Insect Physiology: A Comprehensive Guide to Internal Systems

Posted on May 13, 2024 in Biology

Insect Physiology

Digestive System

Structure

A tube of epithelial cells, with the foregut and hindgut lined with cuticle that must be molted.

Segments

  • Foregut
  • Midgut
  • Hindgut

Digestion

The breakdown and absorption of chemical energy and the elimination of waste.

Dietary Necessities

  • Chemical energy
  • Protein
  • Fats and Lipids
  • Essential vitamins
  • Minerals
  • Water

Foregut

Ingestion, initial digestion, and storage. Consists of the mouth, esophagus, salivary glands, crop, and proventriculus.

Crop

Food storage.

Proventriculus

Modifications for solid food (grinding organ) in beetles, cockroaches, bees, and fleas.

Midgut

Digestion and some absorption. Consisting of the gastric caeca, ventriculus, and peritrophic membrane.

Gastric Caeca

Increases the surface area of food and could contain symbionts.

Peritrophic Membrane

Semi-permeable membrane lining the midgut (sieve).

Midgut Digestion and Absorption
  • Enzymes – secreted by ventricular cells (amylases, proteinases, peptidases, lipases)
  • pH conditions
  • Passive diffusion
  • Active transport

Hindgut

Excretion and water ion balance. Consisting of the Malpighian tubules, anterior intestine, rectum, and anus.

Malpighian Tubules

‘Kidney’ absorbs solutes, water, and wastes from surrounding hemolymph (nitrogenous wastes).

  1. Junction of Midgut and Hindgut
  2. Outgrowths of the Alimentary Canal
  3. Filter out Solutes
  4. Creates Primary Urine

Feeding Behavior

  • Continuous vs. discontinuous
  • Adaptations to the gut for constant processing or storage.

Digestive Symbionts

  • Obligate (must have) or facultative (helpful, not required)
  • Microorganisms (Protozoa, Fungi, and Bacteria)
  • Symbionts may be housed in specialized chambers (e.g., the “fermentation chamber”)

Fat Body

Not a discrete “organ”; found throughout the body.

  • Storage organ
  • A site of many metabolic processes
  • In some insects: storage of metabolic wastes and light-producing organ

Insect Survival Needs

  • Food
  • Oxygen
  • Eliminate waste
  • Movement
  • Receive and respond to stimuli
  • Reproduction

Excretion and Osmoregulation

Adaptations for Terrestrial Life

  • Impermeable exoskeleton
  • Sophisticated excretory system

Excretory System Functions

  1. Excretion – ridding of potentially toxic waste
  2. Osmoregulation – maintaining a sufficient balance of water and ions

Main Organs of Excretory System

  • Malpighian tubules
  • Rectum
  • Anus

Malpighian Tubules

  • Junction of midgut & hindgut
  • Outgrowths of the alimentary canal
  • Filters out solutes
  • Creates primary urine

Rectum

Water and solute resorption from excreta creates secondary urine.

Anus

Elimination of food wastes.

Nitrogenous Waste Products

  1. Ammonia (aquatic insects)
  2. Urea (most mammals)
  3. Uric Acid (terrestrial insects, solid form, requires the least water, most expensive metabolically)

Insect Diets

Herbivores (Phytophagous)

  • Plant eaters
  • Generalists vs. Specialists
Monophagous

One food, especially herbivores.

Oligophagous

Few foods (common).

Polyphagous

Many foods.

Carnivores

Predators, parasitoids, parasites, and blood feeders.

Saprophages

Dead organic matter.

Modifications to Liquid Diet

Liquid diet eaters have an esophagus with powerful muscles (pharynx).

Modifications to Solid Diet

Solid diet eaters have mandibles and an internal gizzard (proventriculus).

Modifications to Hemiptera (Plant-Feeding on Phloem)

Filter chamber to remove excess water and sugars (honeydew).

Circulatory System

Structure

  • Dorsal Blood Vessel (open; a single continuous tube called the heart in the posterior, aorta in the anterior)
  • Hemocoel (closed) aids in the transport of nutrients

Hemocoel

‘Blood cavity’ where blood sloshes around.

Dorsal Vessel

  • Heart: pumps, has ostia to allow blood in, closed to force blood forward when contracted.
  • Aorta: does not pump, usually no ostia.

Insect Blood

  • Blood liquid = hemolymph
  • Blood cells = hemocytes

Diaphragms

  • Dorsal and Ventral – Septum in Legs
  • Antenna help the flow of blood

Accessory Pulsatile Organs

In wings and antennae to help circulate blood.

Circulation

Blood is pushed through the dorsal blood vessel from tail to head dorsally and returns ventrally. Pushed into limbs and wings by accessory pulsatile organs.

Liquid Circulatory System Functions

  1. Transport of nutrients, minerals, hormones, metabolic wastes
  2. Hydrostatic Skeleton (for inflating the body at molts; locomotion of some larvae)
  3. Thermoregulation
  4. Defense (lymph) phagocytic cells (engulf or encapsulate foreign bodies) and coagulation (blood clot)

Tracheal (Respiratory) System

Structure

  • Spiracles (1 pair per body segment on the side) – openings to allow for water to keep in; oxygen passively comes in and out in response to the insect’s response to the environment. Spiracles are normally “closed” (to prevent water loss).
  • Tracheae inside (series of tubes)
  • Large tubes lined with Taenidia (chitinous, molted)
  • Small tubes = tracheoles
  • Enlargements of Tracheae = air sacs
  • Tubes extend to all cells in the body

Function

Gas exchange – this is how oxygen gets to cells and CO2 gets out.

Why a Gas System?

  • Gas exchange primarily from passive diffusion
  • Diffusion of gasses is 10,000X faster than via liquids
  • Limits the size of insects (SA to Vol Ratio)

Active Gas Movement

  • Ventilation: movement of the abdomen and thorax helps exchange gas
  • Rate: proportional to temperature and activity

Aquatic Insect Respiration

  • Gills
  • Breathing tubes
  • Carry air bubble
  • Cuticular respiration

Muscles

Structure and Function

  • Muscle cells are elongate and multinucleate, forming fibers.
  • Muscle cells (fibers) are bundled into groups (muscles)
  • FUNCTION IS CONTRACTION ONLY (pulling)

Muscle Strength

  • The number of fibers in the muscle determines its strength.
  • Power is proportional!

Muscle Cells

  • Elongate & multinucleate
  • Forming “fibers”

Resilin

Protein found in insects such as fleas and dragonflies, an elastic energy store. Cross-linked protein rubber has a resilience (or efficiency) of 97%. Can be found in the labrum.

Insect Flight

Paleopterous Insects

(The insects that cannot put their wings away and the wings are put backward) Direct flight muscles. Downstroke muscle(s) act directly on the wing base with this action causing the wing to go down and the side of the wing (pleuron) acting as a fulcrum.

Indirect Flight Muscles (The Rest of the Insects)

Upstroke (Dorsoventral)
  • Muscles pull the top of the thorax (and base of the wing) down
  • This causes the wing to go up
  • Dorsoventral muscles; run from top (dorsal) to bottom (ventral)
  • Odonata & Ephemeroptera
Downstroke (Dorsal Longitudinal Muscles – Run Lengthwise)
  • Contract
  • Squeezes the top of the thorax UP
  • This causes wings to go down

Forms of Locomotion

  1. Jump – hind legs, hind coxa or femur, spring
  2. Walk – like 2 tripods (4 legs in praying mantises)
  3. Crawl
  4. Swim

Nervous System

Ventral Nerve Cord

(Bottom of insect, centrally located)

Central Nervous System Structure

  • Brain (head; dorsal)
  • Subesophageal Ganglion
  • Thoracic Ganglia
  • Abdominal Ganglia

Brain Function

(Several fused ganglia): nerve center for the front of the head – eyes, antennae, learning, memory.

Subesophageal Ganglion

Controls mouthparts, salivary glands, and neck muscles.

Thoracic Ganglia

Controls legs and wings.

Abdominal Ganglia

Movements of abdominal muscles.

Peripheral Nerves

Branch throughout the body from the central nervous system, sensory and motor nerves, inner neurons.

Nerves

Enclosed bundle of axons (long projection), consisting of neurons or nerve cells connected to long projected axons, with dendrites (finger-like endings). Signals are transmitted one way.

Variety of Nerves

  • Unipolar
  • Bipolar
  • Multipolar

Transmission of Nerve Signals

  • Electrical within the nerve due to changes in the cell membrane, flow of charged ions.
  • Between nerves, there is a gap = synapse. The signal is transmitted across the synapse by a chemical called a neurotransmitter (acetylcholine and its enzyme that breaks it down).

Reflex Arc

  1. Sense Structure – Hair (seta): sensory neuron
  2. Sensory neuron conducts a signal to the central nervous system (ganglion) (interneurons or association nerves)
  3. CNS sends a signal to muscle – motor neuron

A ‘ganglion’ (plural ‘ganglia’) is a collection of neurons or nerve cells in a single place.

Sensory Receptors

Connected to sensory neurons & can take many forms.

  • Mechanoreceptors: Touch (hairs & spikes)
  • Chemoreceptors: Taste & Smell (mouth & antennae)
  • Photoreceptors: Light (vision) (compound eyes & simple eyes)

Gland Systems

  • Endocrine and exocrine glands

Endocrine Glands

  • Internal; release hormones (travel in the hemolymph)
  • Coordinate long-term processes (reproduction, molting, overwintering, migration..)

Exocrine Glands

  • Scent glands – release pheromones that affect the behavior of other organisms.
  • Venom glands & Salivary glands

Reproductive Systems

Male Reproductive System

  • Paired testicular glands (produce sperm)
  • May be stored in the seminal vesicle until mating
  • Accessory glands add fluid
  • Some insects produce a spermatophore
  • Sperm usually transferred to the female using the aedeagus (=genitalia)

Spermatheca

When insects mate, sperm is stored in the spermatheca (helps keep them alive).

Female Reproductive System

  • Paired ovaries produce eggs, move down the lateral oviduct
  • Eggs are fertilized in the median (common) oviduct (one sperm comes out of the spermatheca)
  • Accessory glands add glue, coatings, etc.

Mating and fertilization are separate!

Oviposition

Egg-laying (females store sperm until they find the best place to lay eggs).

Types of Oviposition

  1. Oviparity
  2. Ovoviviparity
  3. Viviparity

Oviparity

Female lays eggs (usual condition).

Ovoviviparity

Female retains eggs until they hatch inside and live young are birthed. Not nourished by the mother.

Viviparity

‘Live birth’ female retains the egg and young (nourishes young).

Ovipositors

A tubular structure that is used for laying eggs. The ovipositor is attached to the abdomen of insects and the eggs pass down the tube.

Parthenogenesis

Development without fertilization (asexual).