Mexican red-kneed tarantula - not a spider?

What makes a spider a spider?  First off, they have to have something called chelicera.  These mouth appendages are present in all arachnids but also are found in horseshoe crabs and sea-spiders, which are not spiders at all.  In spiders, including tarantulas, the chelicera is a pair of scissor-like appendages that open and close sideways.  The tips of the chelicera are equipped with fangs that can (for most species) deliver venom.  Spiders have more flexible chelicera than tarantulas and can open them from side to side when ready to strike.  Tarantulas tend to fold their fangs underneath.

A second characteristic of all spiders is that they have two body segments (a cephalothorax and head) and eight legs.  The pedipalps, the leg-like segments just behind the chelicera, are used in feeding, mating, and web construction.  You can see these in the front of the above image, they look like short legs.  Tarantulas also have these in common with spiders. You can see a diagram below showing the anatomy of a red-kneed spider.

So, what makes a tarantula different from other spiders?  First off, tarantulas are hairy, very hairy.  Tarantulas actually can shed their hairs from their abdomen as a defensive measure.  Hairs called urticating hairs, have sharp barbs on them that can penetrate body tissues of would-be aggressors.  Itching powder is actually made of tarantula hair.  The barbs get into the skin and cause great discomfort.  

Spiders, including tarantulas, spin webs.  General spider anatomy shows that they have six spinnerets that produce silk.  However, tarantulas only have four of them.  Also, most spiders can spin several varieties of silk while tarantulas can only produce one kind that they use to line their tunnels (it also transmits vibrations if something is just outside their burrow).  

Spiders breath using something called a book lung.  All spiders have one, while tarantulas have two.  All spiders have claws at the end of their feet; most, including tarantulas, have a pair.  Tarantulas however can retract their claws to aid in climbing.  Their feet have hundreds of tiny bristles called setules.

In all, tarantulas are different from their spider cousins and are estimated to be an older group of chelicerates.  I believe that tarantulas are spiders but come from a primitive offshoot different from the branch that led to regular spiders.  So, it's not wrong to call a tarantula a spider, it's just that there are a lot of differences between them and the others in their group.

I hope you found that interesting.  I learned a few things putting this together.

Thanks for reading.

Eric Svendsen     www.ericspix.com