Unit 3: Insect Anatomy

1. Metamorphosis and Diapause

Metamorphosis

Metamorphosis is the marked change in form and structure that an insect undergoes during post-embryonic development (from hatching to adulthood). It involves histolysis (breakdown of old tissues) and histogenesis (formation of new tissues).

Types of Metamorphosis:

1. Ametabola (No Metamorphosis):

   • The insect hatches from the egg resembling a miniature adult.
   • It undergoes moulting only to increase in size and reach sexual maturity.
   • Examples: Silverfish (Thysanura), Springtails (Collembola).
   • Stages: Egg Young Adult.

2. Hemimetabola (Incomplete Metamorphosis):

   • Paurometabola (Gradual Metamorphosis): Nymphs resemble adults but lack wings and external genitalia. They live in the same habitat as adults. (e.g., Grasshoppers, Bugs, Cockroaches).
   • Hemimetabola (Proper): The immature stage is aquatic (called a naiad) and breathes via gills, while the adult is terrestrial. (e.g., Dragonflies, Mayflies).
   • Stages: Egg Nymph/Naiad Adult (Imago).

3. Holometabola (Complete Metamorphosis):

   • Involves four distinct stages. The larva is morphologically, physiologically, and ecologically distinct from the adult.
   • Includes a resting stage called the pupa.
   • Examples: Butterflies, Beetles, Flies, Bees.
   • Stages: Egg Larva Pupa Adult.

Hormonal Control:

• Prothoracicotropic Hormone (PTTH): Stimulates prothoracic glands.
• Ecdysone (Moulting Hormone): Triggers the moulting process.
• Juvenile Hormone (JH): Determines the character of the moult. High JH = Larva to Larva; Low JH = Larva to Pupa; Absence of JH = Pupa to Adult.

Diapause

Diapause is a period of arrested development or suspended animation controlled by physiological factors (hormones) and environmental cues (photoperiod, temperature, food quality).

• Obligatory Diapause: Genetically determined; occurs at a specific stage in every generation regardless of environment (common in univoltine insects).
• Facultative Diapause: Induced by adverse environmental conditions.
• Stages: Diapause can occur in the egg (silkworm), larva (corn borer), pupa (moths), or adult (certain beetles) stages.

2. Types of Larvae and Pupae

Types of Larvae (Immature stage of Holometabola)

1. Protopod: Embryonic form; appendages are rudimentary. Occurs in endoparasitic Hymenoptera.
2. Polypod (Eruciform): Cylindrical body with distinct thoracic legs and abdominal prolegs (pseudolegs).
   • Caterpillar: 3 pairs of thoracic legs, 2–5 pairs of prolegs (Lepidoptera).
   • Semilooper: 3 pairs of prolegs are missing (Lepidoptera).
3. Oligopod: Well-developed thoracic legs; no abdominal prolegs.
   • Campodeiform: Active, predatory, flattened body (Ladybird beetle).
   • Scarabaeiform: C-shaped, sluggish, fleshy body (White grubs/Dung beetles).
4. Apod: No legs.
   • Eucephalous: Well-developed head capsule (Mosquito wriggler).
   • Hemicephalous: Reduced head (Honey bee).
   • Acephalous: Head absent, mouth hooks present (Housefly maggot).

Types of Pupae (Resting stage)

1. Obtect: Appendages (legs, wings, antennae) are firmly glued to the body.
   • Example: Moths and butterflies (Chrysalis).
2. Exarate: Appendages are free and not glued to the body.
   • Example: Beetles, bees, wasps.
3. Coarctate: The pupa is enclosed within the hardened last larval skin, known as the puparium.
   • Example: Diptera (Houseflies).

3. Digestive System (Alimentary Canal)

The alimentary canal is a long tube extending from the mouth to the anus, divided into three regions:

1. Stomodeum (Foregut)

• Origin: Ectodermal (lined with cuticle called intima).
• Pharynx: Muscular pump to ingest food.
• Oesophagus: Tube conducting food.
• Crop: Dilated sac for the temporary storage of food.
• Proventriculus (Gizzard): Contains chitinous teeth/plates for grinding solid food. Contains the stomodeal valve to prevent regurgitation.

