Summer World_ A Season of Bounty - Bernd Heinrich [46]
The first four instars of all abbotti caterpillars are chalky white and thus conspicuous on green grape leaves. However, they curl up to resemble larvae of a cimbicid wasp. Cimbicid larvae are chemically protected—they can squirt a defensive fluid from glands along their sides. Apparently the young abbotti caterpillars mimic this distasteful model, because they do not have a “horn” at the end of the abdomen like other sphinx moth caterpillars (hence the common name, hornworms); instead, that structure is altered to look like a yellow translucent drop of fluid. It is unlikely that color, structure, and behavior would all converge to accidentally mimic the wasp larva, especially since the caterpillar’s appearance changes not partially but drastically when it molts into its last larval instar.
More startlingly, rather than just changing into a single totally different “uniform,” it morphs into either of two possible forms that are not only different from the previous form but also strikingly different from each other. One form is brown and streaked with black. This form is nearly invisible on a background of brown grape bark, where it hides in the daytime. At night it comes up to the leaves, and after finishing feeding on a grape leaf it snips off the remainder of the uneaten leaf and crawls back down and hides, staying immobile throughout the day while pressing itself tightly against the old growth of flaking grapevine bark. The other, rarer form of the same (fifth) instar of the same caterpillar on the same plant has large luminous pea green patches on its back and along its sides. This form feeds in the daytime and does not perch on old grapevines that have bark; instead, it stays on the young, smooth green grape stems.
The adaptive significance of simultaneously having two wildly contrasting forms of the abbotti sphinx moth in the last instar caterpillars on the same food plant is obscure. The brown form is clearly, in both appearance and behavior, adapted to hide on bark of grapevines (and Virginia creeper, another of its food plants). But the visually striking form with the green patches appears to be an anomaly with an as yet unknown selective advantage. I speculate that it is so different from the other that a predator who finds one form may be too distracted to search for and see the other. As already mentioned, a bird that has found a particularly juicy morsel will search for others that look like it. If a bird finds one caterpillar form on a grape plant—say, the bark mimic on a grape stem—it will search for others of the same type and in the same setting. It will thus, by knowing what it is looking for, more easily miss what is different. This is indeed the effect I tested with the students and the poplar sapling: some students searched an hour before finding the first caterpillar, but then after they found the first one they almost immediately found the second. A very common caterpillar, no matter how well camouflaged, is likely to be found eventually, and it is thus dangerous for an edible caterpillar to be on a bush with other edible company of the same appearance. However, one that is wildly different from that company has a good chance of not being noticed.
Fig. 19. The hind end of most sphinx moth caterpillars has a “horn,” as in this tobacco hornworm, Manduca sexta.
Neat as such tricks of the caterpillars in their game of hide-and-seek with predators and parasitoids might be, the question of mechanism always arises. How can two different morphs of the same age manage to be on the same food plant at the same time? Do some moths lay eggs that develop into only one form, whereas other individuals