Crab spiders don't make good hunters
4/9/2001Misumena vatia | Crab spider
The woman swivels idly on her stool at the lunch counter. There are no pies revolving in the pie case. There are no salads in the crisper. The window to the kitchen frames gleaming aluminum, too clean to be in use. Every once in awhile, our customer thinks she smells food. But the thought, or the aroma that gave rise to it, passes.
Still she waits. Other hungry customers come in, look around, and move on. Who stays in a restaurant with nothing to eat? But the woman is not alone. A little guy sits at the counter. An artsy couple smokes cigarettes at the corner table. A woman in head-to-toe black munches a Saltine. Maybe she brought it in her backpack.
This is what Dr. Douglass Morse of Brown University in Providence saw among the crab spiders - OK, it wasn't really a diner, more like a cafeteria or buffet. Whatever it was, some 30 percent of the adult spiders he studied stayed in spots where there was little more to eat than an occasional Saltine. Or, given the spiders' carnivorous habits, maybe we should say, an occasional Slim Jim.
Like almost all spiders, crab spiders are predators. Unlike most others, though, they build no web. Instead, they lurk in flowery blinds and wait for the hapless insect to fly too close. Camouflaged by skin pigment that changes to match the bloom, the spiders are nearly invisible to fat bumblebees. Once the bee bumbles forward, the hunter scuttles sideways in attack, snatching the succulent creature
with a crab-like foreleg and quickly sucking it dry.
Properly fed, a female crab spider can increase her size between 500 and 1,000 percent from hatch to adulthood. She'll go from 0.6 milligrams to 400 milligrams in two years. That would be like an 8-pound baby turning into an 8,000-pound baby - adding a truly terrible dimension to that second year of life. But the female spider is eating for future generations. The fattest females will have the most eggs in their brood pouch - up to 300 - come mid-June.
Which brings us back to what Dr. Morse saw. Given these kinds of dietary demands, why did a significant percentage of spiders refuse to move to better hunting grounds, generally no more than an inch away?
“It's counterintuitive,'' Dr. Morse says of the behavior of these hungry ninnies. “Our geneticists tell us that animals who don't choose better than this would have disappeared from the population long ago.''
In other words, those unable to hunt wisely would fail the reproductive race. Over time, the gene or genes for bad hunting would vanish in the scrap heap of other not-ready-for-prime-time traits.
In a series of painstaking experiments, Dr. Morse is attempting to figure out the genetic survival of the poor hunters. It's not a simple question. One possibility is the notion that the poor hunting practices we're seeing in adulthood stem from what the spiders learned as babies.
Here's how it works. Female spiders reach adulthood through a series of seven steps, each involving the loss and replacement of exoskeleton every time the spider grows out of its old skin. Each of these stages, which begin in the egg, is called an instar. But between the first six instars and adulthood, a lot happens to a spider. (Males, who remain a mere speck compared to their female companions, have only four instars.)
What females learned as spiderlings may play a role in how they fare as adults. Not surprisingly, Dr. Morse says, we know “precious little” about spider education.
To resolve this, he's studying the behavior of freshly hatched spider siblings. He exposes some to one kind of plant/prey situation, while others are put on different plants where different prey is available.
He's particularly interested to find out if anything learned in an early instar remains in a later stage. When spiders molt to enter the next phase of development, they skip meals for a few days. In that time, they may forget everything they learned about hunting.
“That's the critical issue I'm testing,'' Dr. Morse says. “I'm only partly through that.''
If spiders are learning, and carrying that learning from one instar to the next, it may be a classic case of being too smart for one's own good.
Hunting skills they learned as youngsters may not serve them well as adults. If they are highly successful at capturing small prey early in the season, they may be ill prepared to hunt for late-season delicacies like the energy-packed bumblebee, the Breakfast of Champions for spiders.
But, since their youth was well spent, they still have enough weight to lay some number of eggs. Not as many as the biggest spiders, but some. So most are not complete losers. They are just winners on a more modest scale.
So, if you're chewing on the notion of survival of the fittest, and wondering about how the genes of a dysfunctional collection like your family - or mine - managed to make it this long in the history of mankind, look to the spider in the diner, spinning on her counter seat, waiting patiently for a meal that will never be served.
If her genes can make it, who's to say ours can't? By comparison, we're brilliant. At least you remembered to eat today.