GS3007: The Global Conquest of the Insects

Arthropods

  • Arthropods are invertebrate animals that have segmented bodies, jointed appendages, and exoskeletons.
  • Marine arthropods make up the largest proportion of carbon weight in the Animal Kingdom (e.g., crabs, lobsters, shrimp, barnacles, sea spiders).

80% of all animal species today are arthropods.
* Basic body plan includes a segmented body and appendages.
* 3 main body segments.
* Tough exoskeleton, typically chitin (sometimes hardened by minerals).
* First occurrence in the early Cambrian.
* First to walk on land: Euthycarcinoidea (~440 Ma).
* Possible reasons for terrestrialization: reproduction, avoiding competition, strolling.

Insects

56% of all described species are insects.

  • ~75% of all animal species are insects.
  • Insects dominate terrestrial & freshwater food chains.
    • Swallows eat ~200,000 insects each.
    • Great tit chicks eat 100 caterpillars every day.
  • Major ecosystem players:
    • Pollinators: the majority of flowering plant species.
    • Decomposers: recycle nutrients, enrich soils, dispose of carcasses and dung.
    • Primary consumers: herbivores, including those that eat leaves, fruits, nectar, and wood.
  • Total global economic value of insect pollination: €153B (Gallai et al., 2009).
    • Nearly 10% of total agricultural production value!
  • Bark beetles cause > US$2B economic loss in the USA each year (Grégoire et al., 2015).
  • Hallmann et al. (2017) discovered a 75% decline in flying insect biomass over 27 years in nature reserves in Germany.
  • Sánchez-Bayo & Wyckhuys (2019) found insects declining approximately 2.5% per year worldwide.
    • Major cause is agricultural intensification, such as the disappearance of field margins and the use of pesticides.
  • Three major body regions (head, thorax, abdomen).
  • Six legs (Hexa-, -pod).
  • Divided into distinct elements.
  • Common in derived groups:
    • Reduced abdominal segments.
    • Wings.
    • Common presence of ovipositor (especially in flying insects).

Insect Diversity

  • Irish and World insect diversity.
  • Creator has an inordinate fondness for beetles.
  • Lepidopterans (e.g., butterflies, moths).

Insect Armageddon

  • Windshield phenomenon in the UK (2004 / 2019): insect decrease by 50%.
  • Protection measures include organic farming and habitat reconstruction.

Insect Classification

  • Phylum: Arthropoda
  • Subphylum: Hexapoda
  • Class: Insecta

Insect Phylogeny

  • Misof et al. (2014) study on insect evolution.
  • Bootstrap Support:
    • 98%
    • 96%
    • >90%
    • >85%
    • >75%

Insect Fossils

  • Insect fossils can be found in amber and compression fossils.
  • Taphonomic biases and preservational quality affect fossil records.
  • DNA can be extracted from fossils up to 1 Ma (mammoths) or 2 Ma (sediment).

Insect Trackways

  • Insect trackways can be found in Permian.

Insect Ecology

  • Mating.
  • Ecological niches and behaviors.
  • Phoresy (hitching a ride).
  • Parasitism.
  • Blood feeding.
  • Egg laying.
  • Color.
  • Pollen collecting.
  • Traces.

Insect Origins

  • Traditional view: ‘Freshwater first hypothesis’ of arthropod terrestrialization.
  • Some anomalies:
    • Oceanic sister groups to insects (Remipedia / Cephalocarida).
  • Giribet & Edgecombe (2019) molecular clock estimates:
    • Hexapoda sister group = Remipeda.
    • Hexapoda: ~500 Ma.
    • Insects: ~465 Ma.
  • First fossil record of insects in the Devonian of Scotland, Rhynie Chert: ~410 Ma.
    • Rhyniognatha hirsti (pair of mandibles).
    • Similarities to Pterygota.
  • Oldest (undisputed) insect: bristletails (~385 Ma).

Hexapod Gap

  • 385–323 Ma (~62 Myrs): No known hexapod fossils!
  • Devonian-Carboniferous: “Hexapod Gap”.
  • Other arthropods exist but are rare (myriapods, scorpions).
  • Wings crucial to insect success? Very low abundance and diversity before the evolution of wings!

Carboniferous: The First Fliers

  • Wings evolved only once as outgrowths of the thorax, NOT modified limbs.
  • Wings secondarily lost or modified (e.g., fleas, beetles, flies).
  • Original functions: thermoregulation & gliding; later: co-opted for active powered flight.
  • Folded flight wings appeared in the Early Late Carboniferous (~320 Ma).
    • Advantages:
      • Flight mechanics à greater maneuverability à predator avoidance.
      • Secure locking for greater protection.
      • Smaller size for hiding.

