Phylum Arthropoda

Phylum Arthropoda • The Arthropoda (from the Greek Arthron, joint and podus, foot) are the largest group of organisms and they occur in all environments on earth. • The group includes: spiders, ticks, mites, centipedes, millipedes, crustaceans, insects and others.

Characteristics of the Arthropoda • Bilaterally symmetrical with a segmented (metameric) body divided into head, thorax and abdomen; cephalothorax and abdomen; or fused head and trunk.

Characteristics of the Arthropoda • Jointed appendages. Primitively one pair per segment, but number often reduced. • Appendages often greatly modified for specialized tasks.

Characteristics of the Arthropoda • Exoskeleton of cuticle. • Exoskelton secreted by underlying epidermis. Made of chitin, protein, lipid and often calcium carbonate. • Exoskeleton is shed periodically (ecdysis) as the organism grows.

Characteristics of the Arthropoda • Muscular system is complex and muscles attach to the exoskeleton. • Striated muscles for voluntary movement and smooth muscles for the viscera. • Coleom is reduced. Most of body cavity is a hemocoel (sinuses or spaces) filled with blood.

Characteristics of the Arthropoda • Complete digestive system. Mouthparts are specialized being modified from appendages and specialized for dealing with various types of food. • Open circulatory system with a dorsal contractile heart arteries and hemocoel.

Characteristics of the Arthropoda • Respiration occurs in multiple possible ways • across the body surface • via a system of tracheal tubes • gills • book lungs

Characteristics of the Arthropoda • Nervous system is similar to that of annelids. A dorsal brain connected via a ring around the pharynx to a double ventral nerve cord.

Characteristics of the Arthropoda • Sexes are usually separate with internal fertilization. • Can be oviparous or ovoviviparous. • Offspring often go through process of metamorphosis. • Parthenogenesis occurs in a some species (e.g. aphids).

Classification of Phylum Arthropoda • Subphylum Chelicarata: horseshoe crabs, spiders, tick, mites, scorpions, • Subphylum Crustacea: crabs, lobsters, copepods, barnacles, pill bugs • Subphylum Myriapoda: millipedes and centipedes • Subphylum Hexapoda: springtails, insects

Classification of the Arthropoda • Subphylum Chelicerata • Class Merostomata • Subclass Xiphosurida: Horseshoe crabs • Class Pycnogonida: sea spiders • Class Arachnida: spiders, scorpions, ticks, mites, harvestmen, whip scorpions.

Subphylum Chelicerata • Body is divided into two tagmata (fused segments) head and abdomen. • Six pairs of appendages: • a pair of chelicerae, first pair of appenadges used for feeding. • a pair of pedipalps (not in horseshoe crabs) • 4 pairs of walking legs (5 in horseshoe crabs). • No mandibles or antennae.

Subphylum Chelicerata • The chelicrates are a very ancient group that includes the extinct Eurypterids (giant water scorpions (200-500 mya), which were the largest known arthropods reaching up to 3m long.

Eurypterus (left) from the Silurian of New York State. Eurypterus remipes was voted the New York state fossil in 1983. Mixopterus (right) from the Devonian reached about one meter in length

Subphylum Chelicerata • Subclass Xiphosurida: Horseshoe crabs. These are an ancient group that date from the Cambrian (543-495 mya). • Five living species. Limulus, which lives in shallow waters on the Atlantic coast of the U.S. is almost unchanged since the Triassic (251-206 mya).

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Subphylum Chelicerata: Horseshoe crabs • Horseshoe crabs have an unsegmented carapace, which is sort of horseshoe shaped, hence the name. • There is also a wide abdomen with a long spinelike tailpiece (telson). • Flat leaf-like gills, called book gills, are present on the underside.

Horseshoe crabs • Horseshoe crabs feed on foods such as worms and molluscs, such as clams, which brings them into conflict with fishermen. • The synchonized breeding of horseshoe crabs which come to spawn on the beaches of the mid-Atlantic coast of the U.S. at the lunar high tides is a striking sight in summer. • The eggs produced in the millions are an important food source for migrating shorebirds such as Knot, which flock to areas such as the Delaware Bay to fatten up for migration.

Horseshoe crabs • Horseshoe crabs are harvested commercially for bait and also for their blood, which is used in laboratory testing for endotoxins in medical products. • Currently, the horseshoe crab is at the center of a series of legal battles about the size of harvests and their sustainability.

Subphylum Chelicerata: Class Pycnogonida: sea spiders • Another name used for these animals is Pantopoda (“all legs”), which is an excellent description. • The body is greatly reduced in size, whereas the legs are long and clawed. In some species modified legs called ovigers are used by males to carry egg masses. • There is a long proboscis on the head, which the pycnogonid uses to feed on soft-bodied invertebrates, especially cnidarians.

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Class Pycnogonida: sea spiders • Most pycnogonids are small being only a few millimeters long, but a few reach 70cm measured by leg spread. • Pycngonids are exclusively marine (occuring from the intertidal to the deep seas) and there are about 1000 species.

Subphylum Chelicerata: Class Arachnida: Order Aranae: spiders • Spiders are a very large group with more than 35,000 described species that occur worldwide except in Antarctica. • The body is divided into a cephalothorax and an abdomen. • Spiders breathe using book lungs or tracheae. Book lungs are unique to spiders and consist of a large number of air pockets extending into a blood-filled chamber. Tracheae are tubules that carry air directly from outside to tissues. They can be closed to prevent excessive water loss.

Spiders • Spiders and insects have an excretory system that uses structures called Malphigian tubules. • Potassium, other salts, and other waste solutes drain into these tubules, which empty into the intestine. Rectal glands selectively reabsorb most of the potassium and water leaving behind nitrogenous wastes in the form of uric acid, which requires little water for its excretion.

