Arthropoda: Arachnida Anatomy

Arthropoda

• Classified into subphyla: Trilobita, Chelicerata, and Myriapoda
• The class Arachnida includes scorpions, spiders, mites, ticks, harvestmen, and pseudoscorpions, exhibiting significant anatomical diversity.
• Diverse taxa vary significantly in form and appendages.
• Most arachnids are free-living, thriving in warm, arid climates, with over 80,000 species described and are among the first arthropods to colonize terrestrial habitats.

Arachnid Anatomy

• Possess two tagmata: a cephalothorax (head and thorax) and an abdomen (segmented or unsegmented).
• The abdomen houses reproductive and respiratory organs like tracheae and book lungs.
• The cephalothorax has a pair of chelicerae, pedipalps, and four pairs of walking legs.
• Most are predaceous with fangs, claws, stingers, or venom glands; fangs from modified chelicerae and claws from pedipalps.
• Typically possess a powerful sucking pharynx to ingest fluids and soft tissues from prey.
• Spiders have spinning glands as a unique adaptation.

Arachnid Interaction with Humans

• The majority are harmless to humans, aiding in the control of harmful insects.
• Arachnids digest prey by secreting digestive enzymes and then consuming the pre-digested liquid.
• Black widow and brown recluse spiders are exceptions, possessing dangerous bites
• Scorpion stings are painful, and certain species can inflict fatal stings.
• Disease vectors: ticks and mites, while others cause irritation and damage to plants.

Order Araneae: Spiders

• Large arachnid group of around 40,000 species worldwide
• They possess a compact body with a cephalothorax (prosoma) and an abdomen (opisthosoma) which are unsegmented
• Body joined by a slender pedicel
• Segmented abdomens found in some spiders are considered ancestral
• They have anterior appendages with chelicerae, venom ducts, and leg-like pedipalps with sensory functions

Spider Anatomy and Feeding

• Pedipalp bases can manipulate food
• Four pairs of legs terminating in claws for locomotion

Spider Hunting

• All spiders are predators that primarily kill insects through venom injection
• Some spiders hunt, some ambush, and some ensnare prey using silk nets
• Venom liquefies prey tissues, which are then ingested
• Some spiders chew prey with teeth-like structures on chelicerae, enhancing digestion

Spider Respiration And Sensory

• Gas exchange occurs through book lungs, tracheae, or both
• Book lungs comprised of air pockets that extend into a blood-filled chamber
• Tracheae formed of air tubes that carry air from a spiracle
• Eight simple eyes with lenses, retinas, and optic rods perceive motion (hunting spiders can form images)

Spider Adaptations

• Depend on the fine setae on their legs to detect vibrations due to poor vision
• Web-spinning is a key adaptation
• Spinnerets connect to abdominal silk glands and secrete a scleroprotein
• Spinnerets produce silk stronger than steel and second only to fused quartz fibers

Spider Webs and Diversity

• Diverse silk webs range from simple strands to geometrical orb webs
• Spiders use silk for nest lining, egg sacs, bridge lines, draglines, warning and molting threads, attachment discs, and prey wrapping
• Wolf spiders, jumping spiders, and fisher spiders chase prey and have lost the ability to produce silk

Spider Venom and Interactions With Humans

• Spiders are timid creatures that are allies in controlling insects, not dangerous enemies
• Venom produced is usually harmless to humans
• The most poisonous spiders only bite when threatened
• American tarantulas are not dangerous
• Two genera with severe bites: Latrodectus (black widow, five species) and Loxosceles (brown recluse, 13 species)
• Black widows identified by red hourglass marking, contain neurotoxic venom, potentially fatal
• Brown recluse spiders identified by the violin-shaped stripe, contain haemolytic venom, mildly to seriously toxic
• Dangerous Spiders: Atrax spp. (funnel web spiders in Australia) and Phoneutria genus (South and Central America)

Order Scorpiones: Scorpions

• Ancient terrestrial arthropods, totaling around 1400 species globally
• Most commonly found in tropical and subtropical regions, some in temperate zones
• Generally secretive, hiding during the day and hunting at night
• Sense prey with compound slit sensilla on their legs

