Subphylum Chelicerata Subphylum Chelicerata n n 70 000

Subphylum Chelicerata

Subphylum Chelicerata n n 70, 000 species Spiders, scorpions, mites, and horseshoe crabs Most are terrestrial Very diverse, with many interesting adaptations to terrestrial existence

General Body Form n Possess two tagmata n n n Cephalothorax Abdomen No antennae, but typically two compound eyes and four ocelli Chelicerae – most anterior appendages; used for feeding Pedipalps – second appendages with various functions Walking legs – four pairs Abdomen n Typically lacks appendages Houses many visceral organs Respiratory structures

Class Xiphosura n Horseshoe crabs n Largest chelicerates n n Up to 75 cm long Only three genera, comprised of four species n n Limulus is the most famous; lives off N. American Atlantic coast Remaining three species inhabit the Indo-Pacific Ocean

Body Form n Cephalothorax is shaped like a horse’s hoof n n n Dorsal exoskeleton is called a carapace Two large lateral compound eyes and two medial ocelli Appendages are located ventrally; most have chelae n n Chelicerae Pedipalps Three pairs of walking legs Pusher legs for pushing soft sediment

n Abdomen n n Ventral concavity that forms branchial chamber One pair of large flat genital opercula, with two gonopores (either male or female) Five pairs of book gills are housed here Long tail spine (telson) that extends posteriorly from last abdominal segment n n n Used for pushing Righting body Anus located near base of tail spine

Nutrition n Omnivorous n n Eat molluscs, worms, algae, etc. Walking leg coxae have gnathobases Medial space between gnathobases forms a food groove Gnathobases crush food and particles are moved towards mouth

Digestive System n n n J-shaped gut Mouth opens posteriorly at anterior end of food groove Stomach n Has two pairs of large digestive ceca n n n For extracellular digestion and absorption May be white as they take up extra calcium from blood, which is later voided with feces Not edible; some have high concentrations of saxotoxin

Hemal System n Hemocoel is main body cavity n Heart has 9 pairs of ostia n Blood contains: n n Hemocyanin Two types of hemocytes (for clotting and producing more hemocyanin)

Respiratory System n n Book gill surface area is approximately 2 m 2 Gill movements ventilate branchial chamber Inhalant aperture on each lateral aspect of carapace (at junction of cephalothorax and abdomen) Exhalant aperture at both sides of tail spine

Excretory System n n n Ammonotelic, so wastes removed across gill surface Four pairs of saccate nephridia assist with osmoregulation in brackish water Two nephridiopores located near the last pair of walking legs

Nervous System n CNS n n Large brain Two longitudinal nerve cords Five pairs of segmental ganglia that serve book gills Sensory structures n Lateral compound eyes n n n 1000 ommatidia each Do not form images, but detect movement Flabellum n n Located on pusher leg, in the path of the inhalant current Chemosensor that monitors water quality

Reproduction n Gonochoric External fertilization via flagellated sperm Have specific mating seasons in spring and summer n n n Influenced by lunar cycles and tides Mating occurs near shore Smaller male climbs onto female’s dorsal abdomen and claps her with pedipalps Female digs a depression in the sand oviposits 2000 – 30, 000 eggs Male releases sperm into depression

n Larvae are called trilobite larvae n n n One cm long Small tail spine and two pairs of book gills Burrow in sand 9 – 12 years to reach sexual maturity May live up to 19 years

Class Arachnida n n 70, 000 described, with an estimated one million more undescribed Spiders, scorpions, solifuges, and mites Almost entirely terrestrial Very well adapted for terrestrial existence n n Waxy epicuticle Book gills transformed into book lungs for breathing air Malpighian tubules Possess other interesting adaptations n n Silk Poison glands

Body Form n n Two tagma are sometimes connected by a pedicel Similar appendages in all n n n Chelicerae – one pair that may have fangs and poison glands Pedipalps – one pair that have many functions, and may have gnathobases Walking legs – four pairs that may terminate in claws

Nutrition n n Mostly carnivorous, but some are ectoparasites Digestion begins extraorally n n n Prey is captured and killed Chelicerae and gnathobases of pedipalps macerate Digestive enzymes digest Liquids are ingested Gut is adapted for a liquid diet n n n Either pharynx or stomach functions as a muscular pump Gut has a very small diameter No gizzard

Gas Exchange n Small arachnids use body surface n Large arachnids have two types of organs n Book lungs n n n Invaginations of ventral abdominal wall with a slit-shaped spiracle Atrium contains lamellae supported by pedestals Muscle contractions expand atrium to facilitate ventilation Atrium surrounded by pulmonary sinus Trachea n n n Do not make direct contact with cells as in insects Blood delivers gasses (possesses hemocyanin) Tracheae conserve more water than book lungs (huge SA)

