Insect Body Wall: Cuticle, Epidermis & Membrane

• Insect body wall consists of the cuticle, epidermis, and basement membrane.
• The epidermis and cuticle together form the integument.
• The insect body wall is a segmented cylinder, with segments movable relative to each other.
• The tergum and sternum are tougher than the pleural membrane.
• Pleural membrane is made of infoldings of the body, allowing expansion during ingestion or when females are gravid.

Epidermis

• Single layer of glandular cells beneath the cuticle.
• Secretes the cuticle (including enzymes for production/digestion during molting) and the basement membrane.
• Neurons and support cells of sensory receptors associated with the cuticle are derived from epidermal cells.

Basement Membrane

• Produced by epidermal cells and plasmatocytes.
• Components include fibrous protein, collagen, glycoproteins, and glycosaminoglycans.
• Epidermal cells are anchored by hemidesmosomes.

Cuticle

• Secretion of the epidermal layer, covering the entire insect body and lining ectodermal invaginations.
• Inert layer with internal extensions forming the exoskeleton, apodemes (for support/muscle attachment), and wings.
• Functions as a barrier between living tissues and environment.
• Apodemes have fossae i.e. modified surfaces for muscle attachment.
• Lines tracheal tubes, some gland ducts, and parts of the digestive tract internally (foregut and hindgut).
• Ranges from rigid (beetles) to flexible (larvae).
• Critical function is restricting water loss.
• Two major regions: procuticle and epicuticle.
• Epicuticle is thin (0.1 µm - 3 µm), consisting of inner, outer, and superficial layers.
• Superficial layer often has a lipid/wax layer.
• Oenocytes in the epidermis synthesize cuticle lipids (hydrocarbons).
• Epicuticle chemistry prevents dehydration through hydrophobic lipids, especially hydrocarbons.
• Surface waxes deter predation, provide mimicry/camouflage patterns, repel rainwater, reflect radiation, or give olfactory cues.
• Epicuticle is inextensible and unsupportive.
• Procuticle gives support, differentiating into a thicker endocuticle and a thinner, sclerotized exocuticle.
• Newly formed region is called the procuticle.
• Procuticle is 10 µm - 0.5 mm thick, made of chitin complexed with protein.
• Epicuticle lacks chitin.
• Chitin is an unbranched polymer of N-acetyl-d-glucosamine units.
• Extensive hydrogen bonding of chitin chains and sclerotization provide strength.
• Sclerotization is irreversible, darkens the exocuticle, and makes proteins water-insoluble.
• Sclerotization results from phenolic bridges (quinone tanning) or dehydration of chains.
• Only exocuticle becomes sclerotized.

Process and control of Moulting

• Growth is limited by the cuticle.
• Marked increase in insect size requires shedding the old cuticle and replacing it.
• Insects attain the new size before the new cuticle hardens.
• Shedding the old cuticle and replacing it with a new one is called moulting.
• Moulting involves apolysis and ecdysis.
• Apolysis is the separation of the old cuticle from epidermal cells.
• Apolysis signals the beginning of moulting process
• Ecdysis is the shedding of the old cuticle remnants.
• Moulting involves hormonal, behavioral, epidermal, and cuticular changes.
• Epidermal cells break down the old cuticle and form the new one.
• Moult commences with retraction of epidermal cells from the old cuticle, usually antero-posteriorly.
• Muscles and sensory nerves retain a connection with the old cuticle
• Apolysis is associated with mitotic division of epidermal cells.
• The cells become closely packed and there is a net increase in the number of cells per unit area such that the overall area of the epidermis is increased.
• Separation creates an apolysial or exuvial space filled with inactive molting fluid (ecdysone).
• Molting fluid contains chitinase and proteinase, which are activated after the new epicuticle is laid down.
• Activation of the two enzymes is associated with active transport of potassium into the space.
• Activation prevents digestion of the new cuticle.
• Endocuticle of the old cuticle is lysed and resorbed.
• New cuticle is deposited as undifferentiated procuticle.
• Digested components are recycled by epidermal cells.
• Digestive enzymes don't affect exocuticle, muscles, and nerve connections initially.
• Connections are broken at ecdysis.
• Old cuticle becomes thin along the ecdysial line.
• Insects emerge through this line, leaving behind the old skin.
• Ecdysial line position varies; in locusts, it's an inverted Y-shape (epicranial suture).
• Ecdysis follows completion of the new endocuticle.
• Pre-ecdysis behavior loosens the old cuticle, varying among species.
• Insects split the old cuticle by increasing pressure from within.
• They swallow air/water to increase haemolymph pressure so that special muscles pump more blood to the part of the body where the ecdysial line is situated.
• The process leads to expansion and splitting along weakness lines.
• Insect emerges, usually head and thorax first, then abdomen.
• Suspension from a support aids in emergence.
• Newly ecdysed insect expands the new cuticle by swallowing air/water and increasing haemolymph pressure.
• Expansion smooths out wrinkles and stretches the procuticle.
• Body surface may become sclerotized, stiffening and darkening the procuticle to form exocuticle.
• The insect attains a new size for further growth before the new cuticle hardens (sclerotized).
• Shed old cuticle is the exuvium (exuviae), consisting of indigestible protein, lipid, and chitin of the old epicuticle and exocuticle.
• Newly eclosed insect's cuticle is pale, tanning within hours.
• Wing expansion, exocuticle sclerotization, and postmoult endocuticle deposition are controlled by bursicon.
• Following ecdysis, more proteins and chitin are secreted, adding to the endocuticle.
• Hormones act on epidermal cells to effect cuticular changes and on the nervous system to coordinate ecdysis behaviors.
• Three major hormone types control moulting and metamorphosis: neuropeptides, ecdysteroids, and juvenile hormone (JH).
  • Prothoracicotropic hormone (PTTH) induces ecdysteroid secretion from prothoracic gland.
  • Ecdysis triggering hormone (ETH) initiates events at ecdysis.
  • Eclosion hormone (EH) controls events at ecdysis; eclosion is adult insect emergence.
  • JH regulates metamorphosis and reproduction.

Explore the insect body wall structure, including the cuticle, epidermis, and basement membrane. Learn about the integument, the role of the epidermis in cuticle secretion, and the composition of the basement membrane. Understand the function of the pleural membrane and the segmentation of the insect body.