Irradiated by LabRat
Friend Matt tracked me down earlier today to ask me a simple question, and if it was worth answering, it’s worth writing down here as well, seeing as I’ve no brighter ideas today.
So, the question is: Why are there no flying spiders?
The glib answer is that there certainly are, and they travel hundreds of miles, but that’s only flight in the sense that they travel distances by air- they have no control at all over where or how far they go, and they’re using their webbing to catch air rather than any other part of themselves for more-or-less controlled flight, the way flying squirrels, snakes, or lizards do.
The real answer is that, while far from outside the space of the evolutionarily possible, flight is an unlikely path for a spider to evolve along. Arachnids in general and spiders in particular are ground-dwelling specialists; their territories are small, with the definition of “small” ranging from a burrow covering a few square inches to a territory of a few hundred square feet for the most adventurous of wolf spiders. While wolf spiders and jumping spiders do range and stalk prey, the vast majority of the group puts a great deal of investment into a single home location, with many good hiding places for concealment from prey and predators alike, and lets their food come to them. When they do “fly”, it’s to set up a new territory to make a home of. While there are plenty of flying animals with permanent to semipermanent homes, they do far more wide ranging in search of food during their active hours than a typical spider ever will or would ever need to.
Aside from this, there’s not much in the arachnid body plan that provides good material to adapt into flight surfaces. Flying squirrels have membranes of skin between their limbs, bats have membranes of skin between what began as fingers, birds have long, layered feathers making up a flight surface, flying snakes and lizards have elongated ribs with yet more skin making up flaps between them, and insect wings… are complicated. Arthropods don’t have loose skin or scales that can be readily exapted to serve other purposes, and their legs are extremely robustly and well adapted to being legs. The most mobile of spiders, the jumping spiders, rely on control of the blood pressure in their legs to approximate hydraulic pressure, and add safety to their leaps with a drag line- high mobility that can even be used to catch flying prey, but relying on the historical spider strengths in their webbing and legs rather than going for flight*.
Evolution offers almost limitless possibilities for animals, but in practice some possibilities are much easier and likelier to access than others. If you think of a given critter as sitting on a surface representing its possibilities, things that are logical extensions of the critter’s current lifestyle and specializations are “downhill” on this surface- easy, likely paths of evolution. Options that are neither extensions of its current lifestyle nor particularly in opposition to it are on the same plane- neither the path of least resistance nor greater, still relatively easy paths but not necessarily likely without some additional advantage, like a new niche opening up or a lateral move from a niche that is collapsing. Options that are possible but in some ways opposed to its current lifestyle are “uphill”- they’re developmental possibilities, but would require development along ways that contradict its current specializations and body plan; like flight for an animal that has specialized in ground dwelling, concealment, and capture for millions of years. They’re technical possibilities, but before they became a likely path for evolution that animal’s current solutions to problems- like escape from predators by going for immediate concealment rather than flying or draglining away- would have to become non-options, and the animal would have to escape extinction in the interim, so the selective pressure could not be *too* strong.
We don’t have any flying spiders, even though we theoretically could, because flight is uphill for a spider.
*There is one species of jumping spider that has abdominal flaps whose first descriptor claimed were like the flaps on a flying squirrel or lizard and enabled it to lengthen and control its leaps. On further study, it appears they’re only used by males in courtship and it jumps about just like every other jumping spider, no flight involved. In any event it appears spiders CAN develop something akin to a proper control surface, but they still don’t use it in that fashion.