Excretion n Uricotelic Possess either saccate nephridia, Malpighian tubules, or both Also possess nephrocytes n n n Large phagocytic cells that police the hemocoel Sequester metabolic wastes indefinitely May detoxify wastes and return to blood

Nervous System n Highly cephalized n Sensory organs n Trichobothria n n n Slit sense organs n n n Setae found in high concentrations on legs Can sense air currents from fly wings, and aid in prey capture Slits in exoskeleton with a thin membrane covering Detect strain on exoskeleton May be found in clusters called lyriform organs Some have as many as 3000 slit sense organs Flagellum n Lack antennae, so some posses a posterior sensory flagellum

Reproduction n Always gonochoric Always internal fertilization Sperm transfer n Direct n n Penis Indirect n Spermatophore placed on ground, or transferred by pedipalp

Order Scorpiones n n 1200 species Found on all continents except Antarctica n n Secretive and nocturnal n n Baja California has greatest diversity Hide under rocks, logs, or in burrows during the day Emerge at night to hunt Fluoresce green under UV light Most are 3 – 9 cm, but Hadogenes from Africa can reach 21 cm

Body Form n Long segmented abdomen n Posses a telson (sting) n n Pedipalps are enlarged to form pincers No pedicel Two comb-like sensory structures (pectines) on ventral surface Several segments possess spiracles that open into book lungs

Nutrition n n Feed on other inverts, especially insects and arachnids Ambush predators n n Wait at burrow entrance Claws open and ready to strike Some dig up prey Some can catch prey in midair

n n Prey is held with pedipalps and stabbed with sting (if needed) Metabolic rates are among the lowest recorded for animals n n Conversion of prey to biomass is very efficient n n Can survive over one year between meals One meal can increase body weight by 1/3 Poison glands n Only 25 species are potentially lethal to humans n n Centruoides in SW US Androctonus in N. Africa

Organ Systems n Respiratory n n Hemal n n Four pairs of book lungs, with spiracles located ventrally Standard Excretory n n Two pairs of Malpighian tubules One pair of saccate nephridia

n Nervous system n Ventral nerve cord n Seven segmental ganglia n n Giant axons for rapid closing of claws and stinging motion Pectines n n Sensory role is unclear Sensitive to ground vibrations Suspected chemosensory role in spermatophore deposition Ablation of pectines results in failure of male to deposit spermatophore

Adaptations to Arid Existence n Lowest evaporative water loss of any invertebrate n 0. 01% body weight / hour n Tolerate up to 40% body water loss n Obtain water from food and rarely drink n Uricotelic n Impermeable exoskeleton n Burrow up to one meter in some n Nocturnal n Raise bodies off of ground (stilting)

Reproduction n Indirect sperm transfer via spermatophore Usually a courtship ritual where claws are grasped and the two walk around as if dancing Male deposits spermatophore n n Stands approximately 6 mm high Consists of a lever, stalk, ejection apparatus and sperm reservoir Male positions female over it Female pectines cause sperm reservoir to eject into open gonopore

n Viviparous n Direct development n n n 1 to 95 young depending on species Juveniles crawl onto mother’s back and remain through first molt (4 weeks) High degree of parental care Six months to six years to reach sexual maturity May live 25 years

Order Uropygi n n n 100 species of whip scorpions Some such as the North American vinegaroon (Mastigoproctus giganteus) can reach 8 cm in length Interesting structures n n n Large pedipalps with pincers Flagellum Repugnatorial glands near anus, that spray acetic acid at predators In some, males use pedipalps to transfer spermatophore to female’s gonopore Females lay eggs in a sac that she attaches to her body

Order Amblypygi n Whip spiders n Secretive and nocturnal n 4 to 45 mm n Flattened bodies that share characteristics of spiders and crabs n Often move sideways like crabs n Lack poison glands n Large spiny pedipalps n First legs are extremely long and thin n Sensory One pointed forward and one sideways while animal moves Have a pedicel

Order Araneae n 40, 000 species of spiders n Huge population densities n n Silk is probably most important adaptation to spider existence n n n Five million / hectare in one field in Great Britain Only arachnids with silk spinning appendages (spinnerets) Possess a pedicel, which allows abdomen to move independently of cephalothorax, in order to distribute silk Carnivorous

Body Form n Large variation in body size (0. 5 mm to 9 cm) n Typically have eight eyes n Chelicerae possess fangs and poison glands n n Distal tips of male pedipalps are knoblike and modified for sperm transfer Four pairs of walking legs that terminate in either two (cursorial) or three (web building) claws Gonopore and book lung spiracles located ventrally Tracheal spiracle located posteriorly, near spinnerets

Silk n n n Produced by abdominal silk glands Extruded from spigots on distal tips of spinnerets Different types of glands (six known) that produce different silks It is a protein that is emitted as a liquid , but hardens as it is drawn out Strong, but resistance to breakage is mainly due to its elasticity Many functions: n n n Prey capture (web building) Shelters Egg cases Draglines Balooning in spiderlings

