Springtails under magnification. Note the “furcula” on the left springtail. Photos by Gustav W. Verderber.
Given an estimated around-the-globe average of 100,000 of them per cubic meter of soil, it seems astonishing that springtails (the tiny creatures often called “snowfleas”) are frequently overlooked. Spotty distribution isn’t to blame for their invisibility. They occur in woodland and in grassland, in deserts and on permanent Arctic snowfields, in caves and greenhouses, even on fresh and salt water. Nor are populations thin. One study in Denmark concluded that 16 million individuals of a single species inhabited a 1-acre forest plot.
We go about our outdoor business blithely unaware of springtails because they are tiny, silent, and live for the most part hidden in duff and leaf litter and in the top inch or so of loose soil. Most species are dark of hue – black, gray, dark blue – which is why casual encounters hereabouts are likely to occur on warmish winter or early spring days when springtails are active on the snow surface, where they appear to wide-eyed, first-time observers as animated pepper grains.
But what exactly is a springtail? Your natural history handbook or insect field guide probably identifies springtails as a primitive order of insects. But specialists in the field are now largely agreed that springtails aren’t insects at all but rather organisms forming an order – even a class – of their own. They do share characteristics with insects – most notably three body parts and six legs – but they differ from them in important regards. For example, their mouthparts – external in insects – are enclosed in the head. And unlike insects, typical springtails absorb oxygen directly through the soft cuticle and feature either small patches of simple eyes (which provide poor vision) or no eyes at all.
With the aid of magnification, one can see toward the rear of a springtail the celebrated action feature, the “furca” or “furcula.” This two-prong-tipped organ is normally held forward against the underbody, secured by a catch called the “tenaculum.” When alarmed (or merely traveling), the springtail releases the catch, and the tensed furca drives down and back into the ground (or log or leaf or snow or water), catapulting its owner up and away, antennae over tea kettle. We are likely to notice these minuscule soft-bodied creatures only en masse. When an assemblage has detected your presence, they will begin to jump, and because the human eye can’t follow them in flight, the pool of organisms will seem simply to evaporate. It is, as far as I know, the only demonstration in nature of the reverse of spontaneous generation.
Taken as a group, springtails consume a remarkably varied diet. A few are predatory, hunting down tardigrades, nematodes, rotifers, even other springtails. Some cave-dwelling species may live entirely on soil bacteria, while those roaming the surface of stagnant pools are probably grazing on algae. Certain highly specialized springtails live their entire lives on cleanup patrol in ant or termite nests, where they are thought to subsist on bits of food dropped or regurgitated by their hosts. Species that periodically or permanently occupy snow or ice fields are assumed to glean wind-blown pollen grains and spores. Many species eat frass (arthropod droppings), while fungi and plant material, both fresh and decayed, are standard food for the majority of species.
Typical insects rely on copulation to initiate the critical business of reproduction. Lacking external genitalia, springtails employ a fastidiously impersonal method. A male secretes a viscous material which, by raising his posterior, is drawn up into a stalk, on top of which he deposits a spherical packet of sperm. This structure, which looks, microscopically, like a golf ball perched on a long and very thin tee, is called a spermatophore, and males produce a number of them during periodic reproductive phases. If a receptive female of the appropriate species happens by, she takes the sperm packet up through her genital slit. Liquids inside her body cause the packet tissue to swell and burst, releasing the sperm for egg-fertilization duty.
If this system for combining precious genetic material seems to us haphazard, it apparently strikes a few springtails that way as well. Males of certain species build a fence of spermatophores around a likely female, a strategy presumably designed to ensure reproductive success by prompting the “eenie-meenie-miney-moe” response. Males of a few species even drag females to their spermatophores. Clearly, however, earth’s environments are yearly strewn with quadrillions of what are destined to be leftover springtail spermatophores. Nothing is wasted in nature, of course, and many of these end up as minute contributions to soil fertility. But others serve springtails more directly. Encountering intact sperm packets – particularly those teed up by other species – foraging males are likely to eat them.