From the Flicker page of Nicky Bay, a photographer from Singapore, we have this beautiful example of mimicry: a spider (not an insect) mimicking a ladybug (“ladybird beetle” to Brits, which is actually more accurate since these insects are members of the order Coleoptera—beetles—rather than that of the “true bugs”, Hemiptera).
Ladybugs are brightly colored with what we biologists call aposematic (“warning”) coloration: a warning to predators to avoid them because they’re bad tasting (ladybugs contain toxic and foul-tasting alkaloids). Such coloration is common: other examples include black-and-orange striped bees and wasps, the orange-and-black monarch butterfly, and the striking pattern of the noxious striped skunk. (I once had a pet striped skunk for several years—descented, of course. It was a lovely animal, bred in captivity, tame, loving and litter-box trained, but I still feel a bit bad about having a pet whose genome was adapted to living in the wild.)
Once an aposematic model species is in place, there is an advantage to tasty and nontoxic species to evolve the patterns and colors of the model, for by so doing they avoid predation. This form of imitation is called Batesian mimicry after the British naturalist H.W. Bates. Here’s one example of a model (this one American) which the spider is likely imitating:
There’s no problem in explaining the evolution of Batesian mimicry, but how aposematic coloration evolved in the models has always been an evolutionary puzzle.
The system works now because predators learn to recognize and avoid the bright coloration after a bad experience tasting the prey (predators may occasionally evolve an innate, genetically-based aversion). But imagine the first mutant ladybug that is somewhat brightly colored. It won’t be avoided by bird predators because they’ve had no experience with the coloration, and will stick out because its color is conspicuous. This mutant individual may attract the attention of predators, and thus be more likely to be eaten than less-colorful individuals in the same species. In other words, natural selection would seem to work against the initial evolution of such colors, even though individuals benefit once the pattern becomes common. But how does it get to be common?
One suggestion is kin selection: mutant individuals may occur in broods of relatives, so an individual “sacrifices” its life to perpetuate the colorful genes of its brothers and sisters, now presumably protected by a bird that’s learned its lesson. And indeed, some aposematic caterpillars tend to stay together as groups of relatives more often than individuals of related species that aren’t colorful. But other evolutionary models show that the bright color could result from individual selection, particularly if the colorful individual isn’t really killed, but only tasted and released. There are other, more complicated models as well. For the time being, the evolution of such coloration remains a bit of a mystery.
And here’s a video showing another “ladybug spider”; I’m not sure if it’s the same species.
Here, from the website “What’s that bug?” is another example of what is likely to be a Batesian mimetic spider.
I found it on Arkive as well; it appears to be a rare, sexually dimorphic species in which the males are Batesian mimics and the females, much larger, aren’t. Here are both sexes with an egg case.