MBIO162 Arthropod Diversity

24 February 2025: Introduction to Arthropoda I

• What is an arthropod?

  • Arthropoda are animals with a segmented body design

  • Bodyplan and unifying features

    • Arthropods share with annelids:

      • Through gut

      • Ventral nerve cord with segmental ganglia

      • Dorsal circulatory system

    • Arthropod skeleton

      • Composed of chitin - a polysaccharide-, proteins and (especially in the Crustacea) mineral salts such as calcium carbonate

        • Made of composite material 

    • Main defining features

      • Segmented body

      • Ventral nerve cord

      • Chitinous exoskeleton

      • Paired, jointed appendages on each body segment (many lost)

• Arthropod diversity 

  • Diversity

    • The Arthropoda is the most diverse phylum of living organisms

    • >1,100,000 described species

      •  86% of animal diversity!

  • Measuring species diversity 

    • Estimates of the real number of extant species span three orders of magnitude using samples

      • 3 – 50+ million species

    • Majority of these are tropical insects

      • Especially beetles, small flies and wasps

        • Many others deep sea/reefs? (maybe not so many!)

  • Major arthropod groups and the relationships between them

• Ecological range

  • Occur in all environments on earth

    • Deep ocean

    • High mountains

    • Ground

    • Forest canopies

    • Glaciers

      • Cryoconite ecosystem – includes many mites (Acari)

    • In caves and deep crevices

      • Family Chelodesmidae

        • Relic species

    • External and internal animal parasites

      • Sacculina - barnacle parasite of crabs

        • Larva settles then the parasite spreads through the host - egg- containing sacs appear externally

      • Pentastomida

        • Lungs & nasal passages of vertebrates

      • Fish louse

        • Argulus

25 February 2025: Introduction to Arthropoda II

• Why are arthropods so successful?

  • Biodiversity:

    • Around 70% of all extant species!

  • Ecological range

    • Arthropods occur in almost all habitats on earth

    • Arthropods occupy a vast range of ecological niches

  • Ecological importance

    • Arthropods are also key players in almost all global ecosystems

  • Historical persistence

    • Arthropod world domination has persisted since (before) the ‘Cambrian explosion’ (540 mya)

  • Body size

    • Cuticle

      • Provides skeletal support

        • Especially important in colonization of land (water 1,000 x denser than air)

      • Relatively impermeable to water

        • Maintaining homeostasis, especially avoiding desiccation on land (epicuticular waxes)

        • Rate of water loss as low as 0.01% of that in soft bodied organisms such as the land snail Helix (Tsetse fly Glossina)

      • Cuticle has proved uniquely flexible

        • Segments and their appendages are modified in a variety of ways both within and between individual arthropod types

    • Flexible bodyplan

      • Arthropod bauplan has undergone various forms of regional specialization

    • Tagmosis and regional specialization

      • Most arthopods have bodies composed of functionally specialized regions (e.g. head, thorax, abdomen)

        • Tagmata

          • Ex. Centipede (Chilopoda) - two tagmata

            • Head (fusion of five segments)

            • Trunk (varying no. of segments)

          • Ex. Locust (Hexapoda: Insecta) - three tagmata

            • Head (fusion of five segments)

            • Thorax (three segments - bears walking limbs)

            • Abdomen (primitively 11 segments, although usually fewer - most segments have lost appendages)

          • Lobster (Crustacea) - three tagmata

            • Head (fusion of five segments)

            • Thorax or pereon (eight segments - partly fused with head - bears walking limbs)

            • Abdomen or pleon (six segments - appendages often modified as gills or for swimming)

      • Modification of appendages

• How can we judge biological success?

  • Organisms on Earth range from 10−17 (Nanoarchaeum equitans) to 109 g (Sequoiadendron giganteum) in carbon weight

    • Intermediate sized organisms, such as arthropods, dominate

    • There are more ecological niches for small sized organisms

      • But there may be more intermediate sized organisms

  • Speciation rate

    • Number of species in a group is a balance between:

      • Speciation and extinction

        • Large organisms tend to be good dispersers

          • Lowers likelihood of allopatric speciation

        • Very small organisms also tend to be good dispersers

          • Lowers likelihood of allopatric speciation

        • So….

          • Arthopods experience a lot of allopatric speciation and thus are found almost everywhere

25 February 2025: Introduction to Arthropoda III

• Classifciation of Arthropoda

  • Crustacea

    • Crabs, shrimps, lobsters, barnacles etc.

  • Hexapoda 

    • Insects and their relatives

  • Cheliceriformes 

    • Spiders, scorpions and their kin

  • Myriapoda 

    • Many legged arthropods: centipedes, millipedes etc.

      • Fraction of past diversity

  • HOWEVER… Should really recognize only three extant subphyla!