Scorpion Anatomy and Feeding

• Predatory arthropods primarily feed on insects and spiders
• Seize prey with pedipalps and shred with chelicerae
• Can locate prey 50 cm away and catch it in three or four quick movements
• Scorpion tagmata: a short cephalothorax (used for sensory) and a pre-abdomen (mesosoma) and post-abdomen (metasoma) which ends in a stinger

Scorpion Distinguishing Features

• Posses chelicerae, pedipalps, and eight-jointed walking legs
• Ventral comb-like pectines are tactile organs used for sex recognition and exploring the ground
• The stinger on the last segment is a bulbous base and a curved barb that injects venom

Order Acari: Ticks and Mites

• Most economically and medically important arachnid group
• Larger than other orders in species and individuals
• Estimated 500,000 to 1 million species
• Live in terrestrial and aquatic habitats: deserts, polar regions, and hot springs
• Most mites are parasitic for at least one stage

Acari Anatomy

• Small, 1 mm or less, except for ticks
• Ticks can expand to 3 cm
• Cephalothorax and abdomen completely fused with no segmentation
• Mouthparts on a capitulum
• Chelicerae function in piercing or tearing food
• Segmented pedipalps lateral to chelicerae

Acari Reproduction

• Adults usually have four pairs of legs, in specialized forms, there are only one to three
• Most transfer sperm directly or use a spermatophore
• Larvae have six legs, succeeding nymphal stages have eight legs
• Dermatophagoides farinae and related species are found in house dust and cause allergies and dermatoses

Acari Ecology

• Most aquatic mites live in freshwater and have hairlike setae and parasitic larvae
• Ecologically important, they can affect food supply and health
• Spider mites (family Tetranychidae) are serious agricultural pests that damage plants
• These mites extract plant cells through webs from silk glands

Class Chilopoda: Centipedes

• Land arthropods with flattened bodies, in moist environments like logs, stones and bark
• Agile carnivores feeding on insects and earthworms
• They use venom claws to kill prey, mandibles to chew
• Scolopendra gigantea is the largest centipede at 30 cm
• Common house centipedes Scutigera contains 15 pairs of legs, catching other insects

Centipedes: Harmlessness

• Most centipedes are harmless: tropical species are dangerous
• Centipede bodies can contain segments from a few to 177
• Each segment has jointed legs except the head and the last two segments

Centipede Anatomical Features

• Appendages of the first segment form venomous claws
• The last pair of legs used for sensory functions are longer than others
• Head appendages: antennae, mandibles, and maxillae, similar to insects
• A pair of eyes on the dorsal side of the head consists of groups of ocelli.
• Digestive is a straight tube empty salivary glands empty at the anterior
• Two pairs of Malpighian tubules are located into the hind part of the intestine

Centipede Body Systems

• Elongated heart with arteries and ostia
• A pair of spiracles in each segment for tracheal system respiration
• Nervous system is arthropodan with a visceral nervous system
• Sexes are separate, with unpaired gonads and paired ducts
• Centipedes lay eggs (oviparous) or bear live young (viviparous), undergo no metamorphosis

Class Diplopoda: Millipedes

• Commonly known as millipedes meaning "thousand feet"
• Inactivity compared to centipedes, walking gracefully
• Prefer dark, moist places such as under logs or stones
• Herbivorous feeding on decayed plant matter
• Millipedes are slow and roll into a coil when disturbed
• They can secrete toxic chemicals as a predation defense mechanism from repugnatorial glands

Millipede Diversity

• Spirobolus and Julus: examples with wide distribution
• There are more than 10,000 species of millipedes worldwide
• The cylindrical body with 25 to more than 100 segments
• Four segments located in the short thorax all bearing one pair of legs

Millipede Anatomy

• Each abdominal segment has two pairs of legs
• Exoskeleton is reinforced with calcium carbonate
• Head anatomy: two clumps of simple eyes, antennae, mandibles, and maxillae; similar to centipedes
• Each abdominal segment contains two pairs of spiracles that connect to tracheal air tubes leading to air chambers
• Appendages of the seventh segment are specialized as copulatory organs

Millipede Reproduction

• Millipedes guard their lays after copulation
• Larval forms have only one pair of legs to each segment.