Spiders • Spiders possess eight eyes, but their image forming ability is limited. • Hairlike setae, however, provide a lot of information about the environment sensing e.g. vibrations and air currents

Spiders • All spiders are predators and their chelicerae function as fangs. • The fangs are connected via ducts to venom glands that produce a lethal venom the spider uses to dispatch its prey. • After killing a prey item the spider injects digestive fluid into the organism and sucks up the resulting soup.

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Spiders • Spiders use a variety of techniques to catch prey. • Some chase their prey or leap on it, some ambush prey often using trip wires, and, of course, many use webs. • One small group, the bolas spiders, capture their prey by deception.

Bolas spiders • Bolas spiders at night spin a line of thread with a sticky globule (the bolas: akin to the bolas used by South American Gauchos to hunt) at the free end. • The bolas contains pheromones which attract certain male moths which think they are approaching a female. When the spider senses the vibrations in the air caused by a nearby flying moth it twirls its bolas in response. • Once the bolas strikes the moth it sticks and the moth is hauled in.

Silk-spinning • Spiders spin silk, which they use to make webs to trap prey. • Webs come in a variety of shapes and sizes and contain and are coated with an adhesive that holds prey. • Webs are typically placed in insect flyways and the spider sits and waits for an insect to become entangled. The vibrations of the struggling prey signal the spider to emerge and dispatch it.

Silk-spinning • The silk is produced by two or three spinnerets, which are connected to abdominal silk glands. The silk is formed from a protein secretion that hardens on contact with air. • The silk is extremely strong (stronger e.g. than steel of equivalent weight) and is being used as a kevlar substitute. Unlike steel, silk can stretch which makes it an extremely useful material.

Silk-spinning • The silk is used to make webs, but also for a variety of other purposes: • Line nests • Form egg sacs • For dispersal by ballooning • To wrap prey

Reproduction in spiders • Courtship rituals are a major feature of spider mating in which the (usually much smaller) male attempts to mate without being eaten by the female. • Males produce a sperm packet wrapped in silk which he holds in a cavity in one of his pedipalps (second pair of appendages).

Reproduction in spiders • The male, if he lives long enough, inserts a pedipalp into a female’s genital opening and she stores the sperm in a seminal vesicle. • The female later fertilizing eggs when she is ready to lay them. Eggs are laid in a silk cocoon where the young hatch and remain for a short time and molt before departing for an independent life.

Toxic spiders • The vast majority of spiders are harmless to humans, but a handful are toxic and potentially deadly. • In the U.S. there are a few venomous spiders: several black widow species (genus Latrodectus), the brown recluse (Loxosceles reclusa) and the hobo spider (Tegenaria agrestis) introduced from Europe to the Pacific Northwest.

Toxic spiders • Spiders eat a liquid diet by injecting digestive juices into their prey and some spider venoms contribute to this process by destroying tissue. • Bites of such spiders can lead to tissue necrosis. This is the type of venom possessed by the hobo and brown recluse spiders.

Hobo spider Brown recluse spider

Toxic spiders • A bite from one of these spiders results in a bite site that develops a painful ulcer where tissue dies (necrosis) and in the case of hobo spiders severe headaches. • One component of brown recluse venom is sphingomyelinase D which attacks and dissolves cell membranes.

Toxic spiders • Sphingomyelinase D and other components in the venom apparently also turn on the patient’s inflammatory defenses against his or her own tissues. • White blood cells destroy themselves releasing other enzymes that attack the victim’s own flesh and blood clots form in tiny vessels cutting off the blood supply to the bite area and causing necrosis.

Toxic spiders • Generally, bites are not fatal but if the venom gets into the bloodstream it may destroy red blood cells or attack the bone marrow, which may lead to fatal complications.

Toxic spiders • Black widow spiders and the funnel web spider (one of the world’s most dangerous species [naturally it’s from Australia!]) produce potent neurotoxins that affect the nervous system.

Female Black Widow spider with egg sac

Toxic spiders • Venom is complex and many components only affect invertebrates, but one substance in black widow venom called alpha latrotoxin stimulates cells to release neurotransmitters such as acetylcholine. • Nerves contract uncontrollably and this can cause small muscle rigidity and intense, agonizing pain.

Toxic spiders • Additional symptoms include nausea and vomiting, increased blood pressure, and the heart may begin racing or slowing significantly. • Brain functions may also be affected producing anxiety, amnesia, and even psychosis.

Toxic spiders • Black widow bites were historically often associated with outhouses, but now usually occur when working in the fields or clearing junk in outbuildings. • Fatality rates have been estimated at 1-5%, but this figure certainly is an overestimate as many people bitten do not seek medical attention. Those most at risk are small children and the elderly.

Toxic spiders • Funnel web spiders (especially the Sydney funnel web spider) are extremely dangerous. • Funnel web spiders inject a venom whose lethal component is called atrotoxin. • The toxin travels in the lymphatic system and binds to nerve endings all over the body where it causes nerves to discharge wildly, especially those of the autonomic nervous system.

Funnel web spiders Funnel web spider burrow site

Toxic spiders • The constant discharging of the autonomic nervous system results in fever, irregular heart rhythm and wild changes in heart rate and blood pressure that can cause respiratory failure, coma and cardiac arrest. • Death may occur anything from 15 minutes to 6 days after a bite.

Toxic spiders • Work by Dr. Struan Sutherland and colleagues in Melbourne has led to success in treating funnel web spider bites and greatly reduced mortality. • Dr Sutherland pioneered the use of pressure bandages to prevent the spreading of the venom and led the team that developed successful antivenins.

Phylum Arthropoda