The name Arachnid refers to any member of a class of small land animals related to insects.
Scientists who study fossils believe that arachnids were among the first animals to live on land
nearly 400 million years ago.
Arachnids belong to the phylum Arthropoda and to the class Arachnida. There are 57,000 named Arachnid
species. The best known arachnids are spiders, ticks, mites, scorpions and harvestmen.
They owe their name to Arachne, a mythological Greek maiden who rashly challenged the goddess Athena
to a weaving contest. Athena, jealous of the maiden’s ability, turned her into a spider. Arachne was
condemned to spin and weave forever.
What do they look like?
Arachnids, unlike insects, have no wings and four pairs of legs. They have no feelers, or antennae.
Arachnid bodies are divided into two main parts, the abdomen and the cephalothorax (head and thorax
joined together). Insects, however, have three main body parts: the head, the thorax, and the
Water striders and other leggy insects resemble spiders, and some bugs and beetles resemble ticks.
But adult insects that have legs always have three—and only three—pairs of legs. All arachnids have
four pairs of walking legs and several have two claw-like appendages called palps, that look like a
Arachnids have a pair of chelicerae (jaws) in front. Each of these has a movable fang often connected
to a poison gland. The next pair of appendages, pedipalps, may resemble legs but have one less
segment. They are rarely used for locomotion. In scorpions, enlarged pedipalps form finger-and-thumb
claws. The tiny pseudoscorpions (false scorpions) lack the scorpion’s tail and have miniature fangs
on their claws.
Harvestmen have an undivided head and body. Many have extremely long, slender legs. They are often
called “daddy longlegs.”
Arachnids have between one and six pairs of simple eyes. Unlike most insects, they have no compound
eyes. Some breathe like insects by means of air tubes. Others have breathing organs called book
lungs. These are small sacs within the abdomen, connected with the outside air by small openings.
Each sac contains many layers of tissue resembling pages of a book. The air enters the sac and flows
through the “pages,” sending oxygen to the blood.
Where do they live?
Arachnids are found throughout the world in nearly every habitat. They reach their greatest size
and are most diverse in warm dry areas and tropical regions.
What stages of metamorphosis do they go through?
Arachnids usually lay eggs which hatch into immatures that resemble adults. Scorpions bear live
young. Many arachnids show parental care by guarding their eggs from predators. Some carry their
young around on their backs after they emerge from the egg sac.
How do they affect people?
Some arachnids cause harm to people, by inflicting poisonous bites or stings. Others suck blood of
humans and animals. The sting at the end of a scorpion’s tail can be dangerous to human life. Several
mite species are parasites, and some carry disease. But many arachnids, especially spiders and daddy
longlegs, are helpful because they eat harmful insects.
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