Unit3 - Subjective Questions

Metamorphosis: It is the abrupt change in the form or structure of an insect during its post-embryonic development as it moves from the immature stage to the adult stage.

Differences between Hemimetabolous and Holometabolous Metamorphosis:

Definition: Diapause is a period of arrested development or suspended animation in insects, characterized by low metabolic activity. It is a physiological adaptation to survive adverse environmental conditions (extreme temperature, drought, or lack of food).

Key Characteristics:

• Hormonally controlled (mainly by Ecdysone and Juvenile Hormone).
• Can occur at any stage (Egg, Larva, Pupa, or Adult).

Types of Diapause:

1. Obligatory Diapause:
   • Occurs in every generation regardless of environmental conditions.
   • Common in univoltine insects (one generation per year).
   • Example: Silkworm (Bombyx mori - univoltine race).
2. Facultative Diapause:
   • Occurs only when induced by environmental cues (photoperiod, temperature).
   • Common in multivoltine insects.
   • Example: Cotton bollworm undergoing diapause in winter.

Insect larvae are broadly classified into three main groups based on appendages:

1. Protopod Larvae:

• Primitive type; embryos hatch early.
• Have ill-defined appendages.
• Example: Parasitic Hymenoptera.

2. Polypod (Eruciform) Larvae:

• Cylindrical body with distinct segmentation.
• Possess 3 pairs of thoracic legs and 2 to 5 pairs of abdominal prolegs (pseudolegs).
• Slow-moving, mostly phytophagous.
• Example: Lepidopteran caterpillars (Butterflies/Moths).

3. Oligopod (Campodeiform/Scarabaeiform) Larvae:

• Possess well-developed 3 pairs of thoracic legs but NO abdominal prolegs.
• Campodeiform: Active predators, flattened body (e.g., Lacewings).
• Scarabaeiform: C-shaped, sluggish, grub-like (e.g., White grubs).

4. Apod Larvae:

• Completely legless (no thoracic or abdominal legs).
• Often live in soil, plant tissue, or decaying matter.
• Example: Maggots (Housefly), Weevil grubs.

1. Obtect Pupa:

• Structure: The appendages (legs, wings, antennae) are firmly glued or soldered to the body wall.
• Mobility: Generally immovable (except abdominal twitching).
• Protection: Often enclosed in a cocoon.
• Example: Moths and Butterflies (Chrysalis).

2. Exarate Pupa:

• Structure: The appendages are free and not glued to the body wall; they hang loosely.
• Mobility: Can show slight movement.
• Example: Beetles (Coleoptera), Bees/Wasps (Hymenoptera).

3. Coarctate Pupa:

• Structure: The pupa itself is exarate but is enclosed within the hardened last larval skin called the Puparium.
• Appearance: Barrel-shaped and smooth.
• Example: Houseflies and Fruit flies (Diptera).

The Foregut (Stomodaeum) is the anterior part of the alimentary canal, lined with cuticular intima (ectodermal origin). Its primary function is ingestion, storage, and grinding of food.

Main Parts and Functions:

1. Pharynx: Muscular pump that sucks food into the mouth (well-developed in sap-suckers).
2. Oesophagus: A simple tube conducting food from the pharynx to the crop.
3. Crop: A dialated storage sac. It holds food temporarily before digestion. Partial digestion may occur here via salivary enzymes.
4. Gizzard (Proventriculus): Contains cuticular teeth or plates.
   • Function: Grinds solid food (in chewing insects like cockroaches) or acts as a valve to filter particles (in honey bees).
5. Stomodaeal Valve: A sphincter separating the foregut from the midgut, preventing regurgitation.

Definition: The Filter Chamber is a specialized modification of the alimentary canal where the anterior part of the midgut and the hindgut come in close contact, often enclosed in a connective tissue sheath.

Mechanism:

• It allows excess water and soluble sugars from the ingested sap to bypass the main absorptive section of the midgut.
• Fluid moves directly from the foregut/anterior midgut to the hindgut.

Insects Involved: Found in fluid-feeding insects of the order Hemiptera (e.g., Aphids, Leafhoppers, Scale insects).

Significance:

1. Osmoregulation: Prevents the dilution of haemolymph by allowing rapid excretion of excess water.
2. Nutrient Concentration: Concentrates the solid nutrients (amino acids/proteins) in the midgut for better enzymatic digestion and absorption.

Open Circulatory System: unlike vertebrates, insects lack capillaries and veins. The blood (Haemolymph) flows freely within the body cavity (Haemocoel), bathing the internal organs directly.