    • Pancrustacea 

      • Crabs, shrimps, lobsters, barnacles etc. PLUS hexapods

        • Insects are crustceans!

    • Cheliceriformes 

      • Spiders, scorpions and their kin

    • Myriapoda 

      • Many legged arthropods: centipedes, millipedes etc.

  • Today the Arthropoda are usually divided into five subgroups:

    • Crustacea 

      • Crabs, shrimps, lobsters, barnacles etc.

    • Hexapoda 

      • Insects and their relatives

    • Trilobitomorpha†

      • Trilobites

    • Cheliceriformes 

      • Spiders, scorpions and their kin

    • Myriapoda 

      • Many legged arthropods: centipedes, millipedes etc.

• Crustacea

  • Crabs, lobsters, shrimps and their many allies

  • Cambrian marine origin

  • 67,000 described living species

    • Many awaiting discovery (rapid rate of current description in many groups)

    • 100 μm - > 3 m

    • Homarus lobsters up to 20 kg in weight

  • Despite the huge range of morphological diversity, all (almost) possess a number of unifying features:

    • 5-segmented head

      • Five segmented head (with two pairs of antennae, mandibles and two pairs of secondary mouthparts (maxillae))

    • Trunk region, usually divided into two tagmata

      • Thorax & abdomen

    • Biramous limbs

      • Two-branched limbs

        • Secondary uniramy (single limb branches) is widespread

    • Carapace

      • Carapace or cephalic shield (reduced in some groups)

    • Nauplius larva

      • Planktonic nauplius larva (lost when development is direct)

        • Nauplius has:

          • Median simple eye

          • 3 pairs of setose appendages (become 2 pairs of antennae and mandibles)

          • Suppressed in many groups which have direct development

          • Ex. isopods & amphipods

  • Making sense of crustacean diversity

    • Crustacean diversity has 3 aspects:

      • 1] Diversity of segments in an adult of one species 

        • Up to 14 distinct types of segments

      • 2] Segment diversity between species

        • A particular body segment takes many forms in different taxa

        • Also tagmatisation can become extensive

          • Ex. in crabs/lobsters etc., thoracic segments become associated with the head, and their appendages function as accessory mouthparts - known as maxillipeds.

      • 3] Diversity of segment morphology during ontogeny

        • Ex. 3 pairs of setose appendages of nauplius larva become 2 pairs of antennae and mandibles

    • Shifts in homeotic gene expression have a major role in all three aspects

03 March 2025: An Introduction to the Arthropoda IV

• Hexapoda

  • The most diverse organisms in the 3 billion year history of life on Earth

    • AND the most ecologically dominant animals on land\

      • Includes:

        • Insects and their allies

          • Three pairs of walking limbs

          • Three tagmata - head, thorax, abdomen

    • Primarily terrestrial

      • Devonian origin ca. 390 MYA (e.g. springtail Rhyniella)

        • Dominated terrestrial ecosystems ever since

    • 950,000 described extant species

      • Average of 7,000 new species described annually!

    • Insect facts:

      • Insects are not only diverse, they are incredibly abundant

        • 200 million insects for every human alive

      • in tropical forests insects make up ca. 40% of animal biomass

      • 3.2 x 108 individual insects per hectare; 60,000 species (W Amazon)

      • Biomass of ants greater than that of mammals

        • Single army ant colony- 22 million workers

          • 1015 individual ants alive at any one time!

  • Insect diversity and abundance

    • Taxonomy

      • Hexapoda

        • Holomenrentoma

          • Entognatha

            • Protura

            • Dipulra

            • Collembola

              • Rhyniella praecursor

                • (Springtail – Collembolla)

                • From the Rhynie Chert, Scotland

                • Close to modern family Isotomidae

          • Insecta

            • Insects are classified into around 30 living orders, including:

              • Odonata 

                • Dragonflies & damselflies

              • Hemiptera 

                • True bugs

              • Contain the most species:

              • Coleoptera 

                • Beetles

              • Hymenoptera 

                • Ants, bees, wasps

              • Diptera 

                • True flies

              • Lepidoptera 

                • Butterflies & moths

              • Mantophasmatodea 

                • Heel walkers

                • New insect order – first recognized in 2002

  • Insect success factors

    • Some apply to all arthropods:

      • Body size

      • Cuticle

      • Flexible bauplan

    • Others are specific to insects:

      • Evolution of flight

        • Wingless insects only account for 1% of extant species

        • Key to success in terrestrial and FW systems

        • Origins of wings

          • Wings are leg exites – basal leg segments are fused into body wall

      • Metamorphosis

        • Three kinds:

