What is an Arachnid?

Taranti la flourished during the Silurian Period (435—400 million years ago); some of these were more than 39in (1m) long. The marine species died out about 250 million years ago, but their descendants have been highly successful on land.

Tin: Pari s oi an Araciinid

An arachnid's body is divided into two parts. The head and thorax are fused together, forming a cephalothorax, or prosoma, which is joined to the abdomen, or opisthosoma. In some, the abdomen is segmented and may have a tail-like extension; spiders' abdomens contain silk glands. An arachnids cephalothorax has six pairs of appendages. The first pair (chelicerae) may be pineer- or fanglike, and are used mainly for feeding. The second pair (pedipalps) have several functions, including capturing prey and fertilizing the female, and may be leglike or enlarged with terminal claws. The other four pairs are walking legs, although the first pair may also carry sensory organs. Gases are exchanged through the trachea or special respiratory organs called book lungs. Most arachnids digest their food outside the body using enzymes, which are pumped into or poured over food. The liquified remains are then sucked up.

ARACHNIDS, which include spiders, lscorpions, ticks, and mites, differ from insects essentially in that their bodies arc divided into two rather than three segments. Their ancestors were marine, scorpion-like creatures, which fourth walking__

third walking ¡eg céphalothorax

> insidk an Arachnid

The cephalothorax houses the brain and sensory organs, as well as the sucking stomach and venom gland. The abdomen is concerned with digestion, gaseous exchange, reproduction and - in spiders (as illustrated here), psendoscotpions, and some mites - the production of silk.

( :i imi \KrnioRw altdomi n brain situated behind the optic nerves sucking stomach draws up liquid n heart pumps he mo lymph from rear to the front venom gland produces toxins that are used to paralyze prey many branches to increase area for digestion and absorption silk gland secretes liquid protein that hardens as if comes out of the body

young scorpions cluster together on mothers back for protection

parental care . Many arachnids, including sonic harvest men and ticks, show parental care by guarding their eggs from predators. Scorpions, whip-spiders, whip-scorpions, and some spiders carry their young around on their backs for a while after they emerge from the egg sac or brood chamber.

Spidkrs' Wkbs

Spiders produce silk to wrap their eggs in, and for lining burrows and making shelters, but the most well-known use is for capturing prey. (Not all spiders catch prey using silk; some simply rely on good eyesight and stealth.) Web-making spiders have evolved various ingenious prey-capturing techniques, several of which are shown below.


I he cephalothorax and abdomen in harvestmen are joined in such a way that they look as though they have only one body section.


I he protruding structure at the front of a tick's body houses barbed mouth parts used to penetrate the host's skin.

Types or Arachnid

Arachnids are a large and diverse group. They are divided into 11 orders, each of which has characteristic features. Sun-spiders, for example, have massive, forward-facing chelicerae. Scorpions are recognizable by their long abdominal "tails," hearing stings, and their large, clawlike pedipalps. Whip-scorpions also have large pedipalps, but they are not clawlike, and the long, whiplike tail is without a sting. Perhaps the biggest variation in appearance is seen in the spiders and the mites. Spiders vary from tiny money spiders with turreted, eye-hearing extensions on the cephalothorax to huge, hairy species, known as tarantulas. The huge number of species that make up the mites and ticks vary from gall-forming mites, which are probably the smallest arthropods in the world at less than Y\:hin (0.1mm) long, to blood-feeding ticks, which can he more than IMin (30mm) long. Some have slender or flattened bodies that allow them to fit inside a human hair follicle or burrow through skin layers.

young scorpions cluster together on mothers back for protection

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