Last post I briefly mentioned my recent encounter with an archaeognathan (specifically, I found it mixed in a specimen vial with a bunch of harvestmen). Archaeognatha are one of the few living orders of wingless insects. In fact, under the current most-commonly-used definition for Insecta (which excludes the entognathous hexapods such as Collembola), Archaeognatha are the basalmost living order. As such, I was pretty excited to finally see one, even if only in the corpse. The Tree of Life page for this order has an absolutely fantastic photo of a live specimen. Archaeognathans are also referred to as bristletails in reference to the long cerci extending behing the abdomen.
The first feature that grabs the attention is the distinct hump that the back makes. This hump contains muscles for the archaeognathan to rapidly bend the abdomen downwards, pushing itself into the air and jumping up to 10cm high. Archaeognathans also have very large forward-facing compound eyes that actually meet in the middle. The maxillary palps are very large and could almost be mistaken for an extra pair of legs coming off the head.
If you want to see what makes archaeognathans really cool, though, you'll have to look a little closer. As befits the basalmost insect order, they retain a few uber-primitive features that have disappeared from other modern insects. They are the only modern insects with monocondylic mandibles - i.e. the mandibles have only one condyle (the socket where they attach and articulate with the head). All other insects have two. And if you were to look underneath the abdomen, you would see that each segment bears a pair of small pointed styles. These styles are moveable by muscles, and are thought to represent reduced legs. Let me repeat that in italics for emphasis - reduced legs. Archaeognathans are the only insects sensu stricto with abdominal styles, though they are also present in diplurans and proturans, two entognathous hexapod orders (I have not been able to find any indication that the styles in these orders are mobile, however).
While they have a fairly wide distribution worldwide, archaeognathans do not appear to be abundant and are fairly localised. I have heard that California is fairly well-blessed with them (big ones, too), but here in Australia they are most abundant in the eastern states.