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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.

wolf-spider-1

Although almost anyone can recognize a spider, there are a number of related arachnids that are less well known.

Scorpions (Order Scorpiones)

scorpion-anatomy

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.

Related posts:

Psuedoscorpions

(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

sunspider1These 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

giant-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)

mite-red-closeThe 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)

daddy-long-legLike 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

spider-books-for-kids

What would you like to know about arachnids?

A few days ago I needed a glass vase for a project. When I pulled it down from the shelf, look who I found inside:

scorpion-clear-stingerFortunately scorpions can't climb glass, so it was definitely trapped in there.

scorpion-close-upAlthough the lighting was less than ideal, it was a good opportunity to see the scorpion parts close up.

scorpion-anatomyIf you are interested in learning more about scorpion anatomy, try our previous post about scorpions.

Scorpions are most active during the summer in Arizona. They hunt for insects at night and hide during the day. People usually don't like them very much because they can deliver a painful - and potentially health-threatening if it is a bark scorpion - dose of venom when they sting.

Scorpions have an intriguing side, however. One really cool thing about scorpions is that they have a natural fluorescence. They glow at night under ultraviolet lights. See, for example, in this video:

Still not convinced scorpions can be interesting? Wired Magazine recently had an article about a doctor who is researching the use of a component of scorpion venom to mark brain tumors. Fascinating!

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Usually scorpions are active at night.

Once in awhile, however, you can spot one during the day.

This is a youngster. I can tell by its size.

Scorpions look very different from their relatives, the spiders.

(Line drawing from Wikimedia Commons)

The parts of a scorpion are not always easy to identify. For example, how many legs does a scorpion have? Does it have eight legs (as it should since it is supposed to be an arachnid), or does it have ten? What do you think after looking at the photographs and line drawing?

Answer:  Those two pincher-like appendages at the front are called pedipalps and are not considered to be legs. Therefore a scorpion does have eight legs.

Another feature of scorpions is that they don't have distinctly separate body regions.

Perhaps this colored diagram will make it easier. In this case, the legs are colored  light green and the pedipalps are blue. The dark green area is the "head" region (called the cephalothorax). Do  you see the black dot that seems to be in the middle of the "head" region in the photographs above? Those are the median eyes, shown as red dots labelled "O" here. The scorpion also has lateral eyes on the front sides of its "head," which you can see in the photographs. The darker blue appendages are the chelicerae, which are mouthparts.

The middle part of the scorpion is colored yellow and is called the mesosoma. The tail, colored in pink, is the metasoma. Finally, the business end is the bright red telson that contains the stinger.

Why do scientists identify and name the various parts? One important reason is to be able to describe characteristics of different species (and higher groups) accurately so that others are able to tell them apart, too.

Bet you didn't think you would be looking this closely at a scorpion today. 🙂

Do scorpions occur where you live?

If you are interested in finding out more, try the middle-grade book Scorpions: Armored Stingers by Sandra Markle (link takes you to a review at Wrapped In Foil).

Reading level: Ages 9-12
School & Library Binding: 48 pages
Publisher: Lerner Pub Group (T) (March 2011)
Language: English
ISBN-10: 0761350373
ISBN-13: 978-0761350378

Scorpions: Armored Stingers (Arachnid World)

(Amazon affiliate link)