Structure of Dorsal Vessel:
It is the main pumping organ located mid-dorsally. It is divided into:

1. Aorta: The anterior tubular part (thoracic region) without valves. It conducts blood to the head.
2. Heart: The posterior pulsatile part (abdominal region), divided into chambers by valves (ostia).
   • Ostia: Slit-like openings that allow blood to enter the heart from the haemocoel but prevent backflow.
   • Alary Muscles: Wing-shaped muscles responsible for the expansion (diastole) of the heart.

Mechanism of Flow:

1. Diastole: Alary muscles contract Heart expands Haemolymph enters heart through Ostia.
2. Systole: Heart muscles contract Ostia close Haemolymph is pumped anteriorly through the Aorta.
3. Circulation: Blood pours into the head cavity flows back through the thorax and abdomen re-enters the heart.

Structure:

• Malpighian tubules are thin, blind-ending, filamentous tubes floating in the haemolymph.
• They arise at the junction of the Midgut and Hindgut.
• The number varies (e.g., 60-70 in Cockroaches, 2-4 in bugs).

Physiological Function:

1. Filtration: Potassium and sodium ions are actively transported from the haemolymph into the tubule lumen. Water follows by osmosis.
2. Waste Absorption: Nitrogenous wastes (mostly Uric Acid) enter the tubules.
3. Precipitation: As the fluid moves toward the hindgut, water and essential ions are reabsorbed (often in the rectum), causing Uric Acid to precipitate.
4. Excretion: The solid uric acid is expelled with feces (uricotelic excretion), which is crucial for water conservation in terrestrial insects.

Insects primarily respire via a network of internal tubes called the Tracheal System.

1. Spiracles:

• External openings located laterally on the thorax and abdomen.
• Usually 10 pairs (2 thoracic, 8 abdominal).
• Equipped with valves to regulate air entry and minimize water loss (Atrium).

2. Tracheae:

• Large air tubes branching from spiracles.
• Lined with cuticle forming spiral ridges called Taenidia, which prevent the tubes from collapsing under pressure.
• They branch repeatedly into smaller tubes.

3. Tracheoles:

• The finest terminal branches (less than 1 micron diameter).
• Site of Gas Exchange: They contact individual cells and tissues directly.
• Fluid at the tip of tracheoles facilitates the diffusion of Oxygen into cells and Carbon dioxide out of cells.

Mechanism: Oxygen enters Spiracles Tracheae Tracheoles Tissues (via diffusion).

Aquatic insects have modified systems to obtain oxygen from water.

1. Tracheal Gills:

• Found in immature stages (nymphs/larvae) like Mayflies and Damselflies.
• These are thin-walled, leaf-like or filamentous extensions of the body wall.
• Richly supplied with tracheae.
• Oxygen dissolved in water diffuses across the gill wall into the tracheal system.

2. Plastron Respiration:

• Found in some aquatic bugs and beetles (e.g., Aphelocheirus).
• Mechanism: A permanent film of air (Plastron) is held against the body by hydrophobic hairs (hydrofuge pile).
• This air film acts as a physical gill: as the insect consumes oxygen from the bubble, the partial pressure drops, causing more oxygen to diffuse from the water into the bubble. It does not need to be replenished at the surface.

1. Central Nervous System (CNS):

• The main integration center, running ventrally along the body.
• Brain (Supra-oesophageal ganglion): Located in the head; coordinates sensory input (eyes, antennae).
• Sub-oesophageal ganglion: Controls mouthparts.
• Ventral Nerve Cord: A chain of paired ganglia linked by connectives, extending through the thorax and abdomen.

2. Visceral (Sympathetic) Nervous System:

• Controls involuntary actions of internal organs.
• Stomatogastric: Innervates the foregut, midgut, and heart.
• Ventral Visceral: Associated with spiracles.
• Caudal Visceral: Innervates the reproductive organs and posterior hindgut.

3. Peripheral Nervous System (PNS):

• Consists of nerves radiating from the CNS ganglia to the muscles and sensory receptors (sensilla) on the body wall.
• It relays sensory information to the CNS and motor commands to muscles.

The insect brain lies in the head, dorsal to the oesophagus. It is formed by the fusion of three pairs of ganglia:

1. Protocerebrum (Forebrain):

• The largest part.
• Innervates the Compound Eyes (via optic lobes) and Ocelli.
• Contains the Mushroom Bodies, which are centers for learning and higher coordination.

2. Deutocerebrum (Midbrain):

• Innervates the Antennae.
• Processes olfactory (smell) and tactile signals.

3. Tritocerebrum (Hindbrain):

• The smallest part.
• Connects the brain to the Sub-oesophageal ganglion (via circum-oesophageal connectives) and the Visceral nervous system (Labrum and foregut innervation).

The insect endocrine system regulates molting (ecdysis) and metamorphosis via hormones.

1. Brain (Neurosecretory Cells):

• Produces PTTH (Prothoracicotropic Hormone).
• Stimulates the Prothoracic glands.

