Friday arachnid blogging, and what's in a name

After last week's grand-guignol, here's a more pastoral scene altogether:

The orangey spider tucked in among the purple petals is one of the 'running spiders' or, more properly, philodromid crab spiders. She is very likely Philodromus rufus though, as she is a juvenile, it is extremely difficult to identify her precisely.

The name 'running spider' might seem odd. After all, most spiders (at least those that do not spend their entire lives on a web) run. But in this case the popular name derives from the Linnaean name Philodromus, 'one who loves to run', and they are quick little things indeed. What is really interesting here is the term 'philodromid crab spider'. They are so called to distinguish them from the thomisid crab spiders. Remember that terminal -d, by the way, because it's going to be important.

If you've gone to look at the thomisids on Ed Nieuwenhuys's excellent website and compared them to my young philodromid, you'll probably have noted that they look very much alike, save for one detail. The thomisids have stubby little hind legs (pairs 3 and 4) while their two forward pairs of legs, especially the first pair, reach heroically Popeye-like proportions. The forward legs of the philodromids are very similar, but their hind legs aren't nearly as stubby; in fact they look more or less 'normal'.

Now, before we contemplate drawing any conclusions about the degree of relation between thomisids and philodromids, we can explain the difference in leg pattern by the respective lifestyles of the two sorts of spider. Philodromids chase down their prey. Thomisids sit stock-still on a plant, often in the middle of a flower, and wait until an insect walks into the grasp of their prodigious forearms; then wham and it's lunchtime.

People used to say 'Well, we have crab spiders; clearly this one sort are different to the other sort, so we'll give them different names, but they're essentially part of the same group'. And so we had all of these spiders in the family Thomisidae, with the short-rear-legged lot called Thomisinae and the longer-rear-legged lot called Philodrominae. The -idae ending denotes a family (one level up from genus in the classical hierarchy), while -inae denotes a subfamily.

What you need to know about taxonomists (people who classify organisms) is that they fall into one two big groups, the lumpers and the splitters. As the names suggest, lumpers are inclined to shove disparate organisms into a single group if they have any shared characteristics; whilst splitters are, shall we say, much given to celebrating diversity.

As far as the crab spiders go, the splitters have tentatively won the day. Today one no longer speaks of Philodromines but of Philodromids. The d makes all the difference; the spider in the picture above is not part of a sub-goup of Thomisidae but rather member of an independent family in its own right, the Philodromidae.

This might strike you as so much hair-splitting, but it isn't. Almost all taxonomists these days have adopted a cladistic worldview. Cladistic systematics is notorious for rigidly dogmatic insistence on an essentially Martian terminology, but brush aside the bizarre words and it's pretty intuitive and (I think) sound: if you want to divide up the world's organisms, the terms you use should reflect the actual evolutionary history of those organisms.

To use a concrete example: as we've all heard, 'birds are really dinosaurs'. This might strike you as odd, given that (i) dinosaurs, as every shoolchild knows, became extinct a squillion years ago and (ii) that thing on your plate is a chicken, for chrissake, not a dinosaur. But to a cladist (and we are all cladists now), the point is that no taxonomic group is 'real' unless it contains all the organisms descended from a single common ancestor, and no organisms not descended from that ancestor. Birds as we know them today arose within one particular group of reptiles; that group was the dinosaurs. So a Tyrannosaurus and a chicken are more closely related to each other than either is to a lizard or snake. They are each members of a single clade, descended from a common ancestor not shared with lizards or snakes. Under the conventions of cladism, therefore, the term 'dinosaur' is only legitimate if it includes the birds. If it does not, then it fails to contain all the descendants of a single common ancestor. If you wish to refer only to the sort of dinosaur that looks like Godzilla and is extinct, cladists would insist that you say 'non-avian dinosaur', which is ideologically pure but, if I may speak frankly, a bit clunky for use in any but a technical context. Birds, lizards, dinosaurs and snakes, of course, are all descended from a more remote common ancestor, so they all legitimately belong to a larger clade than that of the dinosaurs (and birds). Clades fit within clades, a bit like those Russian matrioshka dolls we've all seen. (BTW, cladists wince at the term 'reptile' itself, for the reasons described above. They'd let you say 'reptile' so long as you were willing to include yourself in that group. Otherwise, if you mean what we all mean when we say 'reptile', you had better say 'non-mammalian, non-avian amniote' when you are talking with a cladist.)

When the Philodromines became Philodromids, then, a lot more was going on than the mere swapping out of a couple of letters. The assertion was being made that a biggish group of animals was not a single group at all; that they did not all share a single common ancestor. (Of course the running spiders and thomisid crab spiders do share a common ancestor in any case; but that ancestor was being pushed back at least one level.)

This might all seem very rarified and academic to you, but think of an example a bit closer to home. Are you a chimpanzee? You might be tempted to answer indignantly that you are not. And, in a tightly focused context, you may legitimately distinguish yourself from chimpanzees - today's chimps and bonobos share a common ancestor that we don't have. Yet if you broaden your focus just a little a bit and wish to distinguish yourself from a gorilla, you are going to have to take our chimpanzee cousins along for the ride. For we and they descend from a shared common ancestor that arose after our lineage split from the lineage leading to gorillas. It's understandable, perhaps, that we instinctively divide the primates into two great groups - ourselves and all the others. But there is no systematically legitimate way to hive humans off from other primates unless we take the chimpanzees with us. They are no more closely related to gorillas than we are. Indeed, since the ancestors of (chimps + bonobos) split off from a common lineage after the ancestors of ((chimps + bonobos) + humans) did, from a cladistic perspective it's not so much that humans are a sort of chimpanzee as that chimpanzees are a sort of human.