Insect Morphology and Physiology BZO6T16

Syllabus topics (RTMNU)

• Head and appendages: structure of head; types and functions of antennae; classification and modification of mouthparts
• Thorax and locomotory structures: structure of thorax; wings and basic venation; types of legs and modifications
• Abdomen and external genitalia: segmentation; abdominal appendages including cerci; male and female external genitalia
• Integumentary system: structure and formation of cuticle; chitin and proteins; moulting process; moulting enzymes and hormones

Head and appendages

The insect body has three tagmata — head, thorax and abdomen. The head is a hard capsule (fused segments) bearing compound eyes, ocelli, a pair of antennae and mouthparts. Regions: vertex, frons, clypeus, gena, occiput; sutures (epicranial Y-suture). Head orientation: hypognathous (mouth downward — grasshopper), prognathous (forward — beetles), opisthognathous (backward — bugs). Internally the tentorium braces the head.

Antennae (sensory — touch, smell, hearing, humidity) have scape, pedicel (with Johnston's organ) and flagellum. Types: filiform (thread-like — cockroach, grasshopper), moniliform (bead-like — termite), setaceous (bristle — dragonfly), serrate (saw-like — some beetles), pectinate/bipectinate (comb — male silk moth), clavate (club — butterfly), capitate (knob — some beetles), lamellate (plates — scarab beetle), geniculate (elbowed — ant, honeybee), plumose (feathery — male mosquito), pilose (female mosquito), aristate (bristle on 3rd segment — housefly).

Mouthparts — basic set: labrum (upper lip), a pair of mandibles, a pair of maxillae (with palps), labium (lower lip, with palps) and hypopharynx (tongue).

Thorax and locomotory structures

The thorax has three segments — prothorax, mesothorax and metathorax — each with a pair of legs; wings on meso- and metathorax (pterothorax). Each segment has a dorsal tergum (notum), ventral sternum and lateral pleura; internally apodemes and phragmata for muscle attachment. Spiracles on meso- and metathorax.

Wings: outgrowths of the body wall — two thin layers of cuticle with veins (tracheae, nerves, haemolymph). Comstock–Needham system of venation: costa (C), subcosta (Sc), radius (R), media (M), cubitus (Cu), anal veins (A), plus cross veins and cells. Types: membranous (bees, flies), tegmina (leathery forewings — cockroach, grasshopper), elytra (hard forewings — beetles), hemelytra (half leathery — bugs), scaly (butterflies), fringed (thrips), halteres (hindwings reduced to balancers — Diptera). Wing coupling (hamuli in bees, frenulum in moths). Flight: direct muscles (dragonflies) or indirect (dorso-longitudinal and dorso-ventral muscles distorting the thorax; asynchronous fibrillar muscles allow very fast beats in flies — hundreds per second).

Legs: segments — coxa, trochanter, femur, tibia, tarsus (1–5 tarsomeres) and pretarsus (claws, arolium/pulvilli). Modifications:

• Cursorial (running/walking) — cockroach.
• Saltatorial (jumping; enlarged hind femur) — grasshopper, flea.
• Natatorial (swimming; flattened, fringed hind legs) — water beetle (Dytiscus), backswimmer.
• Fossorial (digging; broad toothed forelegs) — mole cricket.
• Raptorial (grasping prey; spiny forelegs) — praying mantis.
• Scansorial (clinging) — head louse; pollen-collecting (corbicula/pollen basket) — honeybee hind leg; clasping (suckers) — male Dytiscus forelegs.

Abdomen and external genitalia

The abdomen primitively has 11 segments (plus telson); fewer are visible in many insects (e.g. 10 in cockroach, 6–7 visible in bees and flies). Each segment has tergum, sternum and soft pleural membrane bearing spiracles (usually 8 pairs). Adults lack walking appendages on the abdomen; pregenital appendages remain as styli (silverfish) or larval prolegs (caterpillars). Cerci — paired, segmented sensory appendages on the 11th segment (cockroach — wind-sensitive for escape; forceps in earwigs; long filaments in mayflies). Anal styles occur in male cockroach.

