growth barrier. B. Exocuticle: It is a thicker layer below epicuticle and is often darker and harder than the rest. It contributes rigidity and toughness to the cuticle. The exocuticle is wanting or considerably reduced in the regions of integument which are more flexible. The exocuticle structurally consists mainly of chitin and protein. Chitin is a polymerized nitrogenous polysaccharide linked to protein. It is pigmented by a hard brown material as is referred to as tanned. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP Other constituents of cuticle include quinones which polymerized to form dark brown or black pigment known as melanin and in others quinones are utilized in forming sclerotin. C. Endocuticle: The inner undifferentiated part below the exocuticle is endocuticle and is characterized by the presence of chitin. Endoskeleton of insect cuticle provides space for attachment of muscles of antenna and mouthparts, called as Tentorium. Epidermis: It is a cellular layer of one cell thick beneath endocuticle. The epidermal cells are glandular and secrete cuticle and the enzymes for the cuticle formation and digestion at the time of molting. Some of the epidermal cells have specialized glandular function. Basement membrane: The basement membrane is also called basal lamila. It is a noncellular layer beneath epidermis and serves as stable platform where epidermal cells are anchored and muscles are attached. Functions of insect cuticle Being an interface between a living animal and an environment, the cuticle of an insect serves many functions. 1. It limits the dimensions of an exoskeleton and is a basis for muscle insertions (mechanical function and function of locomotion). 2. It is an important element in organism defence against a variety of external factors, such as mechanical stresses, dry, wet, cold or hot environments. 3. It takes part in the transport of diverse epidermal secretions, and serves as a chemical reservoir for the storage of metabolic waste products. 4. A variety of cuticular structures are parts of mechano- and chemoreceptors. 5. The cuticle, its coloration pattern, and chemical components are important for thermoregulation, and are often involved in diverse communication systems. 6. Specialised cuticular protuberances may serve a variety of functions, such as oxygen retention, food grinding, body cleaning (grooming), etc.. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP Molting The molting process is triggered by hormones released when an insect's growth reaches the physical limits of its exoskeleton. Each molt represents the end of one growth stage (instar) and the beginning of another. In some insect species the number of instars is constant (typically from 3 to 5), but in others it may vary in response to temperature, food availability, or other environmental factors. An insect is known as an imago (adult) when it becomes sexually mature. At this point, molting stops and energy for growth is channeled into production of eggs or sperm. An insect cannot survive without the support and protection of its exoskeleton, so a new, larger replacement must be constructed inside the old one -- much like putting an overcoat under a sweater! The molting process begins when epidermal cells respond to hormonal changes by increasing their rate of protein synthesis. This quickly leads to apolysis -- physical separation of the epidermis from the old endocuticle. Epidermal cells fill the resulting gap with an inactive molting fluid and then secrete a special lipoprotein (the cuticulin layer) that insulates and protects them from the molting fluid's digestive action. This cuticulin layer becomes part of the new exoskeleton's epicuticle. After formation of the cuticulin layer, molting fluid becomes activated and chemically "digests" the endocuticle of the old exoskeleton. Break-down products (amino acids and chitin microfibrils) pass through the cuticulin layer where they are recycled by the epidermal cells and secreted under the cuticulin layer as new procuticle (soft and wrinkled). Pore canals within the procuticle allow movement of lipids and proteins toward the new epicuticle where wax and cement layers form. When the new exoskeleton is ready, muscular contractions and intake of air cause the insect's body to swell until the old exoskeleton splits open along lines of weakness (ecdysial sutures). The insect sheds its old exoskeleton (ecdysis) and continues to fully expand the new one. Over the next few hours, sclerites will harden and darken as quinone cross-linkages form within the exocuticle. This process (called sclerotization or tanning) gives the exoskeleton its final texture and appearance. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP An insect that is actively constructing new exoskeleton is said to be in a pharate condition. During the days or weeks of this process there may be very little evidence of change. Ecdysis, however, occurs quickly (in minutes to hours). A newly molted insect is soft and largely unpigmented (white or ivory). It is said to be in a teneral condition until the process of tanning is completed (usually a day or two). Summary 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 Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP Lecture no. 6 Body segmentation: Structure of head & its types Insect have segmented bodies with certain segments fusing to form three usually well defined regions: head, thorax and abdomen. The grouping of segments into functional regions is known as tagmosis. Head consists of 6 segments. Head comprises of mouthparts, compound eyes, simple eyes (ocelli) and a pair of antennae. Thorax consists of 3 segments i.e. prothorax, mesothorax and metathorax. Meso and metathorax are together known as pterothorax. All the three thoracic segments possess a pair of legs and meso and meta-thorax each possess one pair of wings. Abdomen has 7-11 segments with genital appendages. The 8 and 9 segments in female and 8 segment in male insects are modified to bear genital appendices. The outer parts of every segments of body is hard, colored and complex in nature. There are hard, colored and complex plates in each segments of insect body which is called sclerites. The sclerites present on dorsal side is called tergides, Lateral side is called pleurites and Ventral side is called sternites. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP Segments except in head (6 segments fused together to form a capsule) joint together with a flexible soft and white membrane called inter segmental membrane or suture. Insect Head Insect head is a hard and highly sclerotized compact structure. It is the foremost part in insect body consisting of 6 segments that are fused to form a head capsule. The head is connected to the thorax by a flexible neck or cervis strengthened by small cervical sclerites.
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