BIOL 108 Topic 24: Arthropods 2024 PDF

Topic 24: Arthropods Ecdysozoa – Phylum Arthropoda BIOL 108 Winter 2024 © 2024 Neil Harris Arthropods are the most species-rich phylum, accounting for approximately two-thirds of all animal species. − ~1 million extant species have been named, with estimates ranging from 3 to 30 million species in t...

Topic 24: Arthropods Ecdysozoa – Phylum Arthropoda BIOL 108 Winter 2024 © 2024 Neil Harris Arthropods are the most species-rich phylum, accounting for approximately two-thirds of all animal species. − ~1 million extant species have been named, with estimates ranging from 3 to 30 million species in total. − Arthropods exhibit a vast array of body forms and adaptations across terrestrial, freshwater, and marine environments. − Arthropods occupy important ecological roles as decomposers and pollinators. WC 1 Arthropod origins BIOL 108 Winter 2024 © 2024 Neil Harris Arthropod body plan consists of: − A segmented body. − Jointed appendages. − A hard exoskeleton. The arthropod body plan originated in the Cambrian explosion (535–525 mya). − The earliest arthropods showed little variation from segment to segment, e.g. trilobites. Arthropod evolution is characterized by: − A decrease in the number of body segments. − An increase in appendage specialization. − Changes may have been caused by changes in Hox gene sequences or regulation. Fig 33.30 A trilobite fossil 2 Arthropod characteristics BIOL 108 Winter 2024 © 2024 Neil Harris 1. Segmented body. − All arthropod embryos are segmented. https://doi.org/10.1242/dev.129.5.1225  The last common ancestor of arthropods probably had numerous undifferentiated segments, each with a pair of appendages that functioned as limbs. − Body segments and limbs are arranged into functional units (tagmata) with specialized functions.  e.g. the three-part appearance of many insect bodies; the two-part appearance of spiders. Abdomen head and thorax often merged as cephalothorax head: eating thorax: & sensing locomotion abdomen: digestion & reproduction Fig 33.31 3 Arthropod characteristics BIOL 108 Winter 2024 © 2024 Neil Harris 2. Jointed paired appendages. − arthros = joint, pod = foot − Jointed appendages are the most versatile part of the arthropod body, undergoing evolutionary modification for specialized functions such as swimming, walking, feeding, sensory perception, reproduction, and defence. − Appendage specialization varies across tagmata, with distinct roles in the head, thorax, and abdomen:  Head appendages: feeding, sensory.  Thoracic appendages: locomotion.  Abdominal appendages serve a variety of functions, e.g. swimming or reproduction; often lost. 4 Arthropod characteristics BIOL 108 Winter 2024 © 2024 Neil Harris 3. Arthropods possess a sclerotized cuticle composed of chitin. Molting cicada (WC) − The cuticle forms a hard, lightweight, external skeleton (exoskeleton), that provides a supportive, rigid surface for muscles to act against. − Cuticle sclerotization sometimes involves the incorporation of minerals (biomineralization), e.g. calcium carbonate in crustaceans. − When an arthropod grows, it moults its entire exoskeleton. Biomineralization of crab exoskeleton (WC) Ant head: chitin reinforced with sclerotization (WC) 5 Arthropod characteristics BIOL 108 Winter 2024 © 2024 Neil Harris 4. Cephalization. − The head region concentrates neural tissues and is composed of several segments specialized for feeding and sensory perception.  Arthropod heads often have eyes, olfactory receptors, and antennae that function in smell and touch. 6 Arthropod characteristics BIOL 108 Winter 2024 © 2024 Neil Harris 5. Reduced coelom. − The strong, segmented limbs of arthropods eliminate the need for a hydrostatic skeleton (the main ancestral function of coeloms). − Arthropods’ primary internal cavity is a hemocoel.  The hemocoel supports the internal organs. 6. Open circulatory system. − Hemolymph circulates in the hemocoel surrounding tissues and organs.  Hemolymph is the circulatory fluid in the open circulatory system of arthropods; ± respiratory pigments for oxygen transport. − Arthropods have evolved a variety of respiratory systems.  Gas exchange in small aquatic arthropods is by diffusion across the body surface.  Aquatic arthropods (crustaceans) usually have gills that are modified appendages, whereas terrestrial arthropods have book lungs or tracheal systems. 