The Medical Significance of the Arachnida Class of the Arthropods

The class Arachnida is a group of more than 100,000 species, including spiders, scorpions, ticks and mites. Most arachnids are adapted to kill prey with poison galnds, stingers, or fangs. Like crustaceans, arachnids have a body that is divided into a cephalothorax and an abdomen.

Attached to the Cephalothorax are 4 pairs of legs, a pair of Chelicerae, and a pair of appendages called pedipalps. The pedipalps aid in chewing; in some species pedipalps are specialized to perform other functions. Arachnids undergo incomplete metamorphosis. Class Arachnida includes 3 orders of medical importance:

  1. Order Scorpions
  2. Order Araneae (spiders)
  3. Order Acari (ticks and mites)

Scorpions
The scorpions are familiar group of arachnids whose pedipalps are modified into pincers. Scorpions use these pincers to handle their food and tear it apart. The venomous stings of scorpions are used mainly to stun their prey and less commonly in self-defense. The sting is located in the terminal segment of the body, which is slender toward the end. The elongated, jointed abdomens of scorpions are distinctive; in most chelicerates, the abdominal segments are more or less fused together and appear as a single unit. The adults of this order of arachnids range in size from 1 to 18 centimeters. There are some 1200 species of scorpions, all terrestrial, which occur throughout the world, although they are common in tropical, subtropical, and desert regions. The courtship of scorpions is elaborate, with the spermatophores being fixed to a substrate by the male and then picked up subsequently by the female. The young are born alive, with 1 to 95 in a given liter. Scorpions differ from spiders in two ways. Scorpions have greatly enlarged pedipalps, which they hold in a forward position. They also have a large stinger on the last segment of the abdomen. Most scorpions hide during the day and hunt at night. Scorpions seize their prey with their pincerlike pedipalps. Then the fang injects paralyzing venom, the chelicerae tear the prey,, the animal is ingested, and digestion begins. Only a few species have a sting that may be fetal to humans. They do not sting a man unless attacked.

Pathogenicity
The local symptoms of bite include severe pain, inflammation and swelling. Sweating, nausea and vomiting are common systemic symptoms. Muscular spasm and convulsions can occur in severe cases. Fatal outcome is caused by respiratory failure, pulmonary edema and shock.

Control
Spraying of insecticides

Spiders
There are about 25,000 named species of spiders (order Araneae). These animals play a major role in all terrestrial ecosystems, where they are particularly important as predators of insects and other small animals. Spiders hunt their prey or catch it in webs. The silk of the webs is formed from a fluid protein that is forced out of spinnerets, modified appendages on the posterior portion of the spider’s abdomen. Spiders have poison glands leading through their chelicerae, which are pointed and used to bite and paralyze prey. Some members of this order, such as the black widow spider (Latrodectus mactans), Australia black widow spider (Latrodectus seville), caracurt (latrodectus tredicimguttatus), brown recluse (Loxosceles reclusa), tarantula (Lycosa singoriensis), have bites that are poisonous to humans and other large mammals.

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