Entomology Notes

Introduction to Entomology

Overview

• Course: PLNT2011: Plant & Environmental Health
• Instructor: Dr. Anthony Young
• Topics: Evolution, diversity, development, functions, identification, anatomy and physiology, human interactions, and management of insects.

Tribute to Dr. Errol Hassan

• Vale: Dr. Errol Hassan passed away on June 27, 2022.
• Career: Taught entomology at UQ for 34 years.
• Contributions: Named multiple insects and had 5 named after him.
• Research: Conducted eco-friendly insect control research for over 40 years.
• Last taught: PLNT2011 in 2022.
• Student feedback: Highly knowledgeable, passionate, easy to understand, thorough, engaging.

Major Points About Insects

• Insects are incredibly diverse.
• Diversity is driven by specialization of segments.
• They occur virtually everywhere on Earth.
• Insects are critical to ecosystems.
• Insects are critical to humanity.
• Insects are subject to extinction.

Defining Insects

• Domain: Eukarya
• Phylum: Arthropoda
• Sub-Phylum: Hexapoda
• Class: Insecta
• Orders: Approximately 30
• Hexapoda: Includes collembolans, proturans, and diplurans.
  • Insects are arthropods with 3 body segments.
  • Uniramous appendages (typically 6).
  • One pair of antennae.
  • Usually with wings (=ptera) as adults.

Significance of Segmentation

• Segmentation is a key feature in arthropods
• Examples include Polychaete worms, Onychophorans, and Insects

Diversity of Insects

• Currently ~30 Orders of insects.
• Expected to be 5.5 million species (https://www.annualreviews.org/doi/10.1146/annurev-ento-020117-043348).
• Differentiated by anatomy and molecular characteristics.
• Beetles are the most diverse of insects.
  • Approximately 1.5 million beetle species.
  • Weevils are the most diverse of beetles.
  • Coleoptera: Ptilidae.

Insect Functions

• Integral component of food webs.
• Pollinators, seed dispersal.
• Biocontrol agents of other pests.
• Waste removal and nutrient recycling.
• Cultural value

Evolutionary Origins of Insects

• Emerged in Devonian period after land plants.
• Insects have a common ancestor.
• Ancestors had multiple segments.
• Segmental fusion gave 3 ‘segments’.
• Segments then specialised: sensory/dietary, locomotion, reproduction.
• Different groups have different development pathways: ametabolous, hemimetabolous, holometabolous.

Insect Development

• Ametabolous: nymphs look like adults, no wing development
• Hemimetabolous: nymphs look like adults, wing development
• Holometabolous: larvae look unlike adults, wing development
• Neometabolous: pseudo-pupal stage
• Hypermetamorphosis: additional pupa stage; https://doi.org/10.1016/j.jinsphys.2012.02.009

Taxonomy of Insects

• Multiple broad scale groupings
• Approx. 30 Orders, many families/subfamilies
• Genomic molecular systematics is revising our understanding of insect relationships
• Molecular clock has insects pre-Silurian
• Continued work in this space means taxonomy is fluid; https://www.pnas.org/doi/epdf/10.1073/pnas.1817794116

Classification of Insects

• Insects are arthropods that have been on Earth for ~400+ million years
• Insects accompanied plants and other arthropods onto land
• Ancient insects were (are) wingless
  • Archaeognatha (=ancient jaws)
  • Thysanura (=fringe tail)
• Primitive state is WINGLESS and AMETABOLOUS
• Source: Gullan and Cranston (1994) The Insects: An outline of Entomology

Classification: Wings and Metamorphosis

• Wings and metamorphosis came later…
• PTERYGOTA - Insects with wings
• PALEOPTERA
  • Ephemeroptera (=temporary wings)
  • Odonata (=toothed)
• Hemimetabolous and wings do not fold
• Both have aquatic nymphs
• Previously thought that wings evolved as modified gas exchange organs

Classification: Folding Wings

• Folding wings came later
• NEOPTERA - new wings
• POLYNEOPTERA - many new wings
  • Orthoptera (=straight wings)
  • Mantodea (=prophets)
  • Phasmatodea (=phantoms)
  • Blattodea (=cockroaches & termites)
  • Plus several others
• Folding wings allowed for less wing damage
• All hemimetabolous

