Diapause

Down Mexico-way Monarch butterflies that left their chrysalides last September of thereabouts are beginning to emerge, this month, from a curious state of suspension known as the diapause. The word, from the Greek for “stopping,” means that their normal development stopped as the weather turned cool; they stopped consuming food; their sexual development ended; nonetheless they managed to make the huge trek from our northern climes here all the way down to Mexico. Down there they settled in on trees, in masses, to rest from their voyage and to await Springtime.

It is hard to credit that the grey masses in the center are monarchs—especially when glimpsing some obvious monarchs on the left. A close-up makes what we see here more credible:

Monarchs have an average lifespan, as butterflies, lasting between 2-6 weeks. They produce four generations every year of that length. But in the last, the fifth, generation of every season, they have an “active hibernation,” thus a special kind of diapause. They fly great distances before they hang on trees. This, you might call it the Greatest Generation, persists in the butterfly form for 6-8 months. Pondering this I note the fact that the lovely butterfly stage of these insects exists strictly for reproduction. When some external factor—like a prolonged change in temperature—threatens the completion of that mission, they have work-arounds. The diapause is an example, active hibernation a more extreme form of it.

Some species of butterfly—the Red Admiral and the Painted Lady, to name two already mentioned on this blog—overwinter as adult butterflies. Most others do so as pupae. Our own Black Swallowtails are an example. Two of them (Castor and Pollux, as Brigitte named them), formed chrysalides last September 14, 16 (link). They are still with us—waiting—in a big jar out in the garage. I checked on them this morning to see if the warm spells we’d had a ways back had disturbed their suspension in time. All seemed well. The temperature outside right now is 18° F. I expect it will be late March, early May before they cast off their shells and turn to Organic Life’s Job 1: reproduction.

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The first image is from Wikipedia; the trees are called Oyamel. The location is the outskirts of Anagangueo, Michoacán, Mexico. The second image comes from Arizona University (link).

Hibernating?

Learned today that Black Swallowtails do not migrate to warmer climes as the Monarchs do. They over-winter as chrysalides instead—and much more rarely as caterpillars.

The butterfly in chrysalis stage can evidently detects the season from a combination of the temperature and the length of the daylight; it then knows what is coming and begins to prepare. In that process it generates a kind of anti-freeze; the chemical protects it for the duration. If it is still a caterpillar, one source informs me, it will burrow a deep hole and, within that, wrap itself into a leaf. I think we witnessed such a case late last fall when a fairly large caterpillar simply disappeared; but we knew too little. Its feeding plants were then in a large pot full of earth, and it may have buried itself. That pot went outdoors for the winter.

Two quite cold nights—with our furnace coming on—sent me on this investigation. We have two candidates for over-wintering. Brigitte has named them Castor and Pollux (although they might well be Castoria and Polluxina). They formed their chrysalides on September 14 and 16 respectively. We shall see. For starters, I took their container out into the cold this morning lest they be deceived.