7 Phylum Arthropoda BIOL 108 Winter 2024 © 2024 Neil Harris Morphological and molecular evidence suggests three major arthropod lineages: 1. Subphylum Chelicerata (e.g. spiders, scorpions, ticks). 2. Subphylum Myriapoda (e.g. centipedes, millipedes). 3. Clade Pancrustaceans (e.g. crustaceans, insects). 8 Arthropod phylogeny BIOL 108 Winter 2024 © 2024 Neil Harris The phylogeny of extant arthropods is disputed and subject to ongoing reorganization (see Fig 33.37): − Previously, hexapods (incl. insects) and myriapods were grouped due to shared morphological characters. − Molecular studies strongly suggest crustaceans are paraphyletic: hexapods evolved within crustaceans.  Clade Pancrustaceans include crustaceans and hexapods. Placement of extinct trilobites also disputed 9 Subphylum Trilobita BIOL 108 Winter 2024 © 2024 Neil Harris Trilobites are a group of extinct marine arthropods. − Abundant and diverse in Cambrian, flourishing for almost 270 million years. − Became extinct during the Permian mass extinction (252 mya). − ~17k fossil species (easily fossilized exoskeleton!). Segmented body with three distinctive tagmata: Trilobite compound eye − Cephalon (head); thorax (body); pygidium (tail). Sensory organs. − Many trilobites had compound eyes.  Numerous lenses act together to create an image. − One pair of flexible antennae. Each thoracic segment bore a pair of biramous (two-branched) limbs. − Ancestral arthropod limbs seem to be biramous: ventral branch for locomotion, and the dorsal branch forms a gill. − Most derived arthropods lost the dorsal branch → uniramous limbs. https://en.wikipedia.org/wiki/Arthropod 10 Specialization of head limbs in arthropods BIOL 108 Winter 2024 © 2024 Neil Harris The common ancestor of arthropods: one pair of antennae, followed by many pairs of biramous appendages (legs). 11 Chelicerata BIOL 108 Winter 2024 © 2024 Neil Harris Chelicerates (subphylum Chelicerata) are named for claw-like feeding appendages called chelicerae. − Chelicerates include diverse taxa such as spiders, scorpions, and mites. Eurypterids (WC) Chelicerata originated as marine animals during the early Paleozoic era. − The earliest chelicerates were large (up to 2.5m), widespread eurypterids (water scorpions). − Most marine chelicerates (including eurypterids) are extinct, but some marine species survive today, including horseshoe crabs. Fig 33.33 Horseshoe crabs (Limulus polyphemus) 12 Chelicerate characteristics BIOL 108 Winter 2024 © 2024 Neil Harris Chelicerates have two main tagmata: Anterior cephalothorax. − Entirely or partially covered by a carapace, a dorsal exoskeleton section. − 6 pairs of appendages: chelicerae, pedipalps, 4 pairs of legs Jumping spider, Phidippus johnsoni, chelicera (green) and pedipalps (bottom) (WC)  Pedipalps are jointed sensory appendages.  No antennae.  Uniramous appendages/limbs. Pedipalps − Eyes, if present, are located on the cephalothorax. Posterior abdomen. − No appendages. − Contains digestive and reproductive organs. Legs Cephalothorax Abdomen 13 Chelicerate characteristics Three types of chelicerae: jackknife, scissor, and 3-segmented BIOL 108 Winter 2024 © 2024 Neil Harris WC Chelicerae serve as mouthparts adapted for predation or defence. − Chelicerae are modified appendages that appear before the mouth.  Homologous to antennae in other arthropods.  2 or 3-segmented fangs or pincers (chel = claw, ceros = horn). − Chelicerae in spiders are hollow.  Contain venom glands that inject venom into prey or threats. Diversity of chelicerae (WC) 14 Chelicerata Arachnids BIOL 108 Winter 2024 © 2024 Neil Harris Arachnids are the dominant chelicerate group, including spiders, scorpions, ticks, and mites. − ~100k spp., mostly terrestrial with diverse ecological roles. − Most are fluid-feeding predators (do not ingest solid particles). − Simple eyespots (Ocelli) that focus light through a single lens. − Arachnids have an abdomen and a cephalothorax with six pairs of appendages (chelicerae, pedipalps, and four pairs of walking legs). − Separate sexes. Fig 33.35 Anatomy of a spider Fig 33.34 Arachnids 15 Chelicerata Arachnids – Spiders BIOL 108 Winter 2024 © 2024 Neil Harris Spiders (>40k extant spp.) The jumping spider Phidippus audax (WC) − Spiders are fluid-feeding predators.  2-segment chelicerae with poison glands that empty through the fangs of chelicerae.  