Arachnology is the science of arachnids (Arachnida), while Acarology is concerned only with animals with a spider-like appearance. Arachnids belong to the Phylum of Arthropods (Arthropoda) and the Subphylum Chelicerata. Arachnida can be classified as spiders (Araneae), whip spiders (Amblypygi), Solifugae, hooded tickspiders (Ricinulei), harvestmen (Phalangida), mites (Acarina) and ticks (Ixodida). The largest order within the arachnids are the Acarina.
Spider - Arachnid
Arachnids, in contrast to insects, have 8 instead of 6 legs. Instead of compound eyes arachnids have 8 simple eyes. The head and chest are one unit (prosoma). The prosoma is followed by the abdomen which is not divided and mostly appears in a bigger size. They are a member of the subphylum Chelicerata having the chelicerae the appendages which appear before the mouth - which give the group its name. On the end of the Chelicerata are retractable claws, and in their points are poison glands.
Arachnid
The mouthparts (which also are part of the genitals on the male) are to find on the head too. Attached to the prosoma are 4 (six to seven jointed) leg pairs, which (together with the sensitive hairs) build an important sense organ. On the abdomen of the female spiders (Araneae) the genital openings (epigynes) can be found at the bottom. On the underside of the spiders abdomen is the anus surrounded by 4 to 8 warts. These warts belong to the spiders silk-spinning organ (spinneret).
Spider caught in a net
The life span of arachnids, in contrast to that of insects varies widely. Their body growth is greatly limited by a rigid exoskeleton. Arachnids moult, after retreating into hiding and fasting. While most spiders moult hanging down from a thread (thread moulting) to spend the time of moulting on their backs.
Arachnida on cobweb
Almost all spiders are land animals. They often remain hidden during the day and are active only at dusk or at night. The water spider is the only spider that lives in water. A number of species hunt on the water.
Arachnid 1 mm long
Birds are the most common spider predators. Amphibians, reptiles, and bats also prey on spiders on occasion. Some spider species (eg Mimetidae) feed exclusively on other spiders. Other natural enemies are: species of wasps, ants, dragonflies, ball flies, nematodes and mites.
Amongst humans, particularly in western industrialized countries, spiders have a bad reputation. This irrational aversion may in some cases lead to mental illness (arachnophobia). Some species of spiders are in fact dangerous to humans, for example, the black widow. In Cambodia, spiders are brutally killed and eaten. However in many Asian cultures, the spider is considered useful because it kills insects. In West Africa, the spider is considered a deity.
