Order Aranea - Venomous Arachnids

ORDER ARANEA

·              All spiders, with the exception of two small groups, are venomous. There are over 100,000 species of spiders. However, only about 20 species cause serious envenoming in humans, while about 150 to 180 can cause significant toxicity.

·              The common Indian species that cause serious envenoma-tion include Brown Recluse, Black Widow, Wolf Spider, and Tarantula. Other spiders such as Orb Weaver, Running Spider, Hackled-band Spider, Giant Crab Spider, Lynx Spider, Jumping Spider, and Tangleweb Weaver, which are also encountered in India, do not cause significant envenomation. Funnel Web Spider which can cause significant envenomation is found only in the Australian continent.

General Anatomy

·              Anatomically, a spider has a cephalothorax and an abdomen with 4 pairs of legs fanning out from the thorax. Two claw-like fangs called celicera protrude from the head and are connected to venom glands which are under voluntary control.

·              Although the venom is quite potent in many species, serious envenomation is rare because of inadequate injec- tion mechanism, and small quantity injected with each bite.

·              During its normal life span of 1 to 2 years, a spider moults several times, as a result of which there may be periodic changes in colour and markings.

·              Spiders are extremely shortsighted, and depend mainly on sense of touch and vibration. The eyes are on the front part of the cephalothorax. Most spiders have 8 eyes. Their size and position varies by spider type.

·      Some large spiders (e.g. huntsman spider, wolf spider, orb weaving spider), possess large spines on their legs. The spines are raised from a prostrate to a vertical position when the spider is irritated. If the spider is grabbed, picked up, or brushed off, injuries (severe pain, erythema, pruritus) from the spines may occur. These injuries often occur in conjunction with a bite by the same spider, and splinters are usually found at bite sites.