2. Prothoracic Glands:

• Located in the prothorax.
• Function: Secrete Ecdysone (Molting Hormone).
• Role: Ecdysone initiates the molting process (apolysis and formation of new cuticle).

3. Corpora Allata:

• Paired glands behind the brain.
• Function: Secrete Juvenile Hormone (JH).
• Role: Determines the result of the molt.
  • High JH + Ecdysone Larva to Larva molt.
  • Low JH + Ecdysone Larva to Pupa molt.
  • Absent JH + Ecdysone Pupa to Adult molt.

Female Reproductive System:

• Ovaries: Paired organs composed of egg tubes called Ovarioles (produce eggs).
• Lateral Oviducts: Tubes carrying eggs from ovaries.
• Common Oviduct: Formed by the union of lateral oviducts.
• Spermatheca: A pouch to store sperm received during copulation (fertilization is often delayed).
• Accessory Glands: Secrete adhesive to stick eggs to substrates or form ootheca (e.g., Cockroach).

Male Reproductive System:

• Testes: Paired organs containing Testicular Follicles (produce sperm).
• Vas Deferens: Tubes carrying sperm from testes.
• Seminal Vesicle: Dilated part of vas deferens for sperm storage.
• Ejaculatory Duct: Muscular tube leading to the aedeagus (penis).
• Accessory Glands: Secrete fluid for spermatophore formation or sperm nourishment.

Definition: Parthenogenesis is a form of reproduction where the egg develops into an embryo without fertilization by sperm.

Types:

1. Arrhenotoky (Haplodiploidy):

   • Unfertilized eggs develop into Males (haploid).
   • Fertilized eggs develop into Females (diploid).
   • Example: Hymenoptera (Honey bees, Wasps).

2. Thelytoky:

   • Unfertilized eggs develop only into Females.
   • Males are generally absent or rare.
   • Example: Aphids (during summer), Stick insects.

3. Amphitoky (Deuterotoky):

   • Unfertilized eggs can develop into either Males or Females.
   • Example: Some Cynipid wasps.

1. Viviparity:

• The phenomenon where insects give birth to live young (nymphs or larvae) instead of laying eggs.
• The embryonic development takes place inside the mother's body.
• Example: Aphids (produce nymphs directly in summer), Tsetse fly (larviparous - gives birth to full-grown larva).

2. Polyembryony:

• A form of asexual reproduction where a single fertilized egg (zygote) divides repeatedly to produce two or more embryos.
• It is an adaptation for rapid multiplication, often seen in parasitic insects.
• Example: Parasitic wasps like Platygaster (can produce hundreds of larvae from one egg inside the host).

Compound Eye: The primary visual organ in adults and nymphs, consisting of thousands of hexagonal units aggregated together.

Ommatidium (Structural Unit):
Each ommatidium functions as a separate visual receptor. It consists of:

1. Optical Part (Light gathering):
   • Cornea: Transparent cuticular lens.
   • Crystalline Cone: Located beneath the cornea, focuses light.
2. Sensory Part (Light detection):
   • Retinular Cells: Sensory neurons (usually 8) arranged radially.
   • Rhabdom: A central rod-like structure formed by the microvilli of retinular cells; contains visual pigments (rhodopsin).
3. Pigment Cells:
   • Surround the ommatidium to isolate it optically from neighbors (prevents light scattering).

Sensilla: These are the basic structural and functional units of sensory organs in insects, typically consisting of a specialized cuticular structure (hair, plate, pit) and associated sensory neurons.

1. Mechanoreceptors (Touch/Sound/Position):

• Detect physical deformation, touch, vibration, or gravity.
• Trichoid Sensilla: Hair-like tactile organs (touch).
• Campaniform Sensilla: Dome-shaped (detect stress/strain on cuticle).
• Johnston's Organ: Located in the pedicel of antennae (detects flight speed/sound in mosquitoes).

2. Chemoreceptors (Taste/Smell):

• Detect chemical molecules.
• Olfactory (Smell): Usually on antennae (e.g., detecting pheromones or host plant odours). Multi-porous walls.
• Gustatory (Taste): Usually on mouthparts or tarsi (feet). Uni-porous (tip) walls.

Structure:

• The Peritrophic Membrane is a thin, semi-permeable, chitinous mesh-like sheath.
• It is secreted by the cells of the midgut (in some insects) or by the stomodaeal valve.
• It surrounds the food bolus within the lumen of the midgut.

Functions:

1. Protection: It protects the delicate epithelial cells of the midgut from mechanical abrasion by hard food particles.
2. Ultra-filtration: It acts as a sieve, allowing enzymes and digested nutrients to pass through to the gut wall for absorption, while keeping bacteria and large undigested particles inside the lumen to be excreted.
3. Pathogen Barrier: It serves as a partial barrier against infection by certain pathogens.