Arachnids

What are they?

An arachnid is an animal with segmented body and with big heads and external skeletons. It belongs to the group called arachnida.

Characteristics

Their lenght ranges from less than .05 centemetres to close to 20 centimetres. Their body is in two parts - the head or cephalothorax ( head and thorax ) with six pairs of appendages, and the abdomen. The first two pairs of the limbs are called pedipals and chelicerae, used to hold prey. The rest are used to walk. They have no antennae, but rely on simple eyes and sense organs.

They have from one to six pairs of simple eyes. Some have none. Some breathe by air tubes, some have organs like lungs. Spiders generally have both.

Courtship occurs when the male gives off a chemical, or gives visual clues. The male deposits a sperm on the ground or makes a sperm web, and transfers it to the female. Most lay eggs, but some bear live young ones, like scorpions. As arachnids grow, they undergo moulting quite a few times. All but harvestmen (a particular type of arachnids) cannot digest internally, so they inject digestive fluids into prey and the liquified remains.

Many arachnids are found world wide, inhabiting deserts to forests. A few like mites are parasitic. Many posionous spiders, along with scorpions pose threat to humans.

The main species :

1. Scorpiones - scorpions.
2. Psudo-scorpiones - they are 1 to 7.5 mm in length.
3. Opiliones ( harvestmen - the one that can digest internally ) - 1 to 22 mm.
4. Uropygi (whip scorpions or vinegarroons). They are up to 13 cm.
5. Araneida (spiders) - They are half an mm to 9 cm.
6. Parasitiformes (mites and ticks). - they are of varied sizes.