2. Mesenteron (Midgut)

• Origin: Endodermal (Not lined with cuticle; lined with epithelial cells).
• Function: Principal site of enzyme secretion, digestion, and absorption.
• Gastric Caeca: Finger-like projections at the anterior end that increase surface area for absorption.
• Peritrophic Membrane: A semi-permeable mesh produced by midgut cells that surrounds the food bolus, protecting delicate epithelial cells from abrasion while allowing enzymes to pass.

3. Proctodeum (Hindgut)

• Origin: Ectodermal (lined with cuticle/intima).
• Pyloric Valve: Separates midgut from hindgut.
• Ileum & Colon: Anterior parts of the hindgut.
• Rectum: Posterior part containing rectal pads responsible for the reabsorption of water, salts, and amino acids from feces (crucial for water conservation).

4. Circulatory System

Insects possess an Open Circulatory System. The body cavity is called the Haemocoel, filled with blood (Haemolymph).

Haemolymph

• Composed of plasma and blood cells (Haemocytes).
• Functions: Transports nutrients, hormones, and waste; provides hydrostatic pressure for moulting/wing expansion; immune response (phagocytosis/encapsulation).
• Note: Insect blood generally lacks respiratory pigments (haemoglobin) and does not transport oxygen (except in Chironomid larvae).

Dorsal Vessel

The primary pulsatile organ located dorsally.

1. Heart: Posterior portion consisting of segmented chambers. Each chamber has lateral openings called Ostia (valves) that allow blood to enter but not exit.
2. Aorta: Anterior tubular extension that conducts blood to the head.

Accessory Pulsatile Organs

Located at the base of wings, legs, and antennae to ensure circulation in narrow appendages.

5. Excretory System

Malpighian Tubules

• The primary excretory organs (analogous to kidneys).
• Blind-ended tubules floating in the haemolymph, attached at the junction of the midgut and hindgut.
• Mechanism: They filter nitrogenous waste (uric acid), salts, and water from the haemolymph and discharge them into the hindgut.

Nitrogenous Waste

• Uric Acid: The primary waste product in terrestrial insects (Uricotelic). It is non-toxic and insoluble, allowing excretion as a solid paste to conserve water.
• Ammonia: Excreted by aquatic insects (Ammonotelic).

Accessory Excretory Structures

• Nephrocytes: Cells that sieve waste.
• Fat body: Stores metabolic wastes (storage excretion).
• Integument: Accumulates waste which is shed during moulting.

6. Respiratory System

Insects use a Tracheal System for gas exchange, delivering oxygen directly to tissues without using blood.

Structure

1. Spiracles: Lateral openings on the thorax and abdomen (usually 2 thoracic pairs + 8 abdominal pairs). Controlled by valves to prevent water loss.
2. Tracheae: Tubes lined with spiral cuticular thickenings called taenidia (prevents collapse).
3. Tracheoles: Fine, fluid-filled terminal branches where gas exchange occurs directly with cells by diffusion.

Modifications

• Closed Tracheal System: Spiracles are absent or non-functional. Used by aquatic larvae (e.g., damselflies) utilizing Tracheal Gills.
• Plastron Respiration: A permanent film of air held by hydrofuge hairs in aquatic bugs.
• Air Sacs: Dilated tracheae in flying insects (e.g., bees) to reduce specific gravity and store air.

7. Nervous System

1. Central Nervous System (CNS)

• Brain (Supra-oesophageal Ganglion):
  • Protocerebrum: Innervates eyes/ocelli.
  • Deutocerebrum: Innervates antennae.
  • Tritocerebrum: Connects to the visceral system and labrum.
• Sub-oesophageal Ganglion: Controls mouthparts (mandibles, maxillae, labium).
• Ventral Nerve Cord: A double chain of ganglia running along the ventral floor. Typically one ganglion per segment, though fusion is common in advanced orders.

2. Visceral (Sympathetic) Nervous System

• Stomatogastric: Controls the foregut, midgut, and heart.
• Caudal: Controls the reproductive organs and hindgut.