External genitalia (on segments 8–9): female — ovipositor of three pairs of valves (gonapophyses) on the 8th and 9th segments, used for laying eggs in soil, plant tissue or hosts (long in crickets, katydids and ichneumon wasps; modified into a sting in bees, wasps and ants); in flies the abdomen tip telescopes as an ovipositor. Male — aedeagus (intromittent organ/penis) with claspers (parameres) and phallomeres on the 9th segment — hold the female and transfer sperm/spermatophore. Genitalia are species-specific ("lock and key") and important in taxonomy.

Integumentary system — cuticle and moulting

Structure: the integument has a basement membrane, a single-layered epidermis (secretes cuticle; has dermal glands, trichogen cells), and the non-cellular cuticle:

• Epicuticle (outermost, thin, no chitin): cement layer, wax layer (waterproofing — prevents desiccation), outer epicuticle (cuticulin).
• Procuticle: exocuticle (hardened, dark, sclerotised by quinone tanning) and endocuticle (soft, flexible, lamellate); pore canals carry wax to the surface.

Composition: chitin (polymer of N-acetylglucosamine, β1→4 linked — like cellulose) as microfibrils in a protein matrix (arthropodin, sclerotin; resilin — rubber-like elastic protein in wing hinges and jumping legs), lipids/waxes, pigments, and in some calcium salts. Sclerotisation (cross-linking of proteins by quinones) gives hardness.

Moulting (ecdysis): the rigid cuticle limits growth, so it is periodically shed:

1. Apolysis — epidermis separates from old cuticle; moulting fluid (inactive) secreted.
2. Epidermal cells divide and secrete new epicuticle.
3. Moulting fluid activated — chitinases and proteases digest the old endocuticle (up to about 90% resorbed and reused).
4. New procuticle is deposited.
5. Ecdysis — insect swallows air/water, contracts muscles, splits the old cuticle along ecdysial lines and emerges; the cast skin is the exuviae.
6. New cuticle expands, then hardens and darkens (sclerotisation) — controlled by bursicon.

Hormonal control: brain neurosecretory cells release PTTH (prothoracicotropic hormone) via corpora cardiaca/allata → prothoracic glands secrete ecdysone (converted to active 20-hydroxyecdysone) → initiates moult; juvenile hormone (corpora allata) decides the type of moult (high JH — larval moult; low JH — pupal; absent — adult); ecdysis triggering hormone (ETH) and eclosion hormone start the shedding behaviour; bursicon hardens the cuticle.

Likely exam questions

1. (Long, 8 marks) Describe the different types of mouthparts in insects with examples.
2. (Long, 8 marks) Describe the structure of the insect cuticle and explain the process of moulting and its hormonal control.
3. (Short, 4 marks) Describe the types of insect antennae.
4. (Short, 4 marks) Write a note on modifications of insect legs.
5. (Short, 4 marks) What are elytra, tegmina and halteres?

Syllabus topics (RTMNU)

• Respiratory system: tracheal system; spiracles, tracheae, tracheoles; mechanism of gaseous exchange
• Respiratory adaptations: tracheal gills, cutaneous respiration, siphons, plastron, rectal gills; discontinuous respiration in terrestrial insects
• Respiratory metabolism: cellular respiration and energy production; cytochrome oxidase system
• Circulatory system: open type; dorsal vessel; haemolymph composition and functions; haemocytes; haemopoietic organs and immune molecules

Respiratory system — tracheal system

Insects breathe by a tracheal system — a network of air-filled tubes that carries O₂ directly to the tissues, so blood plays little part in gas transport.

• Spiracles: external openings, typically 10 pairs (2 thoracic + 8 abdominal) on the pleura; each has an atrium with filtering hairs and a closing valve worked by muscles to control air flow and reduce water loss. Arrangements: holopneustic (all 10 functional — cockroach, grasshopper), peripneustic, hemipneustic, amphipneustic (only first and last pairs — fly maggot), metapneustic (only last pair — mosquito larva), apneustic (none — aquatic larvae with gills).
• Tracheae: ectodermal tubes lined by cuticle (intima) thickened into spiral taenidia that keep them open; longitudinal trunks and transverse commissures; branches to all organs; shed at each moult.
• Tracheoles: fine (less than 1 µm), intracellular, fluid-tipped endings formed by tracheoblast cells, entering muscle fibres; not shed at moulting. Site of gas exchange.
• Air sacs: thin, dilated parts of tracheae (no taenidia) in active fliers (bees, locusts) — act as bellows, reduce weight, help ventilation and buoyancy.