Some dangerous to humans, e.g. black widows. − Tight constriction between cephalothorax and abdomen. − Gas exchange is via abdominal respiratory organs called book lungs. − 0 – 4 pairs of ocelli. Black widow Fig 33.35 Anatomy of a spider 16 Chelicerata Arachnids – Spiders BIOL 108 Winter 2024 © 2024 Neil Harris Spiders (>40k extant spp.) − Spinnerets: abdominal glands that produce silk, a liquid protein.  The silk is used to capture prey, escape danger, transport, line burrows, and wrap eggs. − Many spiders exhibit maternal care of eggs, and sometimes the young. − Complex courtship, especially in species with good vision.  Peacock spider: https://youtu.be/PQbScg3r1oQ Female black widow spider (Latrodectus mactans) guarding an egg case (WC) Orb weaver spider (WC) Spider ballooning http://tinyurl.com/y2agsrsz 17 Chelicerata Arachnids – Scorpions Scorpions − ~1,750 spp., most in deserts or rainforests. − Pedipalps modified as grasping pincers. − The abdomen is segmented.  Terminates in a poisonous sting.  Many are dangerous to humans. BIOL 108 Winter 2024 © 2024 Neil Harris Indian red scorpion (Hottentotta tamulus; WC) Male and female scorpion mating ‘dance’ (WC) − Scorpions crush or sting prey to death.  Externally digest body contents of prey and ingest the digested fluid. − Undertake complex courtship: the male leads the female in mating ‘dance’. − Female broods eggs beneath her. Female scorpion with young (WC)  Hatchlings are carried on her back. 18 Chelicerata Dust mites (WC) Chigger bites (webmd.com) Predatory mite, Phytoseiulus persimilis Arachnids – Mites BIOL 108 Winter 2024 © 2024 Neil Harris https://doi.org/10.24349/0p4s-gjtm Mites (>40k named species) − Most microscopic (size range: 80 µm to 3 cm) − No separation between the cephalothorax and abdomen.  More like a ball with legs!  No external evidence of segmentation. − 2 or 3-segmented chelicerae. − 0 – 2 pairs of ocelli. − Diverse ecology:  Predators, detritivores, herbivores, parasites.  Soil, water, plants, in and on other animals. Three species live in human skin!  Fluid-feeders; some are detritivores (ingest solid particles). − Most economically important arachnids:  Crop pests, e.g. spider mites feed on 100’s of plant species.  Predatory mites are used as biocontrol agents.  Medically important, e.g. chiggers, ticks, dust mites. Mites as parasites of mites Microscopic mite (Lorryia formosa) WC 19 Myriapods Myriapods (subphylum Myriapoda) include millipedes and centipedes. Myriapod characteristics: BIOL 108 Winter 2024 © 2024 Neil Harris Fig 33.36 Myriapods − Terrestrial; no aquatic myriapods. − Less tagmatized body plan:  Head with simple eyes (ocelli), three pairs of mouthparts, incl. mandibles for main mouthparts (biting/chewing), and one pair of antennae.  Numerous (>3) pairs of legs (myria = a great many): long trunk with numerous pairs of jointed uniramous legs. − Gas exchange via tracheal system (respiratory tubes).  Air enters lateral spiracles, through the trachea, into tracheoles throughout the body; most lack O2-transporting respiratory pigments in the hemolymph. − Direct development (without larval stages). 20 Myriapods BIOL 108 Winter 2024 © 2024 Neil Harris Millipedes (~12k spp., mostly tropical). − Detritivores or herbivores (eat decaying leaves and plant matter). − Two pairs of jointed legs per body segment.  Diplosegments result from the fusion of adjacent pairs of segments (class Diplopoda, diplo = double). Centipedes (>3k spp., mostly tropical). − Fast-moving predators. https://youtu.be/CNlKyJejJps − One pair of jointed legs per segment. − One pair of modified legs on the first trunk segment.  Form venomous claws to subdue prey and fight predators (class Chilopoda, cheilos = lip, pod = foot).  A few species are poisonous to humans. 21 Pancrustaceans Arthropod phylogeny: − Hexapods (incl. insects) and myriapods were previously grouped based on shared morphological characters:  One pair of antennae; three pairs of mouthparts, incl. mandibles for main mouthparts; respiration via trachea; uniramous limbs. BIOL 108 Winter 2024 © 2024 Neil Harris − Molecular studies strongly suggest crustaceans are paraphyletic: hexapods evolved within crustaceans.  Clade Pancrustaceans include crustaceans (subphylum Crustacea; paraphyletic) and hexapods (subphylum Hexapoda).  Myriapods are sister to Pancrustaceans. 22 Crustaceans BIOL 108 Winter 2024 © 2024 Neil Harris Coconut crab Crustaceans (subphylum Crustacea; paraphyletic) include crabs, lobsters, shrimp, and their relatives. − 40-50k extant spp. − Mostly marine, but many freshwater and some terrestrial species. − Crustacean morphology is highly variable. Zooplankton crustaceans  Huge size range: microscopic zooplankton to giant crabs. − Isopods include terrestrial, freshwater, and marine species.  