Springtails (Collembola)

Springtails form the other group of (quasi)aquatic arthropod visitors that aren't crustacea. Springtails are mite-sized six-legged relatives of insects, ranging 1 to 3mm, rarely to 10mm. Til quite recently the Collembola were thought of as primitive insects, though they're wingless and they don't undergo any metamorphosis, unlike all of the real insects. Now most investigators think of them as an independently-evolved parallel line of arthropods. Collembolan fossils go back to the Devonian.
They are found everywhere there is some moisture, even in Antarctica. Their usual habitats are moist terrestrial ones, like leaf litter, but about a dozen species of springtails (in three different families) have adapted to the still surfaces of standing freshwater. Podura aquatica is the most common North American collembolan on pond surfaces. Their waxy cuticle repels water (it's "hydrophobic"), so that surface tension prevents them from falling through the water's surface. Instead they sit high, like a dewdrop on a waxed table. If they're disturbed, they can jump amazingly, considering their size. You see, springtails are utterly devoid of defensive jaws or claws. Instead they have a springlike furculum extending forward under the abdomen, secured by a clasp, the tenaculum ("little holdfast"). When they're alarmed, the furcula snaps from its clasp, bouncing the springtail off the water twenty times its own length. Springtails usually lead a cryptic existence, but Podura aquatica inhabits a very exposed environment at the water's surface, and it protects itself by exuding some exotic aromatic chemicals, like phenols and tetraterpenes, that repel its mite and insect predators and even make fish mostly spit out these springtails when they try them. (Gambusia affinis was the fish used in lab tests.)

Springtail females fight their way through the surface tension (not an easy feat at that scale) to lay golden spherical eggs under the surface, sometimes on the aquarium glass, where I can just make them out at 10x magnification. The eggs will hatch in 2 to 40 days, I'm told. