Description Spider

1 description

1.1 body plan
1.2 circulation , respiration
1.3 feeding, digestion , excretion
1.4 central nervous system
1.5 sense organs

1.5.1 eyes
1.5.2 other senses


1.6 locomotion
1.7 silk production
1.8 reproduction , life cycle
1.9 size
1.10 coloration

description

body plan

spiders chelicerates , therefore arthropods. arthropods have: segmented bodies jointed limbs, covered in cuticle made of chitin , proteins; heads composed of several segments fuse during development of embryo. being chelicerates, bodies consist of 2 tagmata, sets of segments serve similar functions: foremost one, called cephalothorax or prosoma, complete fusion of segments in insect form 2 separate tagmata, head , thorax; rear tagma called abdomen or opisthosoma. in spiders, cephalothorax , abdomen connected small cylindrical section, pedicel. pattern of segment fusion forms chelicerates heads unique among arthropods, , first head segment disappears @ stage of development, chelicerates lack antennae typical of arthropods. in fact, chelicerates appendages ahead of mouth pair of chelicerae, , lack function directly jaws . first appendages behind mouth called pedipalps, , serve different functions within different groups of chelicerates.

spiders , scorpions members of 1 chelicerate group, arachnids. scorpions chelicerae have 3 sections , used in feeding. spiders chelicerae have 2 sections , terminate in fangs venomous, , fold away behind upper sections while not in use. upper sections have thick beards filter solid lumps out of food, spiders can take liquid food. scorpions pedipalps form large claws capturing prey, while of spiders small appendages bases act extension of mouth; in addition, of male spiders have enlarged last sections used sperm transfer.

in spiders, cephalothorax , abdomen joined small, cylindrical pedicel, enables abdomen move independently when producing silk. upper surface of cephalothorax covered single, convex carapace, while underside covered 2 rather flat plates. abdomen soft , egg-shaped. shows no sign of segmentation, except primitive mesothelae, living members liphistiidae, have segmented plates on upper surface.

circulation , respiration

like other arthropods, spiders coelomates in coelom reduced small areas round reproductive , excretory systems. place largely taken hemocoel, cavity runs of length of body , through blood flows. heart tube in upper part of body, few ostia act non-return valves allowing blood enter heart hemocoel prevent leaving before reaches front end. however, in spiders, occupies upper part of abdomen, , blood discharged hemocoel 1 artery opens @ rear end of abdomen , branching arteries pass through pedicle , open several parts of cephalothorax. hence spiders have open circulatory systems. blood of many spiders have book lungs contains respiratory pigment hemocyanin make oxygen transport more efficient.

spiders have developed several different respiratory anatomies, based on book lungs, tracheal system, or both. mygalomorph , mesothelae spiders have 2 pairs of book lungs filled haemolymph, openings on ventral surface of abdomen allow air enter , diffuse oxygen. case basal araneomorph spiders, family hypochilidae, remaining members of group have anterior pair of book lungs intact while posterior pair of breathing organs partly or modified tracheae, through oxygen diffused haemolymph or directly tissue , organs. trachea system has evolved in small ancestors resist desiccation. trachea connected surroundings through pair of openings called spiracles, in majority of spiders pair of spiracles has fused single 1 in middle, , moved backwards close spinnerets. spiders have tracheae have higher metabolic rates , better water conservation. spiders ectotherms, environmental temperatures affect activity.

feeding, digestion , excretion

cheiracanthium punctorium, displaying fangs

uniquely among chelicerates, final sections of spiders chelicerae fangs, , great majority of spiders can use them inject venom prey venom glands in roots of chelicerae. families uloboridae , holarchaeidae, , liphistiidae spiders, have lost venom glands, , kill prey silk instead. arachnids, including scorpions, spiders have narrow gut can cope liquid food , spiders have 2 sets of filters keep solids out. use 1 of 2 different systems of external digestion. pump digestive enzymes midgut prey , suck liquified tissues of prey gut, leaving behind empty husk of prey. others grind prey pulp using chelicerae , bases of pedipalps, while flooding enzymes; in these species, chelicerae , bases of pedipalps form preoral cavity holds food processing.

the stomach in cephalothorax acts pump sends food deeper digestive system. mid gut bears many digestive ceca, compartments no other exit, extract nutrients food; in abdomen, dominated digestive system, few found in cephalothorax.

most spiders convert nitrogenous waste products uric acid, can excreted dry material. malphigian tubules ( little tubes ) extract these wastes blood in hemocoel , dump them cloacal chamber, expelled through anus. production of uric acid , removal via malphigian tubules water-conserving feature has evolved independently in several arthropod lineages can live far away water, example tubules of insects , arachnids develop different parts of embryo. however, few primitive spiders, sub-order mesothelae , infra-order mygalomorphae, retain ancestral arthropod nephridia ( little kidneys ), use large amounts of water excrete nitrogenous waste products ammonia.

central nervous system

the basic arthropod central nervous system consists of pair of nerve cords running below gut, paired ganglia local control centers in segments; brain formed fusion of ganglia head segments ahead of , behind mouth, esophagus encircled conglomeration of ganglia. except primitive mesothelae, of liphistiidae sole surviving family, spiders have more centralized nervous system typical of arachnids: ganglia of segments behind esophagus fused, cephalothorax largely filled nervous tissue , there no ganglia in abdomen; in mesothelae, ganglia of abdomen , rear part of cephalothorax remain unfused.

despite relatively small central nervous system, spiders (like portia) exhibit complex behaviour, including ability use trial-and-error approach.

sense organs
eyes

this jumping spider s main ocelli (center pair) acute. outer pair secondary eyes , there other pairs of secondary eyes on sides , top of head.

spiders have 4 pairs of eyes on top-front area of cephalothorax, arranged in patterns vary 1 family another. principal pair @ front of type called pigment-cup ocelli ( little eyes ), in arthropods capable of detecting direction light coming, using shadow cast walls of cup. however, in spiders these eyes capable of forming images. other pairs, called secondary eyes, thought derived compound eyes of ancestral chelicerates, no longer have separate facets typical of compound eyes. unlike principal eyes, in many spiders these secondary eyes detect light reflected reflective tapetum lucidum, , wolf spiders can spotted torch light reflected tapeta. on other hand, jumping spiders secondary eyes have no tapeta.

other differences between principal , secondary eyes latter have rhabdomeres point away incoming light, in vertebrates, while arrangement opposite in former. principal eyes ones eye muscles, allowing them move retina. having no muscles, secondary eyes immobile.

some jumping spiders visual acuity exceeds factor of ten of dragonflies, have far best vision among insects; in fact human eye 5 times sharper jumping spider s. achieve telephoto-like series of lenses, four-layer retina , ability swivel eyes , integrate images different stages in scan. downside scanning , integrating processes relatively slow.

there spiders reduced number of eyes, of these six-eyes numerous , missing pair of eyes on anterior median line, others species have four-eyes , two. cave dwelling species have no eyes, or possess vestigial eyes incapable of sight.

other senses

as other arthropods, spiders cuticles block out information outside world, except penetrated many sensors or connections sensors nervous system. in fact, spiders , other arthropods have modified cuticles elaborate arrays of sensors. various touch sensors, bristles called setae, respond different levels of force, strong contact weak air currents. chemical sensors provide equivalents of taste , smell, means of setae. pedipalps carry large number of such setae sensitive contact chemicals , air-borne smells, such female pheromones. spiders have in joints of limbs slit sensillae detect forces , vibrations. in web-building spiders, these mechanical , chemical sensors more important eyes, while eyes important spiders hunt actively.

like arthropods, spiders lack balance , acceleration sensors , rely on eyes tell them way up. arthropods proprioceptors, sensors report force exerted muscles , degree of bending in body , joints, understood. on other hand, little known other internal sensors spiders or other arthropods may have.

locomotion

image of spider leg: 1–coxa; 2–trochanter; 3–femur; 4–patella; 5–tibia; 6–metatarsus; 7–tarsus; 8–claws

each of 8 legs of spider consists of 7 distinct parts. part closest , attaching leg cephalothorax coxa; next segment short trochanter works hinge following long segment, femur; next spider s knee, patella, acts hinge tibia; metatarsus next, , connects tibia tarsus (which may thought of foot of sorts); tarsus ends in claw made of either 2 or 3 points, depending on family spider belongs. although arthropods use muscles attached inside of exoskeleton flex limbs, spiders , few other groups still use hydraulic pressure extend them, system inherited pre-arthropod ancestors. extensor muscles in spider legs located in 3 hip joints (bordering coxa , trochanter). result, spider punctured cephalothorax cannot extend legs, , legs of dead spiders curl up. spiders can generate pressures 8 times resting level extend legs, , jumping spiders can jump 50 times own length increasing blood pressure in third or fourth pair of legs. although larger spiders use hydraulics straighten legs, unlike smaller jumping spiders depend on flexor muscles generate propulsive force jumps.

most spiders hunt actively, rather relying on webs, have dense tufts of fine hairs between paired claws @ tips of legs. these tufts, known scopulae, consist of bristles ends split many 1,000 branches, , enable spiders scopulae walk vertical glass , upside down on ceilings. appears scopulae grip contact extremely thin layers of water on surfaces. spiders, other arachnids, keep @ least 4 legs on surface while walking or running.

silk production


an orb weaver producing silk spinnerets

the abdomen has no appendages except have been modified form 1 4 (usually three) pairs of short, movable spinnerets, emit silk. each spinneret has many spigots, each of connected 1 silk gland. there @ least 6 types of silk gland, each producing different type of silk.

silk composed of protein similar used in insect silk. liquid, , hardens not exposure air result of being drawn out, changes internal structure of protein. similar in tensile strength nylon , biological materials such chitin, collagen , cellulose, more elastic. in other words, can stretch further before breaking or losing shape.

some spiders have cribellum, modified spinneret 40,000 spigots, each of produces single fine fiber. fibers pulled out calamistrum, comb-like set of bristles on jointed tip of cribellum, , combined composite woolly thread effective in snagging bristles of insects. earliest spiders had cribella, produced first silk capable of capturing insects, before spiders developed silk coated sticky droplets. however, modern groups of spiders have lost cribellum.

tarantulas have silk glands in feet.

even species not build webs catch prey use silk in several ways: wrappers sperm , fertilized eggs; safety rope ; nest-building; , parachutes young of species.

reproduction , life cycle



mating behaviour of neriene radiata



the tiny male of golden orb weaver (nephila clavipes) (near top of leaf) protected female producing right vibrations in web, , may small worth eating.

spiders reproduce sexually , fertilization internal indirect, in other words sperm not inserted female s body male s genitals intermediate stage. unlike many land-living arthropods, male spiders not produce ready-made spermatophores (packages of sperm), spin small sperm webs on ejaculate , transfer sperm special syringe-like structures, palpal bulbs or palpal organs, borne on tips of pedipalps of mature males. when male detects signs of female nearby checks whether of same species , whether ready mate; example in species produce webs or safety ropes , male can identify species , sex of these objects smell .

spiders use elaborate courtship rituals prevent large females eating small males before fertilization, except male smaller not worth eating. in web-weaving species, precise patterns of vibrations in web major part of rituals, while patterns of touches on female s body important in many spiders hunt actively, , may hypnotize female. gestures , dances male important jumping spiders, have excellent eyesight. if courtship successful, male injects sperm palpal bulbs female s genital opening, known epigyne, on underside of abdomen. female s reproductive tracts vary simple tubes systems include seminal receptacles in females store sperm , release when ready. because sperm stored in epigyne, eggs not fertilized while inside female, during oviposition when stored sperm released chamber. 1 exception spider israel, harpactea sadistica, has evolved traumatic insemination. in species male penetrate pedipalps through female s body wall , inject sperm directly ovaries, embryos inside fertilized eggs start develop before being laid.

males of genus tidarren amputate 1 of palps before maturation , enter adult life 1 palp only. palps 20% of male s body mass in species, , detaching 1 of 2 improves mobility. in yemeni species tidarren argo, remaining palp torn off female. separated palp remains attached female s epigynum 4 hours , apparently continues function independently. in meantime, female feeds on palpless male. in on 60% of cases, female of australian redback spider kills , eats male after inserts second palp female s genital opening; in fact, males co-operate trying impale on females fangs. observation shows male redbacks never opportunity mate, , lucky ones increase number of offspring ensuring females well-fed. however, males of species survive few matings, limited short life spans. live while in mates webs.

females lay 3,000 eggs in 1 or more silk egg sacs, maintain constant humidity level. in species, females die afterwards, females of other species protect sacs attaching them webs, hiding them in nests, carrying them in chelicerae or attaching them spinnerets , dragging them along.

baby spiders pass larval stages inside egg , hatch spiderlings, small , sexually immature similar in shape adults. spiders care young, example wolf spider s brood cling rough bristles on mother s back, , females of species respond begging behaviour of young giving them prey, provided no longer struggling, or regurgitate food.

like other arthropods, spiders have molt grow cuticle ( skin ) cannot stretch. in species males mate newly molted females, weak dangerous males. spiders live 1 2 years, although tarantulas can live in captivity on 20 years.

size

goliath birdeater (theraphosa blondi), largest spider

spiders occur in large range of sizes. smallest, patu digua colombia, less 0.37 mm (0.015 in) in body length. largest , heaviest spiders occur among tarantulas, can have body lengths 90 mm (3.5 in) , leg spans 250 mm (9.8 in).

coloration

only 3 classes of pigment (ommochromes, bilins , guanine) have been identified in spiders, although other pigments have been detected not yet characterized. melanins, carotenoids , pterins, common in other animals, apparently absent. in species, exocuticle of legs , prosoma modified tanning process, resulting in brown coloration.

bilins found, example, in micrommata virescens, resulting in green color. guanine responsible white markings of european garden spider araneus diadematus. in many species accumulated in specialized cells called guanocytes. in genera such tetragnatha, leucauge, argyrodes or theridiosoma, guanine creates silvery appearance. while guanine end-product of protein metabolism, excretion can blocked in spiders, leading increase in storage. structural colors occur in species, result of diffraction, scattering or interference of light, example modified setae or scales. white prosoma of argiope results hairs reflecting light, lycosa , josa both have areas of modified cuticle act light reflectors.