Pill bugs are a well-known group of terrestrial isopods. − Copepods are small crustaceans, many of which are planktonic. − Decapods are relatively large crustaceans, including lobsters, crabs, crayfish, and shrimp. Woodlice (pill bugs) (WC) Microscopic copepods (WC) Fig 33.38-9 Ghost crab and krill, examples of decapods 23 Crustacean characteristics BIOL 108 Winter 2024 © 2024 Neil Harris Prawn carapace (red) (WC) Crustacean body plan includes: Daphnia carapace − Three tagmata:  Head, thorax, and abdomen.  The head and thorax often merge in a cephalothorax (cephalo = head). − Cephalothorax is usually covered by a carapace.  An exoskeleton shield that covers dorsal and lateral sides.  Carapace sometimes also covers the abdomen. − Biramous limbs. − Two pairs of antennae. − Compound eyes (often on stalks). Lobster compound eye https://en.wikipedia.org/wiki/Arthropod Gecarcinus quadratus, a land crab (WC) 24 Crustacean characteristics BIOL 108 Winter 2024 © 2024 Neil Harris Crustacean tagmata typically have branched (biramous) limbs adapted for specific functions: − Head: two pairs of antennae, paired segmented mandibles (main mouthparts; biting/chewing); and 2 or more pairs of maxillae (tasting and manipulating food). − Thoracic limbs are usually used for locomotion.  Crustaceans >4 pairs of thoracic limbs. − Abdominal limbs are used for swimming, gas exchange, filter-feeding, or holding eggs. abdomen cephalothorax carapace compound eye mouth parts antennae abdominal limbs (swimmerets) thoracic limbs (walking) 25 Crustacean characteristics BIOL 108 Winter 2024 © 2024 Neil Harris Exoskeleton is often strengthened by biomineralization. − Calcium carbonate incorporated into the cuticle (crust = crunchy). − In barnacles, a group of mostly sessile crustaceans, the cuticle is hardened into a shell. Small crustaceans exchange gases through the cuticle; larger crustaceans have gills. − Gills are usually modified appendages.  Plate-like or fluffy, well-vascularized, thin unsclerotized cuticle. Fig 33.40 Barnacles − O2 is transported by respiratory pigments (e.g. hemocyanin) in the hemolymph. A few terrestrial crustaceans use tracheae. − Branched tunnels from openings in the body wall deliver oxygen directly into internal tissues. e.g. woodlice/pill bugs. Woodlice (WC) Gills of krill (WC) 26 Crustacean life cycle BIOL 108 Winter 2024 © 2024 Neil Harris Crustacean life cycles are highly variable (hard to generalize!). − Most crustaceans have separate sexes and reproduce sexually.  Except for sessile barnacles, which are hermaphrodites, but rarely self-fertilize. https://youtu.be/zVi8qjb-NuQ  Asexual parthenogenesis (embryonic development without fertilization of diploid eggs) is widespread among crustaceans. Copulating barnacles − Sperm transfer occurs via copulation (sexual intercourse).  The male places sperm on or in the female’s sperm-receiving structure.  Copulation occurs via intromission or spermatophore deposition (sperm capsule; deposited at the female’s genital opening). Daphnia brooding eggs (WC) − Some female crustaceans brood their eggs until they hatch. − Direct (e.g. Daphnia) or indirect development (one or more larval stages; most common). European lobster larva (WC) 27 Crustacean ecology and importance BIOL 108 Winter 2024 © 2024 Neil Harris Crustaceans play crucial roles as detritivores, scavengers, and predators in various ecosystems. Planktonic crustaceans (copepods and larval crustaceans) are food for many vertebrates, e.g. fish and whales. https://youtu.be/1_BqC9IIuKU https://youtu.be/9C6HJfeBkno Some crustaceans engage in filter-feeding, including motile (e.g. krill) and sessile (e.g. most barnacles) crustaceans. Others scavenge on plant or animal matter, while others are predators. https://youtu.be/E0Li1k5hGBE Some crustaceans are parasitic, targeting both vertebrates and invertebrates. − e.g. isopod “whale lice”, and copepod “sea lice” on salmon. 28 Hexapods BIOL 108 Winter 2024 © 2024 Neil Harris Subphylum Hexapoda is an enormous clade including insects and their relatives. − 6-legged arthropods (hex = six).  Three pairs of legs on thoracic segments; no appendages on the abdomen. Protura Collembola Diplura − Class Insecta  Most diverse of all animal groups; >1 million described extant species.  Insects inhabit nearly all terrestrial habitats and some freshwater habitats.  