File: <animalia

The Class Arachnida includes the spiders, horseshoe crab, scorpions and mites. It is a very large class of mostly terrestrial arthropods, with the marine horseshoe crab being an exception.

The general characteristics are the absence of antennae and a body comprised of a cephalothorax and an abdomen, the latter may appear as only a single part without divisions. The cephalothorax bears four pair of walking legs and 6-8 eyes raised on tubercules.

The head appendages include chelicerae, which are jaw like with claws and poison duct openings at their tips. The basal portion of pedipalps serves both feeding and sensory functions.

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Class: Arachnida: Order Araneae includes the true spiders. Segmentation is obscure in the abdomen and there are no obvious appendages except 3-4 pairs of spinnerets at the posterior end of the abdomen that are modified abdominal appendages. Several examples of spiders may be seen in the following diagrams Inv143 - Inv147:

Food & Digestion -- Insects and other small animals are caught in webs. The prey is paralyzed and their liquid contents are moved up through the pharynx and esophagus. A sucking stomach pumps food from the prey through the mouth and into the digestive tract.

Nine diverticulae from the intestine lead to various body parts. There is one located forward and four on each side, which function to increase the surface area. The posterior part of the intestine is surrounded by digestive glands and some food may actually enter the glands. A rectal caecum occurs at the junction of the rectum and intestine.

Circulation -- The heart is long and located in the abdomen. The dorsal aorta in the cephalothorax has subsequent branches to appendages and the brain and eye regions. Some blood is pumped posteriorly to a short posterior aorta. The haemocoel is divided into various sinuses. Blood reaches the book lungs and is aerated after which it returns to the heart.

Respiration -- Air diffuses directly into the book lungs, as the blood does not carry oxygen. Some tracheae may occur but they are never well developed.

Excretion -- Malpighian tubules serve for excretion. Coxal glands that are modified nephridia may also be involved in excretion.

Nervous System -- There is a typical pattern where a great concentration of ganglia occurs in the anterior cephalothorax. Nerves run out to different parts of the body.

Sensory Organs -- There are the eyes, pedipalps and setae all over the body all of which have sensory functions.

Reproduction -- The sexes are separate. Ducts open near the anterior end of the body, but fertilization is internal.

Males use pedipalps to transfer sperm from their genital pore to that of the female. Eggs are laid in silken cocoons and maternal care is common. Development is direct.

Silk Glands -- There are several varieties of silk glands. The silk they produce differs in strength, slipperiness, etc. Different kinds of webbing are produced for particular circumstances. The tips of the legs are modified for walking on the webs.

Economic Importance -- Some species of spiders are poisonous to humans and animals. Spider silk has been used in bombsights during World War II.

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Order: Scorpiones (Scorpionida) -- scorpions: These animals have a well marked cephalothorax and segmented abdomen that is equipped with a sting and poison gland at the posterior end. They can be dangerous in warmer regions. Chelicerae and pedipalps are both chelate. They have book lungs. They feed on other arthropods. They are also viviparous as they bear living young. See Inv150 & Inv151 for examples:

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Order: Amblypygi. (Pedipalpia) -- whip spiders and tailless whip scorpions: There is a long tail, large palps and small chelicerae.

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Order: Pseudoscorpionida -- book scorpions: These are small animals that have the appearance of scorpions because their pedipalps are pincers. The abdomen is rounded but without a sting. They feed on small insects. See Inv152 for example:

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Order: Opiliones (Phalangida) -- harvestmen: Their extremely long walking legs have earned them the name of "Daddy Long Legs." The body regions are all compacted into a single division. They are predators of small insects and other arachnids. See Inv154 for example:

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Order: Acarina -- mites and ticks: The chelicerae and pedipalps are modified into projections called a hypostome. They are parasites and vectors of disease, and serious pests of vegetable and tree crops. See Inv153 for example:

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Class Pycnogonida -- sea spiders: These are tiny marine animals. Included are parasites, commensals and free-living predators.

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Class: Merostomata: Order: Xiphosura -- horseshoe crab: The range is from the East Coast of North America to the coasts of southeastern Asia. These animals have remained essentially unchanged sinde the Paleozoic. They and the Pycnogonida are the only marine arachnids. They are also the only Arachnida with compound eyes. The chelicerae are chelate and the pedipalps look like walking legs. But there is four pair of true walking legs. The abdomen has well developed appendages that have been modified into book gills.

Horseshoe crabs are of course a misnomer as they are not mollusks. Their blood, which is blue in color, is high in metallic copper and is harvested regularly for medical research. See Inv148 & Inv149 for examples:

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Please see following plates for Example Structures of the Arachnida:

Plate 105 = Phylum: Arthropoda, Class: Arachnida, Order: Pseudoscorpionidea -- Menthus rossi

Plate 106 = Phylum: Arthropoda, Class: Arachnida -- Koenenia sp. & Galeodes arabs

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Bibliography

Introduction Hexapoda►