Mechanism of gas exchange: (1) diffusion — sufficient for small or resting insects; O₂ diffuses down the tracheae; (2) active ventilation — in larger, active insects, abdominal pumping movements (dorso-ventral muscles) compress and expand tracheae and air sacs; coordinated opening and closing of spiracles gives a unidirectional flow (e.g. in grasshopper air enters through the thoracic spiracles and leaves through the abdominal). At the tracheole tips, fluid is withdrawn during activity (due to osmotic changes in muscle), so air reaches closer to cells — faster O₂ supply. CO₂ leaves mainly via tracheae, partly through the cuticle.

Physiological significance: very efficient direct delivery allows high metabolic rates in flight muscles (among the highest in animals); but diffusion limits body size; spiracle control conserves water.

Respiratory adaptations in aquatic insects

• Tracheal gills: thin, leaf-like or filamentous outgrowths richly supplied with tracheae in a closed (apneustic) system; O₂ diffuses from water — abdominal gills of mayfly nymphs, caudal lamellae of damselfly nymphs, filamentous gills of caddisfly larvae.
• Rectal gills: tracheated folds inside the rectum of dragonfly nymphs; water pumped in and out of the rectum (also used for jet propulsion).
• Cutaneous respiration: diffusion through thin cuticle in small aquatic larvae — chironomid (bloodworm) larvae (also have haemoglobin), early instars of many insects.
• Breathing tubes (siphons): air-breathing aquatic forms keep contact with the surface — mosquito larva (respiratory siphon with metapneustic spiracles; pupa has respiratory trumpets), rat-tailed maggot (Eristalis, telescopic tube), water scorpion (Nepa, Ranatra — caudal siphon).
• Air stores (compressible gills): a bubble of air carried under the elytra or on body hairs, e.g. Dytiscus, Notonecta; O₂ diffuses into it from water as it is used, extending dive time, but the bubble shrinks (N₂ dissolves) and must be renewed.
• Plastron (physical gill): a thin, permanent film of air held by a very dense pile of fine hydrofuge hairs (or a cuticular network) that resists collapse; acts as a permanent physical gill so the insect can stay submerged indefinitely in well-oxygenated water — Aphelocheirus (water bug), some beetles (Elmis), insect eggs (plastron in egg shells).
• Some insects (e.g. Donacia larvae) tap air spaces of aquatic plants with spines.

Discontinuous (cyclic) gas exchange in terrestrial insects (e.g. pupae of moths, ants, beetles at rest): a cycle of closed phase (spiracles shut; O₂ used, partial vacuum forms), flutter phase (spiracles open slightly and rapidly; air sucked in — little water loss), and open phase (spiracles open widely; CO₂, which accumulated in the haemolymph as bicarbonate, is released in a burst). Significance: water conservation (spiracles closed most of the time), avoiding oxygen toxicity in tissues, and adaptation to hypoxic/hypercapnic burrows.

Respiratory metabolism

Insects obtain energy by oxidising carbohydrates (trehalose — main blood sugar, glycogen), lipids (diglycerides carried by lipophorin — main fuel for long flights of locusts and moths) and amino acids (proline — fuel in tsetse fly and Colorado beetle). Pathways: glycolysis in the cytoplasm, Krebs cycle and electron transport in the numerous, large mitochondria (sarcosomes) of flight muscle. The α-glycerophosphate shuttle rapidly transfers cytosolic NADH reducing equivalents into mitochondria (insect flight muscle produces little lactate). Respiratory quotient indicates fuel: about 1.0 for carbohydrate (bees, flies), about 0.7 for fat (locusts on long flights).

