Insect Physiology: Lab Practical & Systems Overview
Lab Practical and Course Logistics
There will be a lab practical with labeled parts on a dish; plan is for about points (possibly more).
You must bring five insects that you have not pinned yet. You will need to pin these insects correctly.
Only adult insects may be pinned. If the specimen is immature, it should go on ethanol.
If something is a non-insect arthropod, do not pin it.
Goal: bring five unpinned insects (not five mature insects on purpose).
You may use the book during the practical as a resource; computers or the web are not allowed.
The practical will cover pinning, grasshopper parts, and there will be an ID section with questions.
In-class plan: I will select five unidentified insects that you have not seen before and you will identify them to order. The unidentified insects for this exercise come from the BI patch (collecting area).
The unidentified insects are located up in the middle (BI patch). They are samples collected over the summer during research.
You will identify those insects to order; there will also be written questions about collecting or natural history terminology.
Total points for this portion: points.
After the practical, you will pin more specimens and work on your mini collection.
Mini collection is due on Friday; the assignment sheet is on Canvas now.
Mini collection requirements:
Turn in the white box with specimens identified to order, correctly labeled.
Include a collection guide.
Labels must include official locality labels (printed labels from computer; label templates exist).
Rationale for detail: the instructor emphasizes agonizing detail and careful labeling because the process takes a long time; starting early helps avoid last-minute stress.
Next week: a field trip in lab to collect aquatic insects with Dr. Mac. There is a nine-person limit in the van; some students may need to drive themselves. We will discuss logistics next week.
This week focus: insect physiology in the lecture portion; questions and concerns are welcome.
General classroom flow: today we handle the practical first, then proceed with lab activities; floor questions will be addressed as they arise.
Insect Physiology: Overview and SLOs (Student Learning Outcomes)
Chapters 3 and 4 cover insect physiology; intended goal is a comprehensive yet manageable overview of organ systems and their functions.
SLOs (as stated for Peyton):
Describe the characteristics and functions of the different organ systems at an overview level.
Describe unusual or “weird” adaptations in physiological systems across insects.
Apply this knowledge to real-world situations.
Note on endocrine system: not covered in depth due to complexity and ethical constraints around vertebrates; the instructor highlights how well-studied insect endocrinology is (e.g., hormones can be studied via invasive techniques in insects more freely than vertebrates).
Systems to be covered (with emphasis on the main ideas):
Respiratory system
Circulatory system
Digestion
Excretion
Reproduction and nervous/sensory systems (brief; endocrine especially omitted)
Acknowledgement of practical relevance: understanding these systems connects to real-world issues like agriculture, disease vectors, and ecological interactions.
Ethical/philosophical note: entomology allows certain invasive experiments on insects that would be unethical or illegal for vertebrates.
Respiratory System
Core idea: gas exchange in insects occurs via an open tracheal system; there are no lungs like in vertebrates.
Anatomy overview:
Spiracles: paired openings on every body segment acting as nostril-like entry points.
Tracheae: branched tubes from spiracles.
Tracheoles: finer tubes that interface directly with individual cells.
Basement membrane lines the tracheal system; it is a nonliving barrier that is somewhat leaky but supports diffusion.
Gas exchange mechanics:
Gas exchange is largely diffusion-driven across the tracheoles, feeding oxygen directly to cells (notably muscle cells for high-energy activities like flight).
Diffusion operates best over small distances and is efficient for small insects but limits size; larger insects rely on active ventilation.
Modifications and variations:
Tenidia (tenidium): hardened rings around spiracles that keep them open and protect gas exchange pathways; provide a silvery appearance.
Tinidia: structural support to keep tracheal openings open in some insects.
Air sacs: expandable structures that store air and act as a reservoir to support active respiration, buoyancy, and sometimes sound production.
Pseudo-lungs concept: some insects use air sacs to inflate and maintain high oxygen supply during energetic activities.
Specialized aquatic adaptations:
Some aquatic or semi-aquatic insects lack spiracles or have modified openings; others use gills or snorkels.
Gills in aquatic larvae such as damselfly nymphs (Odonata) and other aquatic larvae function by moving water and extracting oxygen from the water.
