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By Root 738 0

from the pupal into the adult stage is normally strictly temperature-dependent: the higher the temperature, the faster they become adult. But the overwintering moth pupae can hold back even if they experience warmth. Their rather amazing block to the developmental process can be removed only by subjecting them to both a sufficient length and a sufficient depth of cold. As shown by brain transplants, the developmental blockage originates in the insects’ brain; implanting a chilled “loose” or unconnected brain that has been subjected to the right day length (depending on the species) into the abdomen of a non-chilled pupa will start the process of development as it releases hormones into its host’s blood. Ultimately the pupae, by not activating their normal brain, are preparing and waiting for the next summer, which is about ten months in the future. And only then, at the right time, does the whole population of millions of them emerge relatively synchronously in a week or two to mate and lay their eggs. They must be timely, and quick; they live for only about a week.

Preparing for summer means being able to anticipate the upcoming season, which presupposes knowing (no consciousness is implied) what season you are in. Perhaps one of the most reliable seasonal cues is photoperiod, the relative hours of daylight versus dark in a twenty-four-hour cycle. Throughout late summer and fall the days get shorter, then lengthen again after the winter solstice. Thus, an organism that sees neither the stars nor the angle of the sun could potentially anticipate the approaching summer by registering day lengths.

To measure day length requires the use of a clock that, on our planet, runs on a twenty-four-hour cycle or period. Biological clock mechanisms with approximately this period have by now been demonstrated in one-celled organisms, plants, insects, birds, and mammals. But a clock, even if it has a correct period, is not sufficient to tell time, any more than a watch that has not been set to the local time. Biological clocks must also be set to the correct local time; to do its job, each clock must be sensitive to and synchronized by signals from the environment, in the same way that we set our watches to the time we may hear announced on the radio, or from some other cue.

Like any good watch, a biological clock doesn’t run faster or slower as temperatures increase or decrease, even though the individual chemical reactions that run it presumably do. However, like the windup wristwatches that we used to wear (before batteries), which would run perhaps a minute or two either fast or slow, so that we had to reset them every few days, biological clocks are never totally accurate and also need frequent resetting to the local solar time. For example, a circadian clock that runs fifteen minutes fast per twenty-four-hour day would be off by an hour within four days. But what are the clocks set to? Most biological clocks are set at the signal of either lights-out or lights-on, which in nature would normally be dusk and dawn, respectively. They thus indicate the actual time relatively accurately, despite the fact that their periods may not be exactly twenty-four hours. Once a clock is set and running, appropriately timed behaviors can be “read” from it and will be close to local time.

One of the first to show that the twenty-four-hour clock could be used by an animal to synchronize to the season was Erwin Bünning, in studies with the common white cabbage butterfly, Pieris brassicae. In the summer the caterpillars of this butterfly proceed without pause from the pupa to the adult stage in a couple of weeks, with the exact duration depending on temperature. In the fall the caterpillars still grow normally; but after they have entered the pupal stage, they stop further development regardless of temperature. If they didn’t stop, they would all hatch out when there is no cabbage for their caterpillars to feed on. So they do not continue developing into adults until the following summer. Bünning asked how the animals “know” what season they are