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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

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What would you like to know about arachnids?

A friend recently asked for some tips how to identify spiders.

jumping-spider-another1. Leg size and position

Often the length and position of the legs are easy to see and can give you a clue. Jumping spiders like this one have short legs that are tucked up under their bodies.

 

flower-spider

Crab spiders have 2 pairs of long front legs. They hold their front legs out to the side with the tips directed forward.

feather-legged-spider-1

Feather-legged spiders also have extra-long front legs. They hold their first pair of legs directly out front of themselves and pressed together.

wolf-spider-23Wolf spiders have legs that are all roughly the same length and shape, as do a few other groups such as fishing spiders.

2. Web pattern

Some spiders are easier to identify by their webs.

garden-spider-orbOrb weavers (also called garden spiders) produce large complex webs, often across paths.

funnel-web-retreatFunnel web spiders produce sheets of silk around a central, hollow retreat.

3. Eye Pattern

If you can get a good photograph from the front of the spider, the size and position of the eyes can aid in identification.

Let's take a quick look at spider anatomy, in case you are not sure which is the front end.

basic-spider-anatomy

Besides the legs, the other parts you see on the spider are the abdomen (body area covered with red arrows), and the combined head and thorax, called the cephalothorax (body area covered with yellow arrows). The two appendages that surround the mouth are the pedipalps. The pedipalps are at the front end of the spider, with the eyes.  (Note:  in newer scientific texts the abdomen may be called an opistosoma and the cephalothorax called a prosoma).

The eye patterns are distinctive and easy to recognize with a bit of practice.

jumping-spider-face-viewLooking face on at a jumping spider you can see they have two large eyes in the front of their cephalolthorax and two smaller eyes on either side.

jumping-spider-side-view-general

They also have two smaller pairs of eyes further back. See the dark, shiny eye about 1/2 way back on the cephalothorax?

crab-spider-on-purple-100In contrast, the crab spiders have a cluster of tiny eyes right in a oval at the top and front. The eyes in this spider are set into the orange-yellow colored area. You can hardly see the eyes in comparison with those of the jumping spider.

Eye_Arrangement_of_a_Wolf_Spider

(Photograph by Thomas Shahan from USA licensed under the Creative Commons Attribution 2.0 Generic license at Wikimedia.)

Wolf spiders also have two large central eyes, but have a downward-curving row of four eyes below them. They also have another pair further back, which you can just see as a dark curves.

This is only a very basic introduction to spider identification. If you would like to learn more about this, BugGuide has set up an excellent page showing all the different eye patterns in spiders.

Spiders have quite a bit going on and are actually quite fascinating once you get to know more about them.