First of all, they’re old. Older than the dinosaurs; older than the oldest mammal; older than corals; older than the continents as we have come to know them. Older, my friends, than bones.

The ancestral arthropods were skittering across the surface of the ocean floor about half a billion years ago, and already they were strange as sin, bodies hung together like exquisite corpses. Spiny things, the first knights — covered in a hard outer skeleton that all their descendants inherited and blessed with an abundance of identical limbs on either side that bent backward and forward like hinged oars and propelled them over the sand.

— from kevinzim - (?)

All arthropods have segmented bodies, and the segments — or tagma — of different subphyla of arthropods have the most fantastic names. Thorax, cephalon, cephalothorax. Pygidium, prosoma, opisthosoma. They are Lego-creatures, kings of specialized functions: each body part playing its part.

All arthropods have exoskeletons, made out of a substance called chitin that is soft and gelatinous when first secreted, and which then hardens to form a tough shell. Because they’re surrounded by this unyielding apparel, arthropods have to moult as they grow, shedding their carapaces and becoming, briefly, tender balloons of squishy vulnerability until their new skins solidify around them. Yes, even spiders do this. Last week I held a tarantula moult in my hand. It was thoroughly strange, and quite wonderfully complete.

All arthropods have many pairs of jointed limbs, which is where they get their name — from words with Greek roots that tell us they are the creatures whose feet fit together. Various evolutionary pressures have resulted in the basic arthropod jointed limb structure being modified into a thousand thousand different forms, from the crowbar-like claws of the coconut crab to the bounding knees of grasshoppers. Opposable thumbs be damned; no other phylum can claim such a glorious array of appendages.

So — older than bones, my friends. And still with us. Stay tuned for more.