Carboniferous-Permian Gigantism

  • Arthropleurids: Mega-myriapods up to 2.5m long!
  • Meganeuropsis permiana: Largest known insect ever (71 cm wingspan).
  • Hyperoxia during Carb-Perm.
    • ↑ O2 à ↑Efficiency of respiration (via tracheal system) à ↑Faster metabolism à Megabugs!
  • Evolutionary arms race (defense against predators).
  • Ecological vacancy (filling new, specialist niches in early forests).

Metamorphosis

  • Holometaboly (= complete metamorphosis) includes egg à larva à pupa à adult stages.
    • Permian: 1st appearance.
    • Triassic: Diversification.
    • now ~85% of all insects!
  • Advantages:
    • Niche partitioning: reduced competition for resources between larvae & adult.
    • Larval environment often protected (vs predators/parasites).
    • Pupae/larvae can suspend development (diapause) during stress.

End-Permian Extinction Event

  • The end-Permian Event (EPE) = the BIG one.
  • Only mass extinction to have a large effect on insects.
    • End of the big fliers: Meganisoptera, Palaeodictyoptera.
  • 30% of species extinct.
    • Less than other animal groups (>80% of marine invertebrates).
  • 8–9 out of ~24 orders extinct!
  • Niche vacancy: rapid diversification of survivors by Middle Triassic (~240 Ma).

Triassic: Rise of the Big 4

  • Facts:
    • Climate: hot and dry at start, cooling later.
    • Pangaea began to split.
    • Many desert environments, climate became humid as Pangaea drifted apart.
    • Period of recovery after P-T extinction.
  • Beetles (Coleoptera) diversify.
  • 1st Diptera (e.g., flies). Appear: Mid. Triassic - Diverse by Late Triassic.
  • 1st Hymenoptera (e.g., wasps, bees, ants) - Appear: Late Triassic.
  • Beginnings of the Lepidoptera (e.g., butterflies & moths).

Jurassic: Big Animals & Opportunities

  • Facts:
    • Climate: warm.
    • Two landmasses Laurasia and Gondwana (formed after Pangaea split).
    • Triassic deserts replaced by rainforests.
  • Vertebrate ectoparasites like Scorpionflies (Late Jurassic), Fleas and Lice (Early Cretaceous).

Cretaceous: Sociality & Symbiosis

  • Facts:
    • Climate: warm.
    • Continents became more like today’s.
    • Diversification of angiosperms.
  • Sociality:
    • subsociality, communality, eusociality

Eusociality

  • Evolved ~20x in insects.
  • E.g., termites, ants, many bee species.
    • Early Cretaceous: Isoptera (termites).
    • Mid-Cretaceous: Ants, bees.
    • Late Cretaceous: First (undisputed) termite mounds.
  • Rapid co-diversification of plant and insect groups with plant-insect mutualism during the mid-Cretaceous à Paleogene.
  • Angiosperms.
  • Diversification of both groups promoted by:
    • Mutualistic co-evolution.
    • Genetic propensity for diversification (both groups).
    • Higher productivity (angios).
    • Warmest (long-term) climate in >200 Myrs à pole-to-pole dispersal.

Mutualistic Co-evolution

  • First signs of mutualistic co-evolution pre-date Cretaceous!
  • Plants—‘Insect-loving’ reproductive organs: Permian (McLoughlin & Prevec, 2021).
  • Insects—Pollen attached to insects: Permian (Khramov et al. 2023).
  • Insects—Mouthparts co-opted for pollination: Jurassic (Ren et al. 2009).
  • Pollination may have occurred independently in other insect groups (with other plants).
  • Jurassic: sawflies with probosci (mouthparts) identical to modern pollinators.
  • Co-occur with gymnosperms: seed ferns, ginkgoes, conifers & gnetaleans.

Paleogene/Neogene

  • Facts:
    • Climate: Overall cooling trend from warm Cretaceous, punctuated by rapid warming in Paleocene-Eocene.
    • Continents close to current configuration.
    • Forested environments.

Insect Secrets to Success

  • Hyperdiverse and hyperabundant, why?
    • Small size
    • Flight
    • Reproduction
    • Cuticle

Summary

  • Arthropods boast a distinctive body plan and a chitin exoskeleton.
  • They exhibit hyperdiversity, especially since colonizing land (via freshwater).
  • Insects play many ecological roles and functions.
  • They evolved on land with MC in the Ordovician, and the first probable fossils in the Early Devonian.
  • The hexapod gap lasted ~ 60 Myrs.
  • Flight evolved once à lots of advantages à ↑diversity + abundance.
  • Carboniferous-Permian gigantism: oxygen + ecological drivers.
  • End-Permian event: the only big one for insects.
  • Mesozoic: Rise of the Big 4, first parasitism, first (confirmed) plant mutualism, eusociality.

Life Lessons from Insects

  • Their loss = our loss
  • Make love not war!