Entomologists, unlike birders, usually do not keep life lists of insects, but seeing my first ice crawler (Grylloblattodea) was akin to checking off the first kiwi on a twitcher’s life list. These insects are a strange breed. Only twenty-eight species are known, and they are found only in colder regions of North America and the Far East of Asia. The American species live mostly along the northern coast of the United States and Canada, in the Cascade and Sierra Nevada ranges. There you can find them on northern talus slopes of mountains, as high as three thousand meters or in ice caves.
These insects clearly love cold weather. Their optimal temperature of activity hovers around the freezing point, and you can easily kill one by touching it due to the warmth of your hand. And yet they are also not really good at surviving in the extreme cold — a drop in the temperature to below -9°C will also cause them to die. Unlike other cold-loving insects they have never evolved the appropriate levels of glycerol and other antifreezing compounds in their blood. And so, like tiny, six-legged Goldilocks, they seem relegated to a life in narrow, fleeting ranges of environmental variables, constantly searching for just the right temperature and humidity.
But a life like this has its advantages: because of their slow, temperature-dependent metabolism, ice crawlers are surprisingly long-lived (for insects) and may reach the ripe age of ten years. Food is usually not a problem as ice crawlers will eat almost anything, ranging from dead insects blown onto the surface of the ice to vegetal matter found under snow. There is not much competition in the subfreezing layers of the soil, especially if you are the largest insect active in this environment, and most predators, such as shrews or toads, tend to stay away from really cold places. Asian species of ice crawlers have slightly higher temperature tolerance and can safely live in the span of 9–15°C. Because many other animals are also active within this range, Asian ice crawlers escape competitors and predators by hiding very deep in the soil or at the bottom of caves.
But in the beginning, the life of these insects used to be very different. Over 250 million years ago, in the Permian, the ancestors of modern ice crawlers were anything but crawlers on ice. For one, all ancient species were equipped with two pairs of large wings. Based on the analysis of the gut content of some remarkably well preserved fossils we know that they fed primarily on pollen of ancient conifers and some of their now-extinct relatives. For millions of years they flourished in hot, tropical parts of the globe, leading a plant-hopping life, quite likely similar to that of many of today’s pollinators. According to some entomologists there used to be as many as 44 families of “ice crawler”–like insects, compared to a single family known today. They are frequently found in rock deposits dating back from the Permian to the Cretaceous.
But then, gradually, they disappeared from the fossil record. Strangely, there is not a single ice crawler known from the period after mid-Cretaceous. Their disappearance coincides roughly with the appearance of flowering plants, or angiosperms, and the nearly concurrent diversification of beetles and other plant pollinators. It seems that rather than allowing themselves to be outcompeted by this new army of more advanced plant-associated insects, the ancestors of ice crawlers found survival in a completely new lifestyle.
They shed their wings and entered a chilly, inhospitable niche along the edges of glaciers, in caves, or deep in the soil. This explains the lack of a fossil record; fossilization usually requires the sinking of a dead organism into fine silt at the bottom of a lake or an ocean, and so the environments favored by ice crawler ancestors made fossil formation difficult.
Although they have been luckier than all their winged, plant-hopping ancestors, the luck of modern ice crawlers may soon run out. Their dependence on a very narrow spectrum of low temperatures, combined with their inability to fly and thus quickly colonize new habitats, may soon spell the end of this ancient lineage in our rapidly warming world. As sometimes happens in evolution, ice crawlers have painted themselves into a corner. During the Pleistocene, when most of the North American continent was covered with a thick ice cap, these insects probably survived only on the edges of the glacier or in underground networks of caves. As the ice receded they followed it and colonized suitably cold (but not too cold) places high on mountain slopes or in icy caves. As a result, populations of these insects tend to be small and genetically isolated from each other due to the island-like nature of cold, alpine habitats. This means that when their environment suddenly becomes uncomfortably warm—and there is already ample evidence of the shrinking of North American glaciers—they have nowhere to go. And even if there still existed available habitats, ice crawlers have no way of getting there because of their inability to fly. Some populations in the Sierra Nevada of California, perhaps entire species of ice crawlers, may already be extinct, as repeated attempts to find them during the last forty years have failed. And when the entire genetic makeup of a lineage consists of only twenty-eight species, every one of them is priceless.
[An excerpt from my book “Relics: Travels in Nature’s Time Machine“]