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the exit valve. It then becomes quiescent and begins to shrink as it reorganizes its body and finally sheds its last larval skin to become the pupa. In the fall and winter after the leaves are down, the cecropia cocoons are conspicuously revealed because they are attached to bare tree branches. The pupae hibernate and can survive in a frozen state, and the moths emerge the following summer provided that the brain has experienced a prolonged period of cold; cold is necessary to activate the brain, telling it that winter has occurred. As a consequence, this moth can produce only one generation per year—unlike some other species that don’t need chilling and that can have at least two broods in the south, where summers are longer.

This scenario of development from egg to moth is the normal one; however, it is actualized in only a small fraction of the caterpillars that hatch, perhaps about one in fifty to 100. Few survive to the pupal stage, and fewer still to the adult stage. We have little idea about the mortality of the eggs and caterpillars, but the pupae can be collected, and they show whether or not a moth emerged and what happened in those cases where no moth emerged. Of the 2,741 cocoons that Marsh collected and analyzed, 10 percent were chewed and pecked open and the contents had been removed; they were the victims of deer mice and woodpeckers. The other 90 percent of the mortality was caused by parasitic flies and wasps. Three percent of the pupae had been destroyed by flies (of the family Tachnidae). Twenty-three percent were victims of a species of ichneumon wasp, Spilocryptus extrematus (the genus is now renamed Gambrus).

Fig. 22. Cocoons of cecropia, polyphemus, and promethean moths. The first has a double wall, is attached to twigs, and has an exit sleeve at one end. The second has a single wall, is built inside a rolled leaf, and has no exit hole—the moth exits by dissolving a hole with enzymes in its saliva. The third is also rolled in a leaf, but it does have an exit hole and it is attached to a twig by a long silk strap (shown is a cocoon that is at least a year old; the silk ring around the twig has constricted its growth).

Each of the 630 pupae killed by Gambrus hatched not just one wasp but, on average, thirty-three. That is, these pupae represent 630 × 33 = 20,790 individual ichneumon parasitoids, which could potentially produce 20,790 × 33 = 686,000 more parasitoids in the next generation, and this wasp can have more than one generation in a year. One might suppose, then, that the Gambrus wasp population would explode quickly enough to eliminate an entire moth population in a year or two. But it doesn’t, because the Gambrus wasps are themselves also “controlled” by parasites. Marsh found, for example, another ichneumon, Aenoplex smithii, attacking the parasitizing Gambrus inside the cecropia cocoons, while the chalcid wasp, Dibrachys boucheanus, entered behind them to attack the A. smithii larvae. Similarly, a parasitic fly in the cecropia cocoons was also controlled by a hyper-parasite wasp, which in turn was the host of another, hyper-hyper-parasitoid wasp.

THE CHECKS AND BALANCES THAT MARSH UNRAVELED BY patient rearing of pupae collected in the field give us a glimpse of only some of the links that make up an ecosystem, which extends from the microscopic to the top predators. He also looked further into the mechanisms of the parasite-host relationships that reveal the subtlety of the tactics in the arms races between parasites and their hosts.

Timing is important. Gambrus wasps, for example, are attracted to their hosts, the caterpillars, by the odor of fresh silk these caterpillars exude while spinning their cocoon. The wasps arrive as soon as cocoon spinning starts, and to have a chance to lay their eggs they must be there while the cocoon is still soft. Otherwise they cannot thrust their ovipositor in to insert their eggs. Nevertheless, Marsh counted more than 1,000 Gambrus eggs in one recently spun cecropia cocoon, whereas on average only thirty-three larvae could grow to adulthood in