Bugs that aren’t really bugs

Springtails are relatively unknown, but not unimportant.

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If you see insects jumping in the sand, they're probably springtails. But they're not insects. – Photo by Ab H. Baas

Here’s another column on an important group of animals that you’ve probably never heard of: the springtails (Collembola to biologists). Tiny and six-legged, springtails are superficially similar to insects, and in the past have sometimes been treated as an order with the class Insecta. But the current view elevates Collembola to a class of its own, a separate lineage that diverged from insects some 400 million years ago, retaining primitive traits from a shared ancestor while the insects evolved more complex forms.

Springtails have simpler bodies than insects, with fewer segments to their legs and abdomens. They lack the external mouth parts of an insect. And critically, they lack any sort of respiratory system; oxygen enters their bodies by diffusing through their “skin.” As a result of this primitive means of breathing, Collembola are necessarily small; diffusion can’t supply oxygen to a large body, and most springtails are only a millimeter or two in length. And because the body covering needs to be permeable to oxygen, it’s permeable to other things, as well, such as moisture: Springtails are largely restricted to moist environments because they’re at constant risk of dehydration. (They’re also vulnerable to insecticides and other toxins, which can readily be absorbed into a springtail’s body.)

Despite these limitations, Collembola occur widely, including on the Vineyard, and are often mind-bogglingly abundant. Along with another primitive life form, the roundworms or nematodes, springtails are considered to be among the most numerous animals on earth, many billions of tiny organisms adding up to a huge amount of living tissue and, therefore, major ecological importance. Collembola live in damp soil, on decaying wood, or sometimes in crevices on tree bark; they live by eating bacterial and decaying organic matter. Operating at such a small scale in moist environments, springtails are in constant contact with fungi, and their most important ecological role is helping spread the spores of fungi, including beneficial ones that form vital partnerships with plants. Without the direct and indirect actions of springtails, the natural world would be a distinctly less fertile place.

Most springtails (here’s how they get their name!) have a spike-like organ, called a furcula, extending from the tips of their abdomens. The furcula normally bends forward under the animal and locks in place against the body, but it can be put under tension and then released: As it snaps straight, the furcula launches the whole springtail into the air. The animal has little or no control of where the furcula will send it, but these leaps are dramatic and help a springtail avoid disturbance or predators. If you encounter a swarm of tiny, leaping, insect-like critters in early spring, odds are good you’ve found some springtails.

On a mild day a couple of weeks ago, I rummaged for signs of life in a narrow strip of litter, exposed as the snow melted, along the walkway in front of my office. A beetle larva, the nymph of some kind of roach, a crab spider … and finally, a tiny, gray object, perhaps two millimeters long, that popped into the air and landed on a flagstone. My first springtail of the season!

The tiny size of the animal challenged the abilities of my camera, and bad photos were all I could manage. But comparing the shots I took to better photos on the Internet, I concluded that I had found the common and widespread springtail Tomocerus vulgaris (like most obscure arthropods, it has no common name). It’s only the second species of Collembola I’ve found on the Vineyard (but then, I’ve barely looked); the other, Hypogastrura nivicola, is commonly known as the snow flea. Even more resistant to cold than most springtails, Hypogastrura gets its name from its habit of swarming (sometimes by the thousands) on the surface of snow.

As far as I know, the Vineyard’s Collembola are essentially unstudied. This is not a large group (depending on the source, there are between 3,000 and 6,000 species worldwide), but I’m sure the two species I’ve found represent a very modest portion of the springtails that are here. Very likely, human activity has significantly altered the Island’s Collembola. For example, because these animals can’t tolerate dry, sunny sites, the sheep-farming days of the 19th century, which largely deforested the Vineyard, must have wrought havoc among our springtails. And because of their soft and permeable body covering, springtails here and everywhere surely experiences stress from acid rain and even background levels of pesticides and other chemicals.

And yet, Collembola has felt little need to evolve during the past 400 million years (the oldest fossils and amber-trapped springtails closely resemble modern species). Despite their small size, simple anatomy, and susceptibility to the chemical environment surrounding their bodies, springtails have found a plan that works for them. They’re here to stay.