          • Ametabolous or no real metamorphosis

            • Juveniles are like small adults

          • Hemimetabolous or incomplete metamorphosis

            • Juveniles resemble adults but lack fully developed wings and sexual structures

          • Holometabolous or complete metamorphosis

            • Holometabolous development allows adults and young to exploit different areas of niche space

            • Avoid damage to developing wings

            • Holometabolous insect orders dominate

  • Insect limitations

    • Size

      • Weight up to 100 g (largest beetles)

      • Wingspan up to 28 cm (Lepidoptera)

        • Carboniferous ‘dragonflies’ - wingspan > 75cm

          • Oxygen levels were higher in the atmosphere

            • Extant species reared under hyperoxia reduce tracheal volume

          • Perhaps insect size is limited by atmospheric  PO2

      • Often stated that this is limited by insect respiratory system

        • Oxygen supplied to tissues directly via diffusion in trachea

          • More difficult in big insects?

      • Diffusion distances increase with body size

      • Does respiration limit the size of insects?

        • Larger species devote proportionately more body volume to tracheal system

          • Tracheal volume scales as mass1.29

          • Particularly true in the legs

            • Legs lack spiracles so rely entirely on trachea entering them from the body

              • X-sectional area of leg orifice scale as mass0.77

              • X-sectional are of trachea in leg orifice mass 1.02

                • Legs get increasingly full of trachea in larger species

04 March 2025: An Introduction to the Arthropoda V

• Cheliceriformes

  • Spiders, scorpions and their kin

    • Body composed of two tagmata 

      • Prosoma (cephalothorax)

      • Opisthosoma (abdomen)

    • No antennae

    • Presence of chelicerae (fangs in spiders) and pedipalps

      • Male pedipalps are modified for sperm transfer

    • Scorpions 

      • Scorpions produce stalked spermatophores

      • Scorpion chelae are also pedipalps

  • Diversity

    • Ca. 80,000 living species

    • Mostly terrestrial and second only to insects in diversity on land

    • Origin

      • Cambrian marine origin (extinct sea scorpions and extant horseshoe crabs (Limulidae) and sea spiders (Pycnogonida))

    • Spiders and mites dominate today

      • Key innovation in spiders is the evolution of silk

        • Silk is produced from spinnerets at the end of the abdomen

        • Fibrous protein rich in glycine, alanine and serine- produced as a water soluble liquid

          • 5x stronger than steel and more elastic than nylon!

• Trilobitomorpha

  • Ca. 22,000 species known

  • Restricted to Palaeozoic seas

    • Dominated Cambrian and Ordovician periods (440-520 mya)

      • Continued to be important up to Permo-Triassic mass extinctions 245 mya

  • Most benthic - up to 70 cm - deposit feeders, scavengers & predators

    • Some apparently planktonic filter feeders

04 March 2025: An Introduction to the Arthropoda VI

• Myriapoda

  • 4 Groups:

    • Chilopoda 

      • Centipedes

        • Largely active predators

        • One pair of legs per segment

        • Phylogeny

          • Scolopendromorpha (common ancestor to taxa below)

            • Active predators

            • Well developed eyes

            • Larger species often have very painful bite

          • Geophilomorpha

            • Largely burrowers 

            • Have ventral glue glands for defence

          • Lithobiomorpha

            • Active predators 

            • Well developed eyes, serrated bases to poison claws

          • Scutigeromorpha

            • Scutigera and relatives have evolved the most rapid gait:

              • Long legs

              • Leg length increases down trunk

              • Dorsal cuticle plates overlap segments

              • Compound eyes (independent origin to those in insects)

              • Tracheal ‘lungs’ connected to the pericardium – unique organs

    • Diplopoda

      • Millipedes

        • Mostly detritivores

        • Burrowers in soil and litter

        • Two pairs of limbs per (visible) body segment

        • Polyxenida 

          • Sister to all other extant millipedes

          • Soft bodied, bristly, sperm deposited as spermatophores

    • Pauropoda

      • Millipede-like

      • Small, stout,

      • Soil-dwelling

      • They all have branched or bifurcated antennae, a defining characteristic

    • Polyxenida

      • Sister to all other extant millipedes 

      • Soft bodied

      • Bristly 

      • Sperm deposited as spermatophores

    • Symphyla

      • Aka garden centipedes or pseudo centipedes

      • Small, long

      • Soil-dwelling

      • Non-venomous 

  • Ca. 14,000 described species (many more await discovery)

  • Origin

    • Cambrian ca. 520 mya

    • Many early terrestrial taxa were large (e.g. Carboniferous ‘millipede’ Arthropleura > 1 m!)

    • Almost all modern taxa are terrestrial

• Why study arthropods?

  • Rarity of species

    • Ex. Deronectes diving beetles

  • Vulnerability to climate change

    • Aquatic insects