Cytochrome oxidase system: the terminal enzyme (Complex IV) of the electron transport chain, containing cytochromes a and a₃ and copper; it accepts electrons from cytochrome c and reduces O₂ to water, pumping protons and contributing to the proton gradient used by ATP synthase. Discovered through studies including Keilin's work on insect flight muscle cytochromes. Its very high activity in flight muscle allows huge ATP production; it is inhibited by cyanide, H₂S and carbon monoxide — the basis of fumigants (HCN) used against stored-grain pests; insecticides like rotenone (Complex I) and some acaricides (mitochondrial complex inhibitors) act on the same chain.

Circulatory system

Insects have an open (lacunar) circulatory system: haemolymph fills the body cavity (haemocoel), which is divided by dorsal and ventral diaphragms into pericardial, perivisceral and perineural sinuses.

Dorsal vessel: a long tube along the mid-dorsal line — posterior heart (in the abdomen, chambered, with paired lateral valved openings called ostia, supported by alary muscles; cockroach has 13 chambers) and anterior aorta (narrow, without ostia, opening in the head).

Mechanism: peristaltic contractions of the heart (systole) from posterior to anterior pump haemolymph forward into the aorta and head; during relaxation (diastole), aided by alary muscles, haemolymph from the pericardial sinus enters through the ostia. It flows backward through the sinuses; accessory pulsatile organs at the bases of antennae, wings and legs push it into appendages. Heart beat is myogenic (in many) with neural modulation (cardioacceleratory peptides).

Haemolymph composition: plasma (about 90% water) with ions (often high Mg²⁺, K⁺ in herbivores), very high free amino acids, sugars (trehalose), proteins (lipophorin, vitellogenin, storage hexamerins), lipids, hormones, uric acid; usually colourless or greenish/yellow (respiratory pigment only in a few — haemoglobin in Chironomus larvae); clotting factors.

Functions: transport of nutrients, hormones and wastes; water reservoir; hydraulic pressure for moulting, eclosion and wing expansion; heat distribution; wound healing and clotting; immunity.

Haemocytes: prohaemocytes (stem cells), plasmatocytes (phagocytosis, encapsulation), granulocytes (phagocytosis, encapsulation), spherule cells, oenocytoids (prophenoloxidase — melanisation), coagulocytes/cystocytes (clotting), adipohaemocytes; in Drosophila — plasmatocytes, crystal cells and lamellocytes.

Haemopoietic organs and immunity: haemocytes are produced in haemopoietic organs (e.g. the lymph gland along the dorsal vessel in Drosophila, near wing discs in Lepidoptera) and by division in circulation. Insects have innate immunity: physical barriers (cuticle, gut lining); cellular responses — phagocytosis, nodulation, encapsulation of parasitoid eggs; humoral responses — the prophenoloxidase cascade (melanin deposition), lysozyme, lectins and antimicrobial peptides (cecropins, defensins, attacins, drosomycin) made by the fat body via Toll and Imd signalling pathways.

Likely exam questions

1. (Long, 8 marks) Describe the structure of the tracheal system of insects and the mechanism of gaseous exchange.
2. (Long, 8 marks) Describe the respiratory adaptations of aquatic insects.
3. (Short, 4 marks) What is a plastron?
4. (Short, 4 marks) Describe the structure of the dorsal vessel of insects.
5. (Short, 4 marks) Write the types and functions of insect haemocytes.

Syllabus topics (RTMNU)

• Nervous system: brain and ventral nerve cord, peripheral and visceral (sympathetic) nervous systems; functional significance
• Physiology of nervous system: nerve impulse transmission; neurons; synaptic transmission and coordination
• Sense organs and biological rhythms: mechanoreceptors, chemoreceptors, photoreceptors, thermoreceptors; circadian rhythms in insect behaviour
• Sensory physiology and behaviour: host-seeking in mosquitoes; pheromones and kairomones in vector management and pest control

Nervous system — organisation

Central nervous system (CNS):

• Brain (supra-oesophageal ganglion) — three fused parts: protocerebrum (largest; optic lobes for compound eyes and ocelli, mushroom bodies (corpora pedunculata — learning, memory, very large in social insects), central complex — locomotor coordination; neurosecretory cells of pars intercerebralis); deutocerebrum (antennal lobes — smell, antennal mechanosensation); tritocerebrum (labrum, links to stomodeal system).
• Sub-oesophageal ganglion — fused ganglia of mandibular, maxillary and labial segments; controls mouthparts and salivary glands; connected to the brain by circum-oesophageal connectives.
• Ventral nerve cord — double chain of segmental ganglia joined by connectives: typically 3 thoracic (legs, wings) and up to 8 abdominal ganglia; tendency towards fusion (cockroach — 3 thoracic + 6 abdominal; housefly — all thoracic and abdominal ganglia fused into one thoracic mass). Ganglia act as local centres — a decapitated insect can still walk.