Phylum Arthropoda: Subphylum Crustacea

• Includes the classes of Crustacea
• Class Malacostraca
• Class Maxillopoda
• Subclass Copepoda

Class Malacostraca

• Largest class of Crustacea (over 20,000 species), exhibiting great diversity
• Classified into subclasses, orders, suborders, infraorders, and superfamilies
• Eight segments located in the thorax and six plus telson in the abdomen
• All segments with appendages, biramous antennules and first thoracic appendages like maxillipeds
• Carapace covers head and part or all of thorax, sometimes absent
• Gills usually thoracic epipods

Malacostraca: Order Decapoda

• Decapoda: shrimps, crabs, lobsters
• Features: thoracic segments fused from carapace; eyes on stalks; the first three pairs of thoracic appendages modified to maxillipeds; five pairs of walking legs
• In crabs: first pair of legs modified to pincers (chelae)
• Size range between millimetres to four metres
• Commercially relevant decapods include crayfishes, lobsters, crabs, and shrimp

Class Maxillopoda

• Contains a number of crustacean groups traditionally considered classes
• They have five cephalic, six thoracic, and usually four abdominal segments plus a telson, reductions common
• No appendages on either the abdomen; an eye is the nauplius

Maxillopoda: Subclass Copepoda

• This group is third only to Malacostraca with billions of tons and species
• Copepods are small (usually a few millimeters or less in length), elongated tapering at the posterior
• Without a carapace with a nauplius eye
• Have a pair of uniramous maxillipeds and flattened thoracic swimming appendages
• Possessed a major articulation on appendage-bearing portion where antennules are employed
• A great number a species diverse in both freeloading and freeliving
• Many parasites so highly modified

Copepod Ecology

• Copepods are have a major contribution at the primary consumer level in aquatic communities
• Highly populated in zooplankton, with a greater proportion of total biomass
• Calanus important dietary component of economically and ecologically fish like herring and sardines for sharks and whales
• Common in various marine invertebrates, economically important
• Copepods serve as hosts for tapeworms and nematodes which show morphoses

Arthropoda: Subphylum Hexapoda: Class Entognatha

• Encompasses entognaths where the base of jawlies within the head
• Features one
• Entomobrya: example species
• Proturans have a minute (1 to 1. 5 mm)
• This species lacks an antennae and lives in humid places in soil
• Diplurids are japygids and campodeids
• This species is eyeless and found in logs
• Collembollanous are jumping eye patches

Hexapoda: Class Insecta

• Insects: in-sekta with two mandibular regions
• Drosophila, Bombus, and Anopheles
• Subclass Apterygota: primitive with no wings
• Thysanura: silverfish and bristletails with long antennae

Pterygota

• This species lives around stones from the around the surrounding environment
• Condition if winged

Pterygota: Paleoptera

• Ephemeroptera features -membranous wings the aquatic -with wings with lateral tracheas
• Lacks odoanta lacks a gill containing labium to acquire food -Dragonflies are nets that long the body

Arthropoda: Neoptera

• Orthoptera: Locust and grasshoppers that fold
• Blattode: Tropicals known as segments are known to exceed houses
• Phasmatodea: herbivore with expanded forms to be long

Factors Contributing to the Biological Success of Arthropods

• Versatile Exoskeleton: strong without preventing mobility
• The cuticle skeleton: layers that secret underlying epidermis -Procuticles are thick and epicuticles are thin with layers -Protein are stabilized with waxes that limit heat loss -Procuticles are secreted and the exocuticles are layered with Chitin, which is a flexible polysaccharide

Segmentation and Appendages contributing to Arthropoda

• Ancestral body plans for appendages that create a wide range
• Combination of segments with tagamata
• Functional groups that contain legs and striated muscles that provide action
• Sensory hairs are also in walking and swimming

Internal Systems

• Air is piped to cells through tubes, aquatic environments are breathable through gills
• Highly developed organs aid in touch-sensitive areas
• Complex behaviour patterns: innate learning controls aspects of activities
• Limiting Intraspecific Competition through Metamorphosis change their structure