We've never discussed arachnids in detail here at Bug of the Week. Let's find out what we've been missing.
Spiders and their relatives belong to the class Arachnida. How can you tell whether a creature belongs to this group? Arachnids have two body regions called the prosoma (part with eyes and legs) and opisthosoma (rear section). Arachnids have eight legs and they do not have antennae, although they do have two short appendages in front called pedipalps that sometimes may be mistaken for antennae.
Although almost anyone can recognize a spider, there are a number of related arachnids that are less well known.
Scorpions (Order Scorpiones)
Scorpions have are pincer-like pedipalps and their opisthosoma is elongated into a relatively thick tail with a stinger on the end.
Like most other arachnids, scorpions are nocturnal. They come out at night to catch insects and other arthropods.
(Public domain photograph by Alex Wild)
These tiny creatures have pinchers like scorpions (red in this photograph), but have no tail and no stinger. They are often found in compost heaps where they feed on even smaller insects and mites.
Wind Scorpions or Sunspiders
These desert arachnids have large chelicerae or jaws that they use for catching insects. They can also use their jaws to stridulate, or make a noise. They don't have a tail.
Related post: Sunspiders
Whip Scorpion or Vinegaroon
Unlike all other arachnids, the whip scorpions have a thin tail at the end of the opisthosoma. The pedipalps are robust. In contrast the first pair of legs are thin and have a sensory function similar to antennae.
Vinegaroons have vinegar in their names because they can spray high concentrations of acetic acid if threatened.
Never seen one before? BBC Earth Unplugged has a video of one spraying.
Mites and Ticks (Order Acarina)
The two body regions are difficult to see in mites and ticks. Their bodies are unsegmented, like small blobs. Also, their legs tend to be quite short relative to their body size.
Certain kinds of mites are among the few arachnids that feed on plants. The red mite in the photograph is an insect predator.
Daddy Long-legs or Harvestman (Order Opiliones)
Like the mites, the harvestman have what looks like a single oval body region. Unlike the mites, they have long thread-like legs.
Harvestmen differ from spiders by possessing fewer eyes, having only two rather than six or eight. They also lack both venom and silk glands.
Some species of harvestmen are omnivores or scavengers rather than true predators.
Want to learn more?
A few spider identification basics
Check out our growing list of children's books about spiders
What would you like to know about arachnids?