IM

Insect Evolution, Biogeography, and Human Contexts

Arthropod origins and insect ancestry

  • Major arthropod groups
    • Myriapods (centipedes, millipedes)
    • Chelicerates (horseshoe crabs, arachnids)
    • Crustaceans (crabs, lobsters, shrimp)
    • Hexapoda (collembola, diplura, and insects)
  • Insects likely evolved from crustaceans; concept of Pancrustacea (Crustacea + Hexapoda)
    • Diagrammatic scenario: Remipedia and Cephalocarida as potential sister lineages to Hexapoda in some evolutionary models
    • Note: None of the depicted relationships are fully resolved by all molecular/morphological data
  • Key molecular/morphological hint for crustacean→insect transition
    • Crustacean cardioactive peptide is a molting hormone used by crustaceans and also involved in insect moulting
    • This supports a deep, shared origin for crustaceans and hexapods
  • Collembola (springtails) are not true insects by strict mouthpart criteria, but occupy a basal position among hexapods and are soil-dwelling

Insect antiquity and fossil context

  • First crustaceans date to ≈505 ext{ MYA} (Upper Cambrian)
  • Hexapods (insects) appear around ≈419 ext{ MYA} (Devonian)
  • Insects evolved from coastal crustaceans; land colonization timing aligns with arthropods moving onto land and with early land plant colonization
  • Oldest hexapod fossils are Collembolans
  • Evidence exists for insect/ mite feeding earlier than the Devonian, but fossil arthropod evidence is sparse

Insects and plant coevolution; radiation patterns

  • Insect antiquity and plant evolution are intertwined
    • The Carboniferous era shows extensive radiation of insects alongside the spread of vascular land plants
    • Major plant groups timeline: ferns, gymnosperms (pine trees), angiosperms (flowering plants), bryophytes
  • Phylogenetic outline (simplified): Hexapoda → Collembola, Diplura, Insecta
  • Timing hints from Fig. 8.2 (in Insects: An Outline of Entomology):
    • Insects’ diversification tracks with major plant radiations; the crown-group insect record is anchored to definite crown-group members; stem-group data are often uncertain
  • Hymenoptera appears comparatively earlier than Diptera in some radiations, with Hymenoptera often treated as among the more evolutionarily advanced insect lineages in some summaries
  • Coevolutionary principle: as flowering plants (angiosperms) become prominent (≈ 146 ext{ MYA}), insect–plant interactions explode in diversity; insects and plants show a tightly coupled evolutionary trajectory
  • Practical example of coevolution and behavior: social insects (hornets, wasps, honeybees) show advanced cognitive/recognition traits (face recognition) and complex chemical signaling (odor-based recognition among conspecifics)

Insect antiquity, diversification, and amber record

  • Notable amber deposits provide fossil evidence of Cretaceous insects; amber preserves can show a wide array of insect diversity from that era
  • Jurassic Park reference: amber fossils prompted discussions about ancient DNA, but authenticated ancient DNA sequences from amber-preserved specimens have not been established due to contamination and degradation

Atmospheric oxygen and insect gigantism

  • Carboniferous and Permian periods show gigantism in insects and other invertebrates
    • Mayfly wingspan ≈ 0.46 m; dragonflies ≈ 0.71 m wingspan (illustrative sizes from the fossil record)
  • Hypothesized driver: high atmospheric O₂ levels in the Carboniferous/Permian enhanced diffusion through tracheal systems, enabling larger body size
    • Key data point: atmospheric O₂ around 30 ext{
      m ext{%}} in the Carboniferous/Permian
    • Higher O₂ promotes greater diffusion via tracheae, reducing the need for a heavy thoracic tracheal system and facilitating flight
    • However, higher O₂ alone does not fully explain all size changes; peak body size did not persist even when O₂ rose again in later periods like the Cretaceous/Tertiary
  • Modern oxygen level context
    • Today’s atmosphere is about 20 ext{
      m ext{%}} ext{O}_2; this limits gigantism relative to ancient high-O₂ periods
  • Additional size constraints
    • Even with high O₂, large airborne arthropods faced competition/predation pressures from birds and later bats, influencing maximum body size

Insect wings, flight, and evolution of wing structure

  • Wing origin and structure
    • Wings developed from thoracic appendages and associated exites/endites; origin hypotheses include a fusion model (fusion of various thoracic and limb segments)
    • Forewings and hindwings can be coupled in flight-capable groups (e.g., Lepidoptera and Hymenoptera) to improve efficiency
    • Diptera evolved halteres (modified hind wings) that act as gyroscopic stabilizers during flight
  • Eight basic wing veins are common across insects; significant variation in wing venation across orders
  • Protowings and wing evolution
    • Early wings likely had non-flight roles (protection of legs and spiracles, thermoregulation, sexual display, camouflage, etc.)
    • Aerodynamic flight arises with wing enlargement and refinement of venation and musculature
  • Wing development and thorax anatomy (for context)
    • Primitive condition shows wing pads on developing thorax (mesothorax and metathorax) in early pterygotes
    • Modern winged insects (pterygotes) show advanced wing structure and venation enabling efficient flight