Insects are the only group of invertebrates that evolved wings and flight.  Insects diversified several times (adaptive radiation), following the evolution of flight, adaptation to feeding on gymnosperms, and the expansion of flowering plants (insect pollinators). − Other small classes of wingless hexapods:  Protura; Collembola (springtails); Diplura (two-pronged bristletails). 29 Hexapod characteristics BIOL 108 Winter 2024 © 2024 Neil Harris Consistent tagmatization (specialization of body parts): − Head with one pair of antennae and 3 pairs of mouthparts, incl. mandibles for main mouthparts. − Thorax with 3 pairs of jointed uniramous legs ± wings. − Abdomen without paired jointed appendages. Gas exchange occurs through a tracheal system via lateral spiracles. − Branched cuticle-lined tubes carry oxygen directly to tissues. − Most hexapods lack O2-transporting respiratory pigments in their hemolymph. 30 WC Insects The plague flea, Xenopsylla cheopis cheopis BIOL 108 Winter 2024 © 2024 Neil Harris WC Bed bug, Cimex lectularius Wings represent a pivotal evolutionary innovation in insects. − Several basal insect groups lack wings. Silverfish  Paraphyletic Apterygota (a = without), e.g. silverfish. − Most insects have wings as adults.  Clade Pterygota (pter = wing). WC  1-2 pairs of wings on thoracic segments. Insect wings are an extension of the cuticle.  Wings evolved only once within this clade. Wings are a shared, derived trait of clade Pterygota. How and why insect wings evolved is not well understood.  Flight is a key factor in the adaptive radiation of insects. Animals that can fly can evade predators, locate food sources, and colonize new habitats efficiently.  Some Pterygota have secondarily lost their wings. e.g. lice, bed bugs, and fleas. Fig 33.42 Ladybird beetle in flight 31 Insect life cycle BIOL 108 Winter 2024 © 2024 Neil Harris Many insects undergo metamorphosis (indirect development), a process involving distinct larval and adult stages with differing morphology, behaviour, and ecology. − In incomplete metamorphosis, the juveniles, called nymphs, resemble adults but are smaller and undergo multiple moults before reaching full size.  Wings develop slowly over several moults, e.g. grasshoppers and dragonflies. − Insects with complete metamorphosis progress through larval stages known as maggots, grubs, or caterpillars.  The larval stage has different morphology and ecology than the adult, e.g. caterpillar vs. butterfly, maggot vs. fly.  Wings develop all at once during a specialized pupal stage (see Fig 33.43). 32 Insect life cycle Most insects have separate males and females and reproduce sexually (no hermaphrodites). Non-insect hexapods and apterygote insects transfer sperm indirectly via a spermatophore (encapsulated package of sperm). All pterygote insects copulate (sexual intercourse). BIOL 108 Winter 2024 © 2024 Neil Harris Springtail with spermatophore − The male places sperm directly on or in the female’s spermreceiving structure. − Individuals find and recognize members of their species by visual, auditory, or olfactory cues. 33 Insect life cycle BIOL 108 Winter 2024 © 2024 Neil Harris During moulting, insects shed their entire exoskeleton. − Includes cuticle invaginations, e.g. linings of foregut, hindgut, and trachea/tracheoles. − The new cuticle is produced before moulting and is soft and wrinkled. − After the moult, the insect swallows air or water to expand the new exoskeleton. http://tinyurl.com/y5ucq3d8 Shed exoskeleton of a larval mayfly: the small filaments are tracheal linings https://www.science.org/content/article/insectmolting-having-your-lungs-ripped-out 34 Insect ecology and importance BIOL 108 Winter 2024 © 2024 Neil Harris Insects predominantly inhabit terrestrial habitats but often have aquatic larval stages. Insects play key roles as detritivores, aiding in decomposition and nutrient recycling. Insects serve as essential food for many animals, including birds, amphibians, reptiles, and freshwater fish. Beneficial insects: − Some insects, like honeybees, are vital pollinators for crops and plants. − Others, such as ladybugs and lacewings, act as biological control agents by preying on pest species.  Some insects are parasites of other insects, regulating pest populations in ecosystems. Harmful insects: − Insects like mosquitoes carry diseases, posing health risks to humans. − Some species, particularly in larval stages, are pests of crops, causing damage to agricultural yields. 35