Snorkel-like structures exist (e.g., rat-tailed maggots) functioning as a butt snorkel to access air.
Physical gills: some aquatic beetles (e.g., whirligig beetles and diving beetles) carry a bubble of air under the elytra (plastron) or on body hairs to breathe underwater.
Plastron system (air-breathing via surface hairs):
Plastron: a surface-covered bubble of air held by specialized hydrophobic hairs that trap a thin air layer against the body.
Some insects possess a plastron that covers the entire body; others have plastron localized to certain areas.
Mechanism: air adherence to hydrophobic hairs allows continuous diffusion of oxygen from water into the insect while submerged.
Notable examples: several aquatic beetles and true bugs that exploit plastron systems.
Notable life forms and examples:
Silk horn moth: Spiracles on every segment; spiracular openings often have waterproof hairs to keep out dust.
Beetles with air-bubble plastrons: silvery appearance due to surface air and hairs; can carry air under elytra.
Damselfly vs dragonfly respiration:
Damselfly larvae have external gills at the end of the abdomen (caudal filaments).
Dragonflies have internal gills; they appear to “move” water through their rectum and out the anus to ventilate.
Ecophysiology and evolutionary notes:
Insects display extreme efficiency in flight muscle oxygen use; tracheal system supports high metabolic demands.
Historical context: during the Carboniferous period, atmospheric O extsubscript{2} was much higher (roughly ), enabling larger insects; today’s atmosphere is around O extsubscript{2}, limiting animal size due to diffusion constraints.
Giant insects (e.g., giant dragonflies) could exist then but not now because of lower atmospheric O extsubscript{2} and differences in diffusion limitations.
Limitations and trade-offs:
Diffusion works best over short distances; larger insects rely on active ventilation or structural adaptations to enhance gas exchange.
Endothermy is not present in insects; most are ectothermic (cold-blooded), making diffusion and diffusion-assisted ventilation more critical.
Practical takeaways:
The respiratory system is primarily a gas exchange system with diffusion driving oxygen to cells.
Muscle demands, especially during flight, push the need for efficient tracheal networks and sometimes air reservoirs.
Circulatory System
Primary role: not gas exchange; the insect circulatory system transports nutrients, immune cells, hormones, and waste—facilitating homeostasis and defense.
Open circulatory system characteristics:
Hemolymph bathes all organs and tissues; there are no fully enclosed blood vessels like in vertebrates.
A dorsal heart with ostia (openings) pumps hemolymph through the body; peripheral pulsatile organs and accessory hearts help drive movement.
Hemolymph composition includes trehalose (blood sugar), free amino acids, organic acids, proteins, lipids, inorganic ions, and hemocytes (immune cells).
Hemolymph lacks hemoglobin in most insects (rare exceptions like bloodworms that use respiratory pigments).
Hemocytes and immunity:
Hemocytes function similarly to white blood cells: phagocytosis, encapsulation, and antimicrobial responses.
Insects possess limited acquired immunity (short lifespan and no long-term memory) but have robust innate and immediate immune responses.
Parasitic defenses: immune cells can encapsulate parasites and melanize foreign bodies or eggs to restrict growth.
Circulatory organization details:
Open system is not a random bath; organs are bathed in hemolymph but the flow is organized via pulsatile heart and ostia.
There are additional smaller pulsatile organs within other segments that contribute to circulation.
The wing tissue is light and requires minimal hemolymph supply; wings have limited circulation to maintain flight efficiency.
Relationship to other systems:
Hemolymph distribution supports transport of nutrients to the digestive tract and delivery of immune defenses to tissues.
Hemolymph movement is slower than vertebrate blood flow and operates under lower pressure.
Practical implications:
The open circulatory system with hemolymph is well-suited to small, active insects, but it imposes limits on pressure-driven delivery and oxygen transport (hence reliance on tracheal system for O extsubscript{2}).
Digestive System and Nutrition
Overall structure: foregut, midgut, hindgut.
Foregut and hindgut:
Both foregut and hindgut are lined with a cuticle (shed during molting).
These regions provide protection and form entry/exit portals for ingestion and excretion.
Midgut details:
Midgut is not lined with cuticle; it is the primary site of chemical digestion and nutrient absorption.
The midgut lacks its own cuticle, creating a vulnerability to mechanical/chemical damage; thus a temporary paratrophic membrane is secreted to protect the midgut lumen during digestion.
Paratropic membrane: a temporary membrane around the food bolus that protects the midgut epithelium; several insects produce different forms of this membrane.
Gastric caeca: finger-like projections that increase surface area for digestion and absorption; food may be directed into these sacs.
Head region and associated glands:
Mandibular glands: produce queen pheromone in honeybees; involved in regulating colony behavior.
Queen substance (pheromone) produced by the mandibular gland in the queen; presence maintains colony cohesion and suppresses queen replacement; reduced queen pheromone triggers queen rearing or swarming.
Pharyngeal glands can secrete royal jelly in honeybees; royal jelly acts as a phagostimulant, making bee larvae hungrier and contributing to queen development when fed to fertilized eggs (haplodiploid system).
Maxillary glands may secrete toxins in some insects (e.g., Neuroptera lacewings use maxillary gland secretions to subdue prey).
Salivary glands are crucial for interactions with hosts and plant tissues; they can contribute to disease transmission in hematophagous insects and to plant damage in herbivores.
Salivary and protective secretions:
Salivary secretions play a key role in feeding on plants (hemipterans use a piercing-sucking mouthpart with a stylet); saliva can lubricate the entry point and modulate host tissue responses.
In hematophagous insects (e.g., mosquitoes), saliva contains anticoagulants enabling feeding and facilitating pathogen transmission (e.g., malaria). Saliva can trigger plant defense pathways in herbivorous interactions as well.
Silk production:
Secreted by salivary glands in Lepidoptera; silk is used by many species to construct tents and cocoons.
Other orders (Hymenoptera, Embioptera) can produce silk through different glands or body parts (e.g., legs in some cases).
Sap-sucking insects and honeydew:
Hemiptera feed through a complex hollow tube (stylet) and secrete saliva that helps guide the stylus into plant tissues; this feeding can elicit plant immune responses and facilitate pathogen transmission.
Honeydew: sugary excretion produced by aphids, cicadas, and similar sap-feeders; can lead to mold growth and ecological consequences (e.g., affecting plants and attracting other insects).
Guts and digestive strategies across insects:
Termites: symbiotic gut microorganisms (protists, bacteria, yeasts) digest cellulose; termites engage in proctodeal trophallaxis, exchanging gut contents with nest mates to repopulate gut microbiota after molts; this may underlie their social colony structure.
Aphids: harbor mycetome (mycetocytes) containing symbiotic bacteria that synthesize essential nutrients missing from plant sap; this symbiosis supports aphid nutrition and contributes to their role as plant disease vectors.
Leafcutter ants: agricultural mutualism with fungus; ants feed fungus by providing leaf material and apply antimicrobial strategies (fungus farming requires co-evolved yeast/bacteria to suppress pathogens).
Nutritional requirements and adaptations:
Insects require carbohydrates for energy and pentose sugars for nucleic acids; they need 10 essential amino acids historically defined for many insects.
They generally do not require vitamins C or K; they also do not require vitamins D or K for reasons tied to their metabolism.
They largely synthesize their own fatty acids but must obtain sterols from their diet; sterols are essential for steroid hormones.
Diet types and digestive challenges vary widely across insects, leading to a diversity of digestive adaptations (see termite, aphid, and sap-sucking insect examples above).
Diet-specific adaptations and examples:
Wood-eating termites rely on gut symbionts (protozoans, bacteria, yeasts) to digest cellulose; repopulation of gut microbes after molts via trophallaxis is essential for termite survival and colony life.
Sap-feeding insects with cryptonephridial or cryptonephridial-like systems can maximize water and nutrient absorption in dry environments; some species exhibit a cryptonephridial arrangement that tightly links Malpighian tubules with the hindgut.
Filter chamber (foregut-midgut separation) seen in sap-feeders like cicadas and aphids allows rapid excretion of excess water and honeydew production; this is a major adaptation to a sugar-rich diet.