From roots of flight to wing control and respiration

  • Wing venation and control
    • Veins function in strengthening wings and modulating airflow during flight
    • In Lepidoptera and Hymenoptera, fore- and hind-wings are coupled; in Diptera, halteres assist flight stabilization
  • Insect respiration and circulatory system (notes from slides)
    • Insects do not have lungs like vertebrates; they rely on a tracheal system with spiracles, tracheae, and tracheoles for gas exchange
    • Hemolymph (insect “blood”) carries nutrients and, to a limited extent, oxygen, but does not transport oxygen as vertebrate blood does
    • Insect heart is tubular and lacks chambers in some descriptions; the tracheal system delivers oxygen directly to tissues
    • When insects emerge from pupation, rapid circulatory and respiratory adjustments help inflate wings and restore normal activity
  • Gills and aquatic respiration
    • Some aquatic nymphs possess gills or gill-like adaptations; certain dragonfly nymphs have gills associated with the rectum (anal gills) in-water respiration in some groups
    • Gills may arise as invaginations or invaginated invaginations of the tracheal system; aquatic respiration in insects is typically limited by diffusion in water and oxygen availability

Holometaboly vs Hemimetaboly: developmental strategies

  • Holometaboly (complete metamorphosis)
    • Life stages: egg → larva → pupa → adult
    • larval and adult forms typically occupy different ecological niches and feed on different resources, reducing intraspecific competition and enabling exploitation of more diverse resources
    • Early fossil record: late Carboniferous (≈ 328 ext{--}318 ext{ MYA}) shows early holometabolous lineages; larval feeding strategies observed include external-feeding caterpillar-like larvae, internal-feeding legless larvae, and predacious larvae
    • After Permian, conifers/dominance of certain plant groups correlated with diversification, with at least ~30 orders emerging
  • Hemimetaboly (incomplete metamorphosis)
    • Life stages: egg → nymph → adult
    • Nymphs typically resemble small adults and share similar diets, leading to more overlapping ecological roles across life stages
  • Diversification consequences
    • The separation of larval and adult resource use in holometabolous insects is a key driver of diversification and ecological success

Permian to Cenozoic insect diversity and key radiations

  • Permian–Triassic boundary (≈ 252 ext{ MYA}): mass extinction but saw rise of major modern orders; lineages including Hymenoptera, Heteroptera (Hemiptera + Homoptera), and Diptera become prominent afterwards
  • Cretaceous (≈ 145 ext{--}66 ext{ MYA}): abundant amber deposits preserve diverse insects; coeval with angiosperm proliferation
  • Angiosperm radiation and insect mouthpart diversification
    • The origin of angiosperms around ≈ 146 ext{ MYA} boosts insect-plant interactions and diversification of mouthparts
  • First bees and modern fauna
    • First bees appear around ≈ 100 ext{ MYA}
  • KT extinction event (≈ 66 ext{ MYA})
    • Dinosaurs go extinct; insect diversity drops but recovers and becomes broadly modern in composition

Insects in amber and the fossil record

  • Insects preserved in amber provide snapshots of Cretaceous ecosystems and their diversity
  • DNA preservation in amber is controversial; although ancient DNA sequences have been claimed, none are universally authenticated due to contamination and DNA degradation

Aquatic origins versus terrestrial origin of insects

  • Most primitive insects are terrestrial; strong evidence for terrestrial origin of insects rather than aquatic origins
  • While mayflies, dragonflies, stoneflies, Megaloptera, and caddisflies have aquatic juvenile stages, their most primitive ancestors did not
  • The evolution of a tracheal system would be challenging in water; terrestrial respiration via tracheae is a defining feature of the group

Biogeography and geographic distribution patterns

  • Biogeography: the study of global distribution of organisms
  • Australia has suitable trees but lacks woodpeckers; American deserts have cacti; other regions show different faunal patterns
  • Darwin is a foundational figure in biogeography; much evidence for faunal drift along with continents comes from entomology and Gondwanan remnants
  • Aquatic insect distribution is not easily explained by trans-oceanic dispersal due to saltwater barriers; disjunct distributions often reflect historical landmasses and vicariance rather than long-distance dispersal
  • The concept of continental drift and historical biogeography helps explain patterns of insect distribution across Gondwana and successor landmasses