Peripheral nervous system: nerves from ganglia to muscles and sense organs; sensory neurons (mostly bipolar, with cell bodies near the periphery) and motor neurons; each muscle receives few motor axons ("fast", "slow" and inhibitory).

Visceral (sympathetic/stomatogastric) nervous system: (1) stomodeal (stomatogastric) — frontal ganglion, recurrent nerve, hypocerebral and ingluvial ganglia — controls foregut, swallowing, heart, linked with corpora cardiaca; (2) ventral sympathetic — median nerves from ventral ganglia to spiracles (opening/closing); (3) caudal — from the last abdominal ganglion to hindgut and reproductive organs.

Functional significance: brain — integration of senses, learning, behaviour and endocrine control; segmental ganglia — local reflexes and rhythmic patterns (walking, flight, ventilation via central pattern generators); visceral system — feeding, digestion, circulation and respiration.

Physiology of the nervous system

Neurons: mostly monopolar (unipolar) in the CNS — cell body in the ganglion cortex, a single process dividing into dendritic branches and an axon in the central neuropile; sensory neurons bipolar or multipolar; neurons are wrapped by glial cells, and the CNS is protected by the neural lamella and perineurium forming a blood–brain barrier (important because haemolymph is often rich in K⁺ and Mg²⁺). Giant fibres (e.g. in cockroach nerve cord) conduct fast escape signals from cercal receptors.

Nerve impulse: resting potential (about −60 to −70 mV) maintained by Na⁺/K⁺-ATPase; a stimulus opens voltage-gated Na⁺ channels → depolarisation (action potential); K⁺ channels repolarise; impulse propagates along the axon (no myelin; speed increases with axon diameter). DDT and pyrethroids keep Na⁺ channels open (repetitive firing, knockdown).

Synaptic transmission: chemical synapses — acetylcholine is the main excitatory transmitter in the CNS (sensory neurons) and is broken down by acetylcholinesterase (target of organophosphate and carbamate insecticides; nicotinic receptors are targets of neonicotinoids such as imidacloprid); at neuromuscular junctions the excitatory transmitter is glutamate and inhibitory is GABA (GABA-gated chloride channels — targets of fipronil, avermectins); neuromodulators — octopamine ("fight or flight"), serotonin, dopamine, neuropeptides. Coordination: reflex arcs, central pattern generators and descending commands from the brain integrate behaviour.

Sense organs and biological rhythms

Most insect sense organs are sensilla — cuticular structures (hairs, pegs, plates, pits) with sensory neurons and accessory cells.

• Mechanoreceptors: trichoid sensilla (tactile hairs), campaniform sensilla (cuticle strain, e.g. on halteres and legs), chordotonal organs (stretch and vibration — subgenual organs in legs; Johnston's organ in the antennal pedicel — hearing in mosquitoes/flight speed), tympanal organs (hearing — on tibiae of crickets and katydids, abdomen of grasshoppers and moths — detect bat ultrasound), hair plates (proprioception), stretch receptors.
• Chemoreceptors: olfactory (smell; multiporous pegs mainly on antennae — pheromone detection by male moths at extremely low concentrations) and gustatory/contact (taste; uniporous hairs on mouthparts, tarsi, ovipositor — a housefly tastes with its feet).
• Photoreceptors: compound eyes made of many ommatidia (each with cornea, crystalline cone, retinula cells with rhabdom, pigment cells) — apposition eyes (diurnal — mosaic image) and superposition eyes (nocturnal — brighter image); detect movement, colour (including UV), polarised light; ocelli (dorsal simple eyes — light intensity, horizon/flight stability); stemmata (lateral eyes of larvae).
• Thermoreceptors and hygroreceptors: on antennae and tarsi; detect heat and humidity — e.g. bed bugs and mosquitoes find warm hosts; thermo-hygro sensilla in antennal coeloconic pegs.