Classification: True Bugs

• Then came the bugs…
• True bugs
• ACERCARIA → No tails
• True bugs
  • Hemiptera (=half wings)
  • Homoptera (=same wings)
• All have specialised sucking mouthparts
• Many have ‘neometabolous’ development

Classification: Holometabola

• HOLOMETABOLA - Insects with complete metamorphosis
  • Coleoptera (=sheath wings)
  • Lepidoptera (=scaly wings)
  • Siphonaptera (=tube without wings)
  • Trichoptera (=hair wings)
  • Hymenoptera (=membrane wings)
  • Diptera (=two wings)
  • Plus others…
• Young are larvae (singular: larva)

Physical Structure of Insects

• Insects have exoskeletons
• Muscles attach inside the exoskeleton
• To grow they have to shed their skin then quickly expand
• Exoskeletons comprised of chitin (same as fungi)
• Insects can also produce silk which is protein based

Basic Insect Structure

• Head
• Abdomen
• Thorax
• Circulatory system
• Digestive tract
• Nervous system
• Brain

Insect Communication

• Antennae = nose and ears
• Tympanum is also an ‘ear’
• Sensilla taste sensors on feet, maxillae etc.
• Compound eye composed of ommatidia
• ‘Vocalisation’ via stridulation or tymbals
• Use of light, color, physical structures

Insect Feeding

• Mouthparts: mandibles, maxillae, labium (fused 2° maxillae), hypopharynx, labrum
• All specialised mouthparts derived from same basic plan
• Insects can be pests in their own right
• Many bugs vector viruses, bacteria and fungi
• Most are beneficial and control other pests; https://www.pnas.org/doi/epdf/10.1073/pnas.1817794116

Insect Digestive System

• Foregut: Ingestion, storage, grinding and transport of food to midgut
• Midgut: Enzymatic (+/- endogenous) breakdown and nutrient absorption
• Hindgut: H_2O absorption, salts and other important minerals, discharge of droppings (frass) through anus.

Insect Excretory Systems

• Malpighian tubules join mid-hind gut junction
• Excrete nitrogenous waste
• Uric acid is nitrogenous waste molecule
• Crystalline white powder
• Uric acid (33%) is not as N rich as urea (47%)
• Does not require H_2O for excretion

Insect Circulation

• Insects have blood (haemolymph)
• Transports nutrients, hormones, enzymes etc., but not O_2
• Open circulatory system
• Multiple ‘hearts’ that pump haemolymph direct to tissues via dorsal vessel
• Wing veins supply sensory organs and other living components

Insect Respiration

• Insects take oxygen directly to their cells
• O2 enters & CO2 exits via spiracles and trachea
• These end directly at cells
• This limits the size of insects
• Contrast with Carboniferous

Insect Reproductive Systems

• Multiple systems exist
• Visual, pheromone cues
• Males typically heterogametic
• Females often possess spermathecae
• Males can partake in sperm wars
• Others parthenogenetic

Insect Nervous System

• Comprised of brain and ventral ganglia
• Ganglia are concentrated nervous bundles
• Can operate in concert or independently
• Many insecticides target the nervous system
  • Anticholinesterase compounds
  • Imidicloprid

Insect Microbe Interactions

• All insects rely on microbes for their proper function
• Many of these are inherited maternally
• Reduced genomes and functions
• Cannot exist outside host
• Can impact fertility (e.g., Wolbachia)

Management Strategies

• Cultural
  • Variety selection
  • Land management systems
  • Rotation crops
  • Burning
  • Corridors for enemies
  • Smoke
• Physical
  • Exclusion (netting, screens, newspaper)
  • Squishing
  • Flooding
  • Solarisation

Management Strategies Continued

• Biological
  • Natural enemies (insects, nematodes, spiders, mites, fungi etc.)
  • Endotoxins
  • Male sterilisation
  • Inundative vs. inoculative
• Chemical
  • Nervous system
  • Digestive system
  • Contact
  • Systemic
  • Different modes of action
  • Natural products

Integrated Pest Management (IPM)

• Uses an appropriate mixture of different management methods
• Based on monitoring, identification, records and economic thresholds
• Prevents development of resistance
• Accepts some loss to prevent major loss
• Maintains natural enemy populations

Conclusions

• Insects are among the first land animals
• Insects conquered the land and air
• Critical services to agroecosystems
• Worldwide insect numbers declining
• Understanding insects is rewarding in its own right, but can help the environment