H6 Mortimer- Arthropods I, II + III. Chelicerates and Myriapods + Insect origins + Insect diversity: wings, larvae and pupae

describe the general features and subphyla of arthropods

• arthropoda is a phylum in the ecdysozoa clade of bilaterian animals

• segmented, with articulate membranes (folded flexible regions) allow inter-segmental articulation

• the ancestor had a series of similar segments (homonomous), but these are often now fused/lost/specialised (heteronomous) eg. head, thorax and abdomen regions grouped by tagmosis (independently evolved) and specialised

• jointed, paired appendages with articles (like segments)- flexible cuticle at the joint with flexor + extensor and protractor + retractor muscles

• exoskeleton/cuticle (tergite, sternite + pleurite) made from chitin polysaccharide with properties controlled by sclerotisation- tanning + cross-linking

• growth is permitted by moulting (ecdysis)

• dorsal + ventral longitudinal muscles, dorso-ventral muscles and extrinsic + intrinsic limb muscles

• antennae with sensory functions

• heart on the dorsal side with ostia and hemocel + ventral nerve cord

this can be split into four extant subphyla:

• hexapoda (insects)

• crustacea

• myriapoda (millipedes + centipedes)

• chelicerata (spiders + scorpions)

the trilobitomorpha is an extinct subphylum

describe the hexapoda

• hexapoda is a subphylum of the arthropoda phylum of bilaterian animals (ecdysozoa clade)

• these are insects including bees, butterflies, moths, ants, wasps, and flies

• almost exclusively terrestrial

• one pair of antennae

• generally three segments from tagmosis: head, thorax (three segments + leg pairs) and abdomen

• tracheal system has spiracles that can close (don’t need humidity)

• they have waxy cuticles and hyperosmotic excreta

• coevolved with flowering plants- phytophagous (plant eating) and pollinators

• can be eusocial, with haplodiploidy

two classes:

• the entognatha (concealed mouthparts)- no metamorphosis

• the insecta/ectognatha (exposed mouthparts), two sublasses:

  • apterygota (wingless)- no metamorphosis

  • pterygota (two pairs of wings, may be reduced)- metamorphosis, four superorders:

    • paleoptera, polyneoptera, paraneoptera and holometabola (largest suborder)

describe the crustacea

• crustacea is a subphylum (paraphyletic group, all pancrustacea but excluding hexapoda) of the arthropoda phylum of bilaterian animals (ecdysozoa clade)

• this includes lobsters, crabs, crayfish, shrimp, krill, barnacles and woodlice- mainly marine, some freshwater or terrestrial

• two pairs of antennae

• direct (hatch with all segments + appendages) or indirect/metamorphosis (nauplius larval stage with just 3 head segments, moults add new segments) development

• no general body plan, but at least two body segments from tagmosis

• biramous or uniramous appendages (branching or no branching) can be specialised, e.g., as gills

• largest class, malacostraca (crabs, krill…), have standardised tagmosis with a head, thorax and abdomen, and a gastric mill in their stomach

describe the myriapoda

• myriapoda is a subphylum of the arthropoda phylum of bilaterian animals (ecdysozoa clade)

• this includes the centipedes and millipedes

• head + trunk, single pair of antennae, open tracheae (need humidity)

• centipedes have an odd number of leg pairs, poison claws on 1st trunk segment, defensive anal legs on the last segment, and are nocturnal, active hunters

• millipedes have diplosegments (two pairs of appendages per segment), and mostly eat rotting vegetation and wood, + secrete noxious chemicals as a defence mechanism

describe the chelicerata

• chelicerata is a subphylum of the arthropoda phylum of bilaterian animals (ecdysozoa clade)

• this includes 5 major subgroups, the ticks + mites, scorpions, spiders (together, the arachnids), horse shoe crabs + sea spiders

• head and thorax region fused into a cephalothorax, + abdomen, 4 pairs of legs

• compound eyes

• no antennae (only arthropod group), but have chelicerae appendages (pincers) and pedipalps (non-locomotory appendages)

• book lungs

• carnivorous with liquid diet

• no antagonistic muscles, use hydraulic pressure from hemocel to extend appendages and muscles to retract them

• spiders produce silk, have poison glands and transfer sperm via pedipalps (almost exclusively predatory)

describe the trilobitomorpha

• trilobitomorpha is an extinct subphylum of the arthropoda phylum of bilaterian animals (ecdysozoa clade)

what is a wing and how did they evolve?

• wings are though to have evolved once in the hexapoda/insects (ectognatha class), in the pterygota subclass

• in many cases the one or both of the two pairs of wings have been lost, reduced or modified

• powered flight, which evolved four times across history, actively generates lift (not gliding etc.), by the power stroke and the recovery stroke- wing changes angle so that the lift isn’t cancelled out

• the top of the wing is the leading edge, and the veins of cuticle strengthen them

wings are not appendages, they evolved de novo, by two theories:

• paranotal theory- wing develops from outgrowths of nota

• endite-exite theory- wing develops from exites of primitive leg segments

they likely evolved from a gliding ancestor:

• fixed angle, non-flapping wing generates lift for gliding

• hinge, venation, muscle and higher respiration/metabolism needed to develop flapping flight

• folding of wings in non-paleopterans opened up more habitats/niches

• asynchronous muscle, which needs just a single nerve impulse, evolved independently multiple times to increase wing beat frequency + decrease body size

how are wings controlled?

in paleoptera (eg. dragonflies):

• controlled by antagonistic direct flight muscles

• elevator vs depressor muscles contract in turn to cause cyclic wing movement

• anterior vs posterior direct muscles control forward and backward movements

in other groups (eg. flies):

• power stroke controlled by indirect vertical flight muscles, which pull on the roof of the thorax

• recovery stroke controlled by indirect longitudinal flight muscles, which pull on the anterior and posterior ends of the thorax

• direct muscles allow for steering

what are the different kinds of development to do with metamorphosis?

in hexapods

what is holometabolous development? what are the advantages of this?

• in holometaboly, the insect (hexapod) undergoes complete metamorphosis- this evolved only once

• the immature larva enters a quiescent pupa stage, from which a very different winged adult then emerges from

• the larva will moult very small amounts multiple times, mostly 5, to get larger (different stages = instars)

• in the pupal stage, the larval organs are hydrolysed and reformed into adult organs from imaginal discs (clusters of embryonic reserve cells)

• the imago is the final adult form

• this process is hormonally controlled

• this allows the larvae and adults to have ecologically different niches and specialisation, leading to resource partitioning and avoidance of competition between life stages

• the speed of development can be controlled dependent on the amount of food eg. through diapause

• it also allows for parasitoidism (longer-term than parasitism + causes death) by larva