Circadian rhythms: about 24-hour cycles of activity and physiology controlled by an internal biological clock located in the brain (clock neurons near the optic lobes — e.g. lateral neurons expressing PDF in Drosophila; the optic lobes in cockroach), driven by clock genes (period, timeless, Clock, cycle) in a feedback loop and reset (entrained) by light via cryptochrome and eyes, and by temperature. Behavioural examples: adult eclosion of Drosophila at dawn, nocturnal locomotion of cockroaches, mosquito biting times (Anopheles night, Aedes day), mating flights and pheromone release at specific hours, time-compensated sun-compass navigation in honeybees, egg hatching rhythms. Also seasonal (photoperiodic) rhythms like diapause.

Sensory physiology and behaviour — host-seeking in mosquitoes

A female mosquito needs a blood meal for egg development. Host seeking is a sequence guided by multiple senses:

1. Activation and long-range attraction: exhaled CO₂ (detected by maxillary palp neurons) activates flight and upwind orientation in odour plumes (anemotaxis).
2. Odour cues: skin and breath volatiles — lactic acid, ammonia, carboxylic acids, 1-octen-3-ol — detected by antennal olfactory receptors (ORs, IRs) guide the mosquito to a host and decide host preference (e.g. Anopheles gambiae prefers humans).
3. Visual cues: contrasting dark objects and movement at medium range.
4. Close range: heat and moisture (thermo- and hygroreceptors on antennal tips) guide landing; tarsal and labellar contact chemoreceptors, then probing with the proboscis guided by blood-borne cues (ATP).

Males find females by hearing their wing-beat frequency with Johnston's organ.

Chemical communication — pheromones and kairomones; use in pest and vector control

• Semiochemicals are message-carrying chemicals. Pheromones act within a species: sex pheromones (bombykol of silk moth — first identified by Butenandt, 1959; pink bollworm gossyplure), aggregation pheromones (bark beetles, rhinoceros beetle), alarm pheromones (aphids — E-β-farnesene; bees — isopentyl acetate), trail pheromones (ants, termites), queen pheromone (honeybee 9-ODA), oviposition and marking pheromones.
• Allelochemicals act between species: kairomones benefit the receiver (e.g. human odours and CO₂ guiding mosquitoes; host-plant volatiles guiding pests; host cues used by parasitoids); allomones benefit the emitter (defensive secretions, repellents); synomones benefit both (floral scents and pollinators; plant volatiles attracting natural enemies).

Importance in vector management and pest control: monitoring with pheromone traps (fruit flies with methyl eugenol/cue-lure, bollworms, rhinoceros beetle); mass trapping; mating disruption (saturating the air with synthetic sex pheromone so males cannot find females — pink bollworm in cotton); lure-and-kill (attract-and-kill) combining lures with insecticide; kairomone-baited traps for mosquitoes and tsetse (CO₂, octenol, lactic acid lures; odour-baited traps), oviposition-pheromone-baited ovitraps for Aedes; repellents (DEET, picaridin) block host-seeking odour detection; push–pull strategies. These methods are species-specific, non-toxic and fit integrated pest/vector management.

Likely exam questions

1. (Long, 8 marks) Describe the organisation of the nervous system in insects.
2. (Long, 8 marks) Describe the types of sense organs in insects and explain the role of senses in host-seeking behaviour of mosquitoes.
3. (Short, 4 marks) Write a note on the structure of an ommatidium.
4. (Short, 4 marks) What are pheromones? Give their uses in pest control.
5. (Short, 4 marks) Explain circadian rhythms in insects with examples.

