Class Arachnida

This class not only includes spiders, but also scorpions, pseudoscorpions, whip scorpions, ticks, mites, daddy longlegs (harvestmen), and others. The differences lie in form and appendages. Mostly free living and more common in warm, dry regions than elsewhere.

Arachnid tagmata are a cephalothorax and abdomen. The cephalothorax usually bear pair chelicerae, pair pedipalps, and four pairs walking legs. Antennae and mandibles are lacking. Most arachnids are predaceous and may be provided w/ claws, fangs, poison glands, or stingers. Usually have sucking mouthparts or strong sucking pharynx.

Some species of arachnids are dangerous, but most are harmless to humans.


Order Araneae - Latin for spider

Large group of 35,000 species distributed all over the world.


Body structure

1) Cephalothorax (prosoma) - fused head and thorax of arthropod

2) Abdomen (opisthosoma) - Both unsegmented and joined by slender pedicel

3) Chelicerae - anterior appendages in pair

a) fangs have poison glands

4) Pedipalps - second pair appendages that contain basal parts with which they chew

5) Four pairs walking legs terminate in claws.

6) Simple eyes contain lens, optic rods, and retina

a) Used chiefly for perception of moving objects, but some actually see images.

7) Sensory setae

a) All used to communicate some information about environment.


Feeding habits

All predaceous, largely feed on insects. Quickly kill prey with fangs and poison. They chase prey, ambush them, or trap them in a net of silk. Unique adaptation of spinning glands. After prey is seized with chelicerae and venom injected, tissues liquefied with digestive fluid and sucked up. Spiders with teeth at bases of chelicerae crush or chew up prey, aiding digestion by enzymes from mouth.



Breathe by means book lungs or tracheae or both. Book lungs are unique to spiders, consisting of many parallel air pockets extending into a blood-filled chamber. Air enters chamber by slit in body wall. Tracheae make up system of air tubes that carry air directly to tissues form openings called spiracles.



Unique excretory system of malpighian tubules, which work with specialized rectal glands. Potassium and other solutes and waste materials are secreted into tubules, which drain fluid into intestine. Rectal glands reabsorb most of potassium and water, leaving behind such wastes as uric acid. Many spiders have coxal glands - modified nephridia which open at coxa , or base, of first and third walking legs.



Important factor in lives of spiders. Two or three pairs spinnerets containing hundreds of microscopic tubes run to special abdominal silk glands. A scleroprotein secretion emitted as a liquid apparently hardens as a result of being pulled from spinnerets and forms a silk thread. Stronger than steel threads of same diameter. Believed to be second only in strength to fused quartz fibers.

Web used for trapping insects is the most familiar, but varies with different species.

Also other uses. Line nests; form sperm webs or egg sacs; build draglines; make bridge lines, warning threads, molting threads, attachment discs, or nursery webs; or wrap up prey securely. Not all spiders use webs either. Wolf spiders, jumping spiders, and fisher spiders simply chase and catch their prey.



Male spins small web, deposits a drop of sperm on it, and then picks sperm up and stores it in the special cavities of his pedipalps. When he mates, he inserts pedipalps into female genital opening to store sperm in his mate's seminal receptacles. Usually a courtship ritual before mating. The female lays her eggs in a silken net, which she may carry about or may attach to a web or plant. A cocoon may contain hundreds of eggs, which hatch in approximately 2 weeks. Young usually remain in egg sac for a few weeks and molt once before leaving it. Several molts occur before adulthood.


Dangerous Spiders

a) Black widow - Latrodectus mactans - venom neurotoxic.

1) Small to moderate in size

2) Shiny black w/ bright orange or red "hourglass" on underside of abdomen


b) Brown recluse - Loxosceles reclusa - venom hemolytic.

1) Smaller than black widow

2) Brown w/ violin-shaped dorsal stripe on its back


Order Scorpionida - scorpions

Found mostly in tropical and subtropical regions. They are secretive, hide in burrows, under objects, and feed at night.



1) Short cephalothorax, bearing appendages.

a) Small, three jointed chelicerae

b) Large, six jointed chelate (pincerlike) pedipalps


2) Pair of large median eyes and two to five pairs of small lateral eyes.

3) Preabdomen has seven segments

4) Postabdomen, or tail, has five segments

a) End has stinging apparatus: bulbous base and curved barb injects venom

5) On ventral side of abdomen are comblike pectines - tactile organs used for exploring ground or in sex recognition



1) Perform complex mating dance.

a) Male deposits a spermatophore and female takes it into her orifice.

2) Either ovoviviparous or viviparous


Order Opiliones: harvestmen

More of a scavenger than regular spiders. Often called daddy longlegs



1) Abdomen and cephalothorax are broadly joined, without constriction of pedicel.

2) Abdomen shows external segmentation

3) Four pairs of long, spindly legs

4) End of chelicerae are pincerlike


Order Acari: ticks and mites

Many acarines are parasitic during one or more stages of their life cycle. Most medically and economically important group of arachnids. Spider mites are serious agricultural pests on many plants. Chiggers cause an irritating dermatitis and some also transmit Asiatic scrub typhus. Other species cause mange in domestic animals. There are additionally ticks which transmit disease, such as Rocky Mountain spotted fever, and cattle ticks.



1) Complete fusion of cephalothorax and abdomen

2) No external division or segmentation

3) Mouthparts on anterior projection are called capitulum

4) A chelicerae can be found on both sides of the mouth

5) Ventral bases of pedipalps are fused to form hypostome

6) Rostrum or tectum - snoutlike projection on the head- extends dorsally over the mouth

7) Four pairs of legs



1) Sperm transferred directly

2) Most species transfer sperm indirectly by spermatophore


Mite structure

1) Long, hairlike setae on legs for swimming (aquatic forms)

2) Convex dorsal surface