Desert and drought-tolerant insects exhibit cryptonephridial systems and other osmoregulatory adaptations to minimize water loss.
Nutritional and disease implications:
Nutritional symbioses (bacteria, fungi, yeasts) are critical for many insects and influence ecology, behavior, and disease transmission.
The ability of insects to adapt their digestion and nutrition to diverse diets underlies their ecological success and makes them important agricultural pests or beneficial pollinators.
Excretion and Osmoregulation
Primary excretory organs: Malpighian tubules and hindgut (rectum) work together to regulate ion balance and water reabsorption.
Nitrogenous waste management:
Insects predominantly excrete nitrogenous waste as uric acid to conserve water; uric acid is energetically costly to synthesize but minimizes water loss, which is critical for terrestrial, flight-capable insects.
Some aquatic and hemi-terrestrial insects may excrete different forms under specific conditions, but uric acid is the general strategy for most terrestrial insects.
Fat body role:
Fat body (trophocytes) participates in nitrogenous waste processing by converting ammonia into uric acid, contributing to overall homeostasis.
Osmoregulation in various environments:
Cryptonephridial system: in dry environments (e.g., some flower beetles) the Malpighian tubules surround the hindgut to maximize water reabsorption; this permits survival in arid habitats.
Cryptonephridial arrangements effectively reduce water loss by tightly reclaiming water from waste.
Filter chamber and rapid nitrogenous waste handling help insects in high-water or sugar-rich diets manage osmoregulation efficiently.
Sap-feeding and honeydew producers:
Filter chamber and related osmoregulatory adaptations enable rapid processing of sugar-rich sap; excess water is excreted as honeydew, while sugars are retained and metabolized.
Blood-feeding challenges and strategies (hemolymph and osmoregulation):
Blood is high in water and salts and contains heme; insects like bedbugs and mosquitoes must rapidly digest blood to remove water and heme toxicity while retaining nutrients.
Heme is toxic; strategies include rapid digestion and sequestration/removal of heme to prevent toxicity.
High water intake from blood requires careful osmoregulation; rapid processing helps maintain mobility and flight capabilities.
Diet-related osmoregulatory strategies:
Terrestrial herbivores often live in potassium-rich and sodium-poor plant environments; they may visit salt-rich locations to obtain essential ions (e.g., butterflies gathering minerals from animal excreta or tears).
Some true bugs (including cicadas and aphids) produce honeydew; their osmoregulatory needs are tied to balancing sugar intake and water losses.
Aquatic osmoregulation:
Insects that live in water can avoid some osmoregulatory challenges, as they are constantly in a water-rich environment and can excrete ammonia through their cuticle or through specialized tissues.
Flower beetle adaptations:
Insects that live in extremely dry environments (flower beetles) demonstrate specialized adaptations such as the cryptonephridial system to extract nearly all available water from ingested material, enabling survival in arid habitats.
Practical implications and examples:
Understanding excretion and osmoregulation helps explain insect distributions, habitat preferences, and responses to environmental changes.
Reproduction and Endocrinology (brief, ongoing exploration)
Endocrine system is not a primary focus in this course; it is acknowledged as fascinating and well-studied but not fully covered here due to complexity.
Bee social regulation and pheromones:
Mandibular gland secretes the queen pheromone (queen substance) that maintains colony organization and suppresses worker reproduction.
As queen pheromone levels decrease, workers may rear new queens or swarm, illustrating endocrine-like regulation of social structure.
Queen development and royal jelly:
Pharyngeal glands may secrete royal jelly, a phagostimulant higher in nutrients and used to feed larvae.
Feeding larvae royal jelly determines queen development in honeybees (fertilized eggs develop into queens when fed certain secretions; fertilized eggs become females; haplodiploidy is noted).
Insect eggs, reproduction, and sensory integration:
Queen pheromone and pheromonal cues regulate colony dynamics, worker behavior, and reproduction.
Note on future topics:
Reproduction and specific endocrinological pathways (e.g., juvenile hormone, ecdysone) are not covered in depth in this module but are recognized as important in insect biology.