Syllabus topics (RTMNU)

• Endocrine system: neurosecretory cells, corpora cardiaca, corpora allata and associated complexes; roles in growth, development and regulation
• Insect hormones: juvenile hormone, ecdysone, neurohormones — sources, functions, mechanisms; roles in moulting, metamorphosis and reproduction
• Reproductive systems: male and female systems with reference to Orthoptera and Diptera
• Reproductive physiology: egg maturation, fertilisation, embryonic development; viviparity, ovoviviparity, polyembryony, parthenogenesis; hormonal control

Endocrine system — organisation

• Neurosecretory cells (NSCs): modified neurons in the brain (mainly pars intercerebralis and pars lateralis of the protocerebrum) and in ventral ganglia that make peptide hormones; axons carry them to neurohaemal organs.
• Corpora cardiaca (CC): paired bodies behind the brain near the aorta; neurohaemal organ storing and releasing brain neurohormones (e.g. PTTH in some insects); intrinsic cells make adipokinetic hormone (AKH) (mobilises lipids and trehalose for flight — like glucagon).
• Corpora allata (CA): paired (fused in some, e.g. Diptera) glands behind the CC connected to them by nerves; secrete juvenile hormone (JH).
• Prothoracic glands (ventral/thoracic glands): diffuse glands in the prothorax of larvae; secrete ecdysone; degenerate in most adults.
• Ring gland (Weismann's ring) in higher Diptera larvae — a combined complex of CC, CA and prothoracic gland around the aorta.
• Others: Inka cells (epitracheal glands — ecdysis-triggering hormone), gonads (ovaries/testes make ecdysteroids), gut endocrine cells, fat body (insulin-like peptides in some), perisympathetic organs.

Roles: growth and moulting, metamorphosis, reproduction (vitellogenesis, accessory glands), metabolism (AKH, insulin-like peptides), water balance (diuretic and antidiuretic hormones), colour change, diapause, caste differentiation in social insects, behaviour.

Insect hormones — sources, functions and mechanisms

Mechanisms: peptide hormones (PTTH, AKH, bursicon) bind cell-surface receptors and act through second messengers (cAMP, Ca²⁺). 20-Hydroxyecdysone (steroid) enters cells and binds the nuclear ecdysone receptor (EcR + USP), activating a cascade of genes (seen as chromosomal puffs in polytene chromosomes of Drosophila/Chironomus salivary glands). JH acts through the intracellular receptor Met (Methoprene-tolerant) and the gene Kr-h1 to block adult development.

Role in moulting and metamorphosis: each moult is started by ecdysone; the JH titre decides the outcome — high JH → larva-to-larva moult; low JH → larva-to-pupa (holometabola) or to the last nymph; no JH → adult moult. Experiments: removing CA (allatectomy) from young silkworm larvae causes precocious pupation (miniature adults); implanting extra CA gives extra larval instars (giant larvae). Insect growth regulators — JH analogues (methoprene, pyriproxyfen) and ecdysone agonists (tebufenozide) and chitin synthesis inhibitors (diflubenzuron) — are used in pest and mosquito control.

Role in reproduction: JH (and/or ecdysteroids in Diptera) stimulate fat body to make vitellogenin and follicle cells to take it up; JH controls male accessory glands and pheromone production.

Reproductive systems — Orthoptera and Diptera

Male: paired testes (each of several follicles/sperm tubes, where spermatogenesis proceeds in cysts), vasa deferentia, seminal vesicles (sperm storage), accessory glands (secrete seminal fluid and the spermatophore wall; in Drosophila "sex peptide" makes females refuse remating and lay eggs), median ejaculatory duct and aedeagus.

• Orthoptera (grasshopper, cockroach): testes of many follicles fused dorsally; cockroach has a "mushroom gland" (utricular accessory gland) and a phallic gland; sperm transferred in a spermatophore.
• Diptera (housefly, mosquito): small, oval/pear-shaped testes, short vasa deferentia, one or two pairs of accessory glands, ejaculatory duct with an ejaculatory bulb (pump); sperm transferred as free seminal fluid; aedeagus with claspers.

Female: paired ovaries, each with several ovarioles (egg tubes) — terminal filament, germarium (oogonia), vitellarium (growing oocytes in follicles), pedicel; ovarioles join the lateral oviducts → common oviduct → vagina/genital chamber; spermatheca (stores sperm, sometimes for years — queen honeybee) and accessory (colleterial) glands (glue for eggs or the ootheca — egg case of cockroach and mantis, froth of grasshopper egg pods).

