The standard division of a year into four seasons is a mighty coarse approximation. Shifts of light, air, and growth as the earth’s orbit progresses call for a much more nuanced taxonomy. This past week, for example, clearly registers in my mind as something like “early late summer” — a period of a week or two, around the third week of August, when you suddenly realize that the show is ending. The focus of the natural world shifts, on balance, from exploiting this year to preparing for next.
As with other subtle seasonal shifts, the onset of this brief moment is evident everywhere I look: hints of color in the trees, the ascension of goldenrod’s yellow on the landscape, the occasional call note of a migrating songbird overhead if you happen to be outside at night. The natural world poises to dive toward the shortest days.
Perhaps because every organism, in its own way, must respond to the approach of winter, this time is of particular interest to the naturalist. Certain of our butterflies, for example, have run their course for the year, at least as adults: they’re still out there, of course, as eggs, caterpillars, or dormant pupae, but the adults have died or, or if they’re still here, it’s only as worn individuals bereft of much of their color. But the biology of the life stages left behind is as elegant, if harder to observe, than the activity of the adults.
Each type of butterfly has its preferred form for overwintering. For some, it’s a simple egg, laid, in a wonderful tribute to optimism, near where the proper food plant for the caterpillar grew this year and might, with luck, appear again next spring when the egg hatches. For other species, partly grown larvae overwinter; in a few cases, like the Orange Sulphur, the caterpillars might even wake up to take advantage of warm days in the winter to feed and grow a little. Still other species overwinter as chrysalids, homely lumps unrecognizable as butterflies, buried in the leaf litter or stuck to senescing stems. When the right time comes next year, an adult butterfly will emerge, unfurl its wings, and seek to pass on its genes.
The complexity of this process of “winterization” is astounding. For many insects, each summer sees just a single generation, and their developing progeny are programmed to cease activity at the right time, hunker down, and wait for spring. But what triggers this response, and what happens, physiologically, inside the young insect to prepare its body for freezing weather? And for butterflies with more than one generation per year, how does the final generation know to shut down rather than mature?
Sure, there are a few general principles at work here. Day length, for example, is a reliable factor, and many plants and animals use it to time their life cycles. Likewise common in the insect world is the production of chemicals that protect cells against damage from freezing. But each species has its own rules, and it seems to me that to fully understand the life of just a single insect species would be the work of more than one human lifetime.
For some insects, late summer is the season for adults. Some of these species eat seeds, and need to wait until plants have done their part by maturing. Other late-season insects probably evolved their life schedule to avoid certain predators or parasites that are more prevalent earlier in the season. Others, like many of our grasshoppers and katydids, simply have large, complex adults that need a full growing season to make it to maturity. But these later-season insects have much to do and not long to live.
I make a sweep through the quasi-meadow in our front yard for insects. Numbers and diversity have dropped precipitously from even just a week ago; many flowers have finished blooming, fading to brownish stems and seeds. Yet bees are still working the flowers that remain in bloom, loading their pollen sacks with yellow powder to provision their young.
One grasshopper, my old friend Chortophaga viridifasciata, is present again in numbers. Adults of this species, which spent last winter half-grown and matured by May, have long since petered out. But the young produced by this year’s generation have just hatched. Tiny nymphs smaller than rice grains are already prodigious jumpers. Some of these same individual insects will likely furnish my first insects of 2015, miniature grasshoppers cavorting in the yard on a warm February day.
And in the state forest I find a Carolina grasshopper, laboriously waggling her abdomen into the ground to lay eggs. It’s hard work; with only her body weight to work against as she digs, she hoists her hind feet off the ground to muster all possible leverage. I watch for 20 minutes, my patience expiring before her job is done. But I’m grateful to her for her efforts, because she’s making provision for the year to come.