Microbiology and Mutualisms: Ecological and Evolutionary Contexts
Symbiotic relationships in digestion and nutrition:
Termites: gut microbes (protozoa, bacteria, yeasts) digest cellulose; replication of gut symbionts after molt via trophallaxis may be essential for colony ecology and phylogenetic co-evolution.
Aphids: mycetome (mycetocytes) house bacterial symbionts that provide essential nutrients missing from plant sap; these symbionts are vital for aphid nutrition and survival.
Leafcutter ants: fungi farming system; ants cultivate a fungus that is the primary food source; ants manage this fungus with antimicrobial secretions and mutualistic bacteria/yeasts that suppress pathogens.
Evolutionary implications:
Mutualisms such as termite cellulose digestion and aphid mycetome endosymbionts demonstrate long-term co-evolution and co-dependence between insects and microbes.
Leafcutter ants illustrate extrinsic mutualisms with fungal partners and additional microbial teammates, forming an advanced agricultural system.
Practical significance:
Mutualisms can influence pest management and ecological dynamics; disruption of symbionts can severely affect insect survival and ecosystem services.
Practical Highlights: Quick Reference of Key Terms and Concepts
Key structures and terms:
Spiracle, Trachea, Tracheoles: components of the insect respiratory system; open tracheal system.
Tenidia (tenidia), Tinidia: structural features that keep spiracles open and support gas exchange.
Sphericals: misnamed term in the lecture for spiracles; used to describe spiracular openings.
Paratropic/paratrophic membrane: protective membrane around the midgut lumen during digestion.
Gastric caeca: finger-like digestive projections with high surface area.
Crop and Proventriculus: foregut storage and mechanical digestion spaces; proventriculus often described as a stomach-like region.
Malpighian tubules: insect kidneys; excrete nitrogenous waste and regulate ions.
Hindgut and Rectum: sites of water reabsorption and final waste processing.
Hemolymph: insect “blood” bathes tissues; contains trehalose, amino acids, lipids, ions, and hemocytes; generally lacks hemoglobin.
Hemocytes: immune cells in insects; responsible for encapsulation and antimicrobial responses.
Mycetome/Mycetocytes: bacterial symbionts in aphids that provide essential nutrients.
Proctodeal trophallaxis: exchange of gut contents among nest mates to maintain gut microbiome after molts.
Cryptonephridial system: osmoregulatory adaptation in dry environments linking Malpighian tubules to hindgut.
Filter chamber: adaptation in sap-feeders to separate sugar-rich fluid from the rest of the gut quickly.
Honeydew: sugary excretion from sap-feeders due to high sugar intake; ecological consequence.
Plastron: air-retaining system on body hair used by aquatic insects to breathe underwater.
Pheromones: queen pheromone (queen substance) and royal jelly-related secretions in bees.
Haplodiploidy: sex-determination system in bees and some other Hymenoptera (fertilized eggs become females; unfertilized eggs become males).
Real-world connections:
Pest management considerations include understanding termite symbionts and the role of aphid mycetomes in plant disease spread.
Agricultural implications of leafcutter ant farming and fungal symbiosis.
Disease transmission vectors (mosquito saliva and pathogen delivery) highlight public health relevance.
Quick Reminders for Exam Preparation
Expect questions about: differences between open and closed circulatory systems, major digestive compartments (foregut/midgut/hindgut), the role of Malpighian tubules and the hindgut in osmoregulation, and examples of unique respiratory adaptations (plastron, gills, snorkels, air sacs).
Be able to discuss evolutionary contexts for insect respiration (e.g., Carboniferous oxygen levels) and how they shaped insect size and physiology.
Remember the key bacterial/ fungal symbioses in termites, aphids, and leafcutter ants and why these matter for nutrition and ecology.
Understand the difference between lymph-like hemolymph and vertebrate blood, the lack of hemoglobin in most insects, and how some insects cope with nitrogenous waste via uric acid.
Review honeydew and its ecological implications, as well as the structural adaptations that allow sap-feeders to manage high sugar intake.
Know the general logic of why certain glands are important (queen pheromone, royal jelly, venom from maxillary glands) and how these relate to insect social behavior and feeding strategies.
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