Summer World_ A Season of Bounty - Bernd Heinrich [47]
Fig. 20. Four disguises of Abbott’s sphinx caterpillar. The typical hornworm horn is adapted to look like a drop of yellow fluid in the young caterpillar; later, it looks like an eye in the mature larva, which mimics a snake.
Both of the final instar forms of the abbotti sphinx caterpillar rely on blending in so as not to be noticed, and this requires that they don’t move. No matter how well camouflaged a caterpillar is, it is likely to become dead meat if it moves when a bird is nearby. But what happens when the larva leaves the food plant and must crawl along the ground in order to search for a pupation site? Remarkably, Abbott’s sphinx caterpillars then take on a fourth disguise: both morphs now switch to the same disguise. The mottled green form darkens, and the other form stays brown. In both forms the “horn” then resembles a reptile’s eye, and the anal flap mimics a reptile’s mouth. The caterpillars also change their behavior to appropriately show off the “reptile” face when they are startled. When touched, they curl the end of their abdomen up and, eerily, look like a snake raising its head when it is ready to strike. To be sure, the caterpillar does not have two “eyes,” but to a small bird, even a one-eyed snake could be startling.
Most hornworms are large, and large size gives a caterpillar the option of mimicking an elongated scary or distasteful vertebrate animal. Abbotti sphinx moth caterpillars are not totally unique; the ground-dwelling caterpillar of the gallium sphinx, Hyles gallii, has one morph that is black with yellow spots, and thus has key features that mimic poisonous spotted salamanders, which are colored in that way as a warning. Among some large tropical sphinx moths, the caterpillars convincingly mimic a snake’s head (Miller et al. 2006), but in this case the effect of scales on the head comes from the folding of the front legs on the caterpillar’s underside, which is turned up in its snake display. The “eyes” (in this case, two of them) are derived from puffing out darkly pigmented skin on the sides of the head end. That is, the head of this snake mimic is fashioned from the front end, rather than the back as in abbotti. The snake head display is, in a butterfly, even found in the next stage, the chrysalis (Aiello and Silberglied 1978).
Coloration is an important component of many disguises, but color as such can also have another, equally important function. Dark coloration serves both to protect an animal from the sun and to increase the absorption of solar energy. Many butterflies are colored in ways that facilitate solar heating, which permits their flight muscles to operate in a cold environment. In caterpillars, instead, elevated body temperature speeds up the growth rate and greatly shortens the time until the relatively safe pupal stage is reached. In caterpillars, growth rate is perhaps one of the most critical factors in avoiding predators, because every day that the caterpillar stage can be shortened is a day when the gauntlet of both parasites and predators is avoided. Being black and exposed to the direct sun is, however, a two-edged sword. At the same time that it reduces the duration of exposure to predators and parasites, it also increases the intensity of selective pressure by making the animal more visible and more accessible to predation.