Summer World_ A Season of Bounty - Bernd Heinrich [45]
9
Masters of Disguise
IT IS RARE FOR ANYONE NOT SPECIFICALLY LOOKING FOR a big plump caterpillar of a sphinx moth to find one. There is one exception, though—the tomato hornworms. We always have a tomato patch in our garden, and we almost always used to find several hornworms in it, although I have not seen any for years. The big green (and sometimes pale blue or black) horntail grub munching on tomato foliage in the garden transforms itself into a mummylike pupa encased in a hard brown shell and then remains in a deathlike stupor for the better part of a year in an underground crypt. The following summer it molts from its shell to be resurrected into a moth that flies only at night and feeds on the nectar of flowers, and superficially looks and acts like a hummingbird. But the moth is substantially more different from a hummingbird than a human is from an aardvark. Because they are unique and represent a strange breed that has been a favorite animal for unraveling many mysteries of development, I am always hoping to find one or several hornworms feeding on our tomato greens.
An insect’s metamorphosis in body and behavior from larva to imago (adult) is amazing, yet it is easily taken for granted because of its inflexible inevitability.
It is difficult enough to envision butterflies exercising choice in behavior, much less to imagine their immature grubs exercising options that determine how they will turn out physically after a molt. However, some insects do exercise options, on the basis of often subtle cues from their environment. For example, many species of aphids have optional wings if they experience the photoperiod of a summer.
The phenomenon of developmental plasticity was first discovered in some butterflies that had been described as different species but were later found to be different forms of the same species, which had experienced different seasons of the year. The summer environment had provided some cue that had switched their developmental patterns. Similarly, when young (nymphs) of the grasshopper Schistocerca gregaria are tickled—as when they encounter each other during high population densities—the adults from such “stimulated” nymphs molt into forms that look like a species totally different from those that grow up alone. Furthermore, the “tickled” nymphs are adapted to wander in search of new feeding areas as local supplies are depleted. Similarly, the caterpillars of some species of moths also respond to their environment by switching one developmental pathway into another one, to produce forms that are adapted to reduce their chances of being eaten by predators.
The appearance of a caterpillar from one instar (a stage separated by a shedding of its “skin,” or armor shell) to the next may commonly differ, but the new “uniform” that it wears is usually specific for all individuals. However, in some species there are two or more options for the “uniform,” depending on what the caterpillar experienced when it was younger. For example, in one sphinx moth species, Laothoe populi, when the caterpillars are raised on a white background they molt from green to white. In another moth, Nemaria arizonaria, when the young caterpillars perch and feed on oak catkins in the spring they resemble what they eat. Later—by summer, when they perch on twigs and eat leaves—they molt into a new form that resembles twigs.
For sheer number and variety of disguises in