Why 8 eyes? I know, I know, “Why not?” and “Because no mutations for another pattern/number of eyes has made arachnids more competetive (cave-dwellers who’ve lost their eyes excepted).” But more than that: cave-dwellers excepted, are there any other animals that have evolved anything substantially different from 2 eyes/eyespots/compound eyes?
And the stuff their bodies are made of. Same as insects or markedly different? And how about muscles and limb movement mechanics–similar to bugs?
I know there’s not really much good reason to assume arachnids and insects should be very much alike, but they are arthropods and one would expect at least some rudimentary similarities; just like humans and frogs are quite different, but as chordates we at least have 4 limbs, 2 eyes, 1 tongue, etc.
And one last…still no wings for spiders? Because even mammals made it to wings. Do they lack the capacity (like, I dunno, maybe the ones that jump can’t move their legs fast enough and long enough and in the right way to make wing-like adaptations more beneficial than leg-like adaptations) of developing wings? Any spideys that seem to be on their way to wings?
It really comes down to what works. The eyes of spiders aren’t like ours. They can’t focus, they have no iris to adjust for light levels and so forth. As such there’s a need for multiple eyes if you want to be able to see both near and far, see under varying light levels and so forth.
This is a classic example of people thinking that whatever mammals do must be normal.
The fact is that animals with two eyes are highly exceptional. In the animal kingdom we are freaks. The vast majority of species have more than two eyes. Starting right down at the jellyfish we find eyes in multiples of four, anything up to eight eyes per animal. Most molluscs have multiple eyes, hundreds in some cases. All insects have five eyes.
And so on and so forth. By my rough estimate, 90% of animals that have eyes have more than two. Us mammals fall into that tiny minority of crippled freaks that have lost most of our eyes.
Different, larger than the gulf between what you are made of and what a starfish is made of.
Radically different. In simple terms, bugs have system of opposed muscles just like us. There is one muscle that straightens a limb/digit and a separate muscle that bends it again. Spiders only have one muscle that bends the limb. They straighten the limb by pumping it full of fluid from the body cavity, which forces it to straighten hydraulically.
IOW bugs are mechanically closer to us in their movement than they are to spiders.
How closely related arthropods are is highly unsettled. In fact there is still debate about whether arthropods even exist as a taxonomic group. It seems likely that the arthropods evolved multiple times from multiple ancestors and are not very closely related at all
The problem is that developmentally and physiologically arthropods are all very different. Far, far more different than any other phylum. Right from the very first cleavage patterns following fertilisation through to the way that teh eyes develop to the way that they manufacture their exoskeletons and the way that they move, the arthropods are more dissimilar to each other than they are to members of supposedly unrelated phyla. Moreover if they are part of the same phylum then multiple complex features must have evolved, then been lost, then re-evolved multiple times both within the phylum and within unrelated phyla. As you point out, in natural phyla you expect a majority of features to be consistent based upon a common ancestry. As far as I know the arthropods don’t share a single common feature beyond having segmented legs, and even their the joints differ radically. Even the presence of an exoskeleton is only common if you use a purely cosmetic definition of exoskeleton. If you apply a biochemical or developmental definition of what an exoskeleton is then many arthropods don’t have one.
At the moment the existence of any arthropod clade is in great dispute. It seems like the arthropods have evolved multiple times. The insects from an onychophoranancestor, the crustaceans from an annelid ancestor and the arachnids from a nematode-like ancestor. They may look superficially similar because of the constraints of a rigid external skeleton, but developmentally and biochemically they don’t seem to have anything at all in common.
Spiders have a problem with evolving wings because they lack the basic starting point. Insect wings seem to have developed from external gills along the side of the body. Spiders and other arachnids, OTOH, have book gillsthat hang down within a protective pouch *underneath *the body. When you start from a gill plate sticking out at right angles from the body, it’s fairly easy to modify it to a flat wing that sticks out at right angles from the body. When you start with plates that hang down beneath the body, it’s much harder to produce a wing.
So if I read that correctly, spiders are wholly alien and were probably deposited here by hostile extraterrestrial life forms? Makes sense to me. But seriously, thanks for the response–marvelous stuff.
Some do, but not generally; for example, most beetles have only two eyes and 25% of all insects are beetles.
Also, having 100 eyes, like some molluscs, isn’t necessarily better, as they can only really distinguish light and dark and movement; similarly, insect eyes aren’t adapted to do what human eyes can, and having to support 100 human-style eyes would be far too much work (some molluscs, like octopuses, have more complex eyes which are superior to human eyes in some aspects, such as a retina in front of the nerves and no blind spot, but only two eyes).
Could you expand on this? I had always just assumed that spiders and insects both had exoskeletons made of chitin, but it sounds like that’s not the case…?
Assuming that the arthropods are three (or more) different clades, where do the 'pedes fit in? I’m guessing that they’re related to the onychophorans, like insects, but that’s just based on superficial resemblance.
And I’m also guessing that the horseshoe crabs are in a completely different clade yet from any of those three?
But seriously, thanks for the response–marvelous stuff.