Humans and modern biogeography

  • Human activity reshapes modern biogeographic patterns and makes some distributions appear cosmopolitan
    • Anthropophilic species (cockroaches, silverfish, houseflies) travel with humans
    • Synanthropic species thrive in disturbed habitats associated with humans
    • Ectoparasites and endoparasites of humans (mosquitoes, bedbugs) are cosmopolitan
    • Agriculture and horticulture create widespread pest distributions
    • Humans have also facilitated introductions of potential biocontrol agents (with ecological consequences)
  • Caution about biological control: introduced species can have unintended ecological effects; host-specificity and regulatory checks are essential

Island biogeography and Pacific examples

  • In the Pacific (e.g., Hawaii, Galapagos), island faunas are highly endemic and show strong lineage–island endemism patterns
  • Drosophilidae in Hawaii: many single-island endemics; colonization and subsequent radiations follow island geology
  • Clade concepts and area endemism
    • A clade is a group consisting of a common ancestor and all its descendants
    • Distribution of Drosophila is tightly linked to geography and island age
  • Area cladograms (e.g., Hawaii) illustrate patterns of founding, diversification within islands, and possible back-colonization events; times are expressed in million years ago (myo)
  • Practical takeaway: island biogeography explains why island endemism is high and why colonization sequences shape present-day diversity

Biological control of pests: concepts and safeguards

  • Three main types of biological control
    • Classical (neoclassical) biological control: introduce natural enemies after the pest is established; goal is to reduce pest to below economic threshold and restore balance (not eradicate)
    • Augmentative release: releasing additional natural enemies to boost control
    • Inundative release: releasing large numbers of agents to overwhelm the pest
  • Key considerations when using biological control
    • Host specificity: ensure control agents target pests but do not threaten non-targets, including endangered species
    • Effectiveness on targeted pest and likelihood of success
    • Costs: initial costs can be high, but long-term costs may be reduced
    • Potential impacts on biodiversity and non-target organisms; risk of beneficials becoming pests
    • Once released, control agents cannot be retrieved; regulatory and ecological oversight is essential
  • Pre-release evaluation in the USA
    • Screen for natural enemies of the target pest in the host country
    • quarantine evaluations and risk assessments in the USA
    • Testing includes host specificity, disease risk, effects on native species, and larval development tests
    • Typical timelines: 2–4 years for parasites/predators; 5–10 years for weeds
    • Administrative steps: TAG (Technical Advisory Group) review, USF&W, Environmental Assessment (EAS), Finding of No Significant Impact (FONSI) under USDA/EPA, state approvals, and finally USDA-PPQ permit for release and evaluation
  • Practical note
    • Photo caption in course material (C. Cheah) illustrates regulatory and ethical considerations for field releases

Key dates and numbers (summary reference)

  • First crustaceans: 505 ext{ MYA}
  • Hexapods appear: 419 ext{ MYA}
  • Carboniferous atmospheric O₂: ext{around }30 ext{
    m ext{%}}
  • Angiosperms appear: 146 ext{ MYA}
  • First bees: 100 ext{ MYA}
  • Cretaceous–Paleogene boundary (KT extinction): 66 ext{ MYA}
  • Flight evolution: roughly 315 ext{ MYA}
  • Notable holometabolous fossil window: 328 ext{--}318 ext{ MYA}
  • Amber fossil evidence: widespread in the Cretaceous (and earlier for various lineages)

Notes on figure and source material

  • Text references to Insects: An Outline of Entomology, Fifth Edition, by Gullan & Cranston (2014) and companion site www.wiley.com/go/gullan/insects
  • Figure references include Fig. 8.1 (Pancrustacea and the hexapod–crustacean relationship), Fig. 8.2 (geological history of insects vs plant evolution), Fig. 8.3 (Stenodictya lobata reconstruction), Fig. 8.4 (stylized tracheal system), Fig. 8.5 (appendages of primitive vs modern pterygotes), Fig. 8.6 (Hawaiian area cladogram)
  • Important caution: some slide claims reflect educational hypotheses and classroom interpretations; consult primary literature for current consensus when required for exams

Connections and implications

  • Evolution is tightly coupled with terrestrial plant evolution; major plant radiations (especially angiosperms) drive insect diversity and vice versa
  • The evolution of flight, metamorphosis, and respiratory adaptations collectively enable the remarkable ecological success of insects
  • Human activity profoundly reshapes insect distributions and ecosystem dynamics; responsible management of biological control requires rigorous ecological risk assessment and regulatory oversight
  • Island biogeography provides powerful explanations for endemicity and patterns of colonization and diversity on archipelagos such as Hawaii