3. Peripheral Nervous System

• Network of nerves connecting the CNS to sensory receptors (muscles and skin).

Neuron Types:

• Sensory (Afferent): Receptor to CNS.
• Motor (Efferent): CNS to effector (muscle).
• Association (Interneuron): Links sensory and motor neurons.

8. Secretory (Endocrine) System

This system comprises ductless glands that secrete hormones into the haemolymph.

Major Endocrine Centers

1. Neurosecretory Cells (in Brain): Secrete Brain Hormone (PTTH).
2. Corpora Cardiaca: Stores and releases PTTH; regulates heartbeat.
3. Prothoracic Glands: Located in the thorax. Secrete Ecdysone (moulting hormone) when stimulated by PTTH. Degenerates in adults (except in Apterygotes).
4. Corpora Allata: Located behind the brain. Secretes Juvenile Hormone (JH). Also regulates yolk deposition in adult female eggs.

Exocrine Glands (Secretions exit body)

• Salivary glands: Digestive enzymes/silk production.
• Pheromone glands: Chemical communication (sex attractants, alarm, trail).
• Wax glands: Honey bees, mealybugs.
• Lac glands: Shellac production.

9. Reproductive System

Male Reproductive System

• Testes: Paired organs containing testicular follicles where spermatogenesis occurs.
• Vas Deferens: Tubes carrying sperm from testes.
• Seminal Vesicle: Dilated section of vas deferens for sperm storage.
• Ejaculatory Duct: Single muscular tube leading to the copulatory organ.
• Aedeagus: The intromittent organ (penis).
• Accessory Glands: Secrete fluid (spermatophore) to nourish and transport sperm.

Female Reproductive System

• Ovaries: Paired organs composed of egg tubes called ovarioles.
• Lateral Oviducts: Tubes from ovaries.
• Common Oviduct: Union of lateral oviducts.
• Spermatheca: A sac-like structure to store sperm after mating. Fertilization occurs as eggs pass this organ.
• Accessory (Colleterial) Glands: Secrete adhesive to glue eggs to substrates or form protective coverings (e.g., Ootheca in cockroaches).
• Bursa Copulatrix: Copulatory pouch (in some insects).

10. Types of Reproduction

1. Sexual Reproduction (Oviparity): Fusion of sperm and egg; eggs are laid and hatch later (Most insects).
2. Viviparity: Giving birth to living young.
   • Examples: Aphids, Tsetse fly.
3. Parthenogenesis: Reproduction without fertilization.
   • Arrhenotoky: Unfertilized eggs produce males (Haplodiploidy in Bees/Wasps).
   • Thelytoky: Unfertilized eggs produce females (Aphids).
   • Amphitoky: Can produce both sexes.
4. Polyembryony: A single egg divides to produce multiple individuals (Parasitic Hymenoptera).
5. Paedogenesis: Reproduction by immature stages (larvae or pupae).
   • Example: Gall midges (Miastor sp.).

11. Major Sensory Organs

1. Photoreceptors (Light)

• Compound Eyes: Aggregation of units called Ommatidia. Responsible for mosaic vision and motion detection.
• Ocelli (Simple Eyes): Usually 3 on the vertex. Measure light intensity (circadian rhythms), not image formation.
• Stemmata (Lateral Ocelli): Visual organs of holometabolous larvae (e.g., caterpillars).

2. Chemoreceptors (Smell/Taste)

• Olfactory (Smell): Sensilla located primarily on the antennae (e.g., plumose antennae in male moths detect pheromones).
• Gustatory (Taste): Sensilla located on the mouthparts, tarsi (legs), and ovipositor.

3. Mechanoreceptors (Touch/Sound)

• Trichoid Sensilla: Hair-like setae (tactile).
• Johnston’s Organ: Located in the pedicel (2nd segment) of the antenna. Detects vibration and air speed (e.g., Mosquitoes).
• Tympanal Organs: "Ears" consisting of a membrane stretched over an air sac.
  • Location: 1st abdominal segment (Grasshoppers) or Tibia of forelegs (Crickets).