• Ovariole types: panoistic — no nurse cells (Orthoptera, Odonata); meroistic polytrophic — nurse cells with each oocyte (Diptera, Lepidoptera, Hymenoptera); meroistic telotrophic — nurse cells in the germarium connected by trophic cords (Hemiptera, Coleoptera).
• Orthoptera: many panoistic ovarioles per ovary (cockroach 8 per ovary), well-developed ovipositor; cockroach forms an ootheca of 16 eggs.
• Diptera: ovaries with polytrophic ovarioles; mosquitoes have 1–3 spermathecae; ovipositor telescopic (housefly); eggs laid singly (Anopheles with floats, Aedes) or as rafts (Culex).

Functional significance: internal fertilisation suited to land life; sperm storage allows mating once and fertilising eggs over long periods; accessory gland secretions protect eggs from desiccation and predators; ovipositor places eggs in safe sites.

Reproductive physiology

Egg maturation: oocytes grow in the vitellarium; nurse cells (in meroistic ovarioles) pour RNA and proteins into the oocyte; vitellogenesis — fat body synthesises vitellogenin (under JH or 20E), which is taken into the oocyte by receptor-mediated endocytosis through gaps between follicle cells (patency) and stored as yolk (vitellin); follicle cells then secrete the vitelline membrane and the chorion (egg shell) with micropyles (for sperm entry) and aeropyles; ovulation into the oviduct. In mosquitoes a blood meal triggers release of ovary ecdysteroidogenic hormone and ecdysone → vitellogenesis (gonotrophic cycle).

Fertilisation: sperm stored in the spermatheca are released as the egg passes the spermathecal opening and enter through the micropyle; meiosis completes, pronuclei fuse. (In Hymenoptera the queen controls fertilisation — fertilised eggs become females, unfertilised males.)

Embryonic development: eggs are centrolecithal; cleavage is superficial — nuclei divide without cell walls (syncytium), migrate to the periphery forming a syncytial then cellular blastoderm; pole cells at the posterior end form germ cells; a ventral germ band forms; gastrulation produces mesoderm; extra-embryonic membranes (amnion, serosa) form; segmentation (controlled by maternal, gap, pair-rule and segment-polarity genes in Drosophila), appendage formation, katatrepsis (blastokinesis), dorsal closure and organogenesis → hatching.

Specialised reproductive strategies:

• Viviparity: young develop inside the mother with nourishment — adenotrophic (tsetse fly Glossina — one larva at a time fed by "milk glands"), pseudoplacental (some aphids, earwig Hemimerus), haemocoelous (Strepsiptera, paedogenetic gall midges).
• Ovoviviparity: eggs with sufficient yolk retained in the genital tract and hatch inside or just at laying — flesh flies (Sarcophaga), some cockroaches (Blaberus), some beetles.
• Polyembryony: one egg divides to form many embryos — parasitic wasps such as Copidosoma (up to about 2,000 embryos from one egg) and Platygaster; useful in biological control.
• Parthenogenesis: development of unfertilised eggs — arrhenotoky (males from unfertilised eggs — honeybee drones, haplodiploidy), thelytoky (only females — many aphids, stick insects), amphitoky/deuterotoky (both sexes); cyclic parthenogenesis in aphids (parthenogenetic and viviparous in summer, sexual in autumn); paedogenesis — larvae reproduce (Miastor gall midge).

Hormonal control of reproduction and importance: brain NSCs and allatotropin regulate CA → JH stimulates vitellogenin synthesis, ovarian patency and accessory glands (in most insects); 20E controls vitellogenesis in mosquitoes and flies; ovarian ecdysteroids, mating factors (sex peptide) and nutrition (insulin-like peptides) synchronise egg production with food and mating; JH deficiency causes reproductive diapause. Importance: matching reproduction to food and season, very high reproductive potential of insects, and targets for pest control (JH analogues, sterile insect technique, mating disruption).

Likely exam questions

1. (Long, 8 marks) Describe the endocrine glands of insects and explain the hormonal control of moulting and metamorphosis.
2. (Long, 8 marks) Describe the male and female reproductive systems of an orthopteran insect.
3. (Short, 4 marks) Write a note on parthenogenesis in insects.
4. (Short, 4 marks) What is polyembryony?
5. (Short, 4 marks) Distinguish between panoistic and meroistic ovarioles.