Prey Capture n Web building spiders n n n Most famous type of web is the orb web n n Webs are strategically placed and oriented according to prey habits Web type differs between species, which partitions resources One plane Radial threads aren’t sticky (spider walks on these) Catching spiral is sticky Spiders are sensitive to web vibrations n Can sense type, size and location of prey

n Cursorial (hunting) spiders n Do not build webs n Ambush or actively forage n Heavier legs n Possess scopulae n n Tufts of setae near claws Each seta may have over 1000 branches (huge SA) Use for adhering to slick surfaces and prey Can walk up walls, glass, or upside-down across ceilings

Nutrition n n Fangs on chelicerae penetrate prey Muscles surrounding poison glands contract Low metabolic rates Can double body weight with one meal Mainly carnivores of insects, but some wasps (tarantula hawks and mud daubers) use spiders as food for larvae

n Most spider venom is not toxic to humans n A few genera are toxic, but rarely lethal n Latrodectus n n n Loxosceles n n Black widows have neurotoxic venom Symptoms of bites include nausea, pain, and muscles spasms Recluses have hemotoxic venom Six eyes, and a violin shape on cephalothorax Causes tissue necrosis A few genera are lethal n Atrax n n n Australia’s funnel-web spider World’s deadliest spider Trechona n Lives in South America

Gas Exchange n n Respiratory structures located ventrally Primitive spiders (such as tarantulas) possess two pairs of book lungs Most possess one anterior pair of book lungs and one posterior set of tracheae (with one spiracle) Some only possess tracheae

Hemal and Nervous Systems n Hemal n n Heart only has 2 or 3 pairs of ostia Spiders lack extensor muscles, so legs are extended by elevating hemocoel pressure n n n Legs are always flexed in dead spiders Jumping spiders can jump 50 X body length by quickly elevating blood pressure Nervous n n Eyes lack ommatidia Vision is important to cursorial spiders n n Jumping spiders have best vision of all arachnids Main eyes form images while secondary eyes detect movement

Reproduction n n Sexually dimorphic (males usually much smaller) Highly modified pedipalps are used for sperm transfer n n n Have a sperm reservoir Male builds sperm web and extrudes sperm (not spermatophore) onto web Dips pedipalp into sperm, fills reservoir, and searches for receptive female

n n Courtship behavior, chemical and tactile cues are important so that the female doesn’t eat the male Orb-weaver males pluck radial threads of female’s web during courtship n n Number, frequency and intensity of plucks is species specific Female lays up to 300 eggs in a silk case

n n Egg case is attached to web, inside of home, or spider itself Spiderlings remain in egg case until completing first molt Disperse via ballooning Most live one to two years, but some have lived for 25 years in captivity

Order Pseudoscorpiones n Small, less than 7 mm n 2500 species n Live in leaf litter n n n Large chelate pedipalps with poison glands Feed on small arthropods such as mites Exhibit a phenomenon known as phoresy n Grab leg of a flying insect and hitch a ride

Order Solifugae n n n Many names: solifuges (with many variations on spelling), sun spiders or wind scorpions Body size: a few mm to 7 cm Large chelicerae with the largest mandibles (relative to size) in the entire animal kingdom n Two median eyes n Long, leg-like pedipalps

n n First legs are used as antennae Carnivorous or omnivorous n n n Many eat termites Tracheae are similar to those of insects Oddly shaped sensory organs n n Called racquet organs Two to five pairs hang from fourth legs

Order Opiliones n n 5000 species of daddy longlegs or harvestmen Lack silk glands and poison glands Many are herbivores or feed on dead organic matter Gut is adapted for ingesting and processing particulate matter

n n Body length sometime reaches 22 mm, but leg length ca reach 160 mm May be brightly colored Long legs are excellent for maneuvering over rough terrain Males possess a penis and females possess an ovipositor

Order Acari n n n 40, 000 species of mites Most mites are free-living, but some are parasites of people, animals, and crops Mites are the smallest arachnids n n n Some can inhabit the tracheae of honeybees Some live in human hair follicles The largest mites (called ticks) can reach 3 cm when engorged with blood

n n Lack segmentation; cephalothorax is fused with abdomen Can be internal or external parasites n n n Larvae of Trombicula are referred to as chiggers Mange mite burrows under skin, lays eggs, and causes a skin condition known as scabies Can also be vectors of diseases n Example – Lyme disease

Class Pycnogonida n n 1000 species of sea spiders Marine, but not really spiders Crawl around on eight long legs One mm to 10 mm long normally, but Colossendeis can achieve leg spans of 70 cm

n n n Mostly feed on sessile animals like hydroids, soft corals, and anemones Some are ectoparasites Ovary has branches that extend into femurs n n Gravid females have swollen legs Males have ovigerous legs (ovigers) that are used to carry egg masses n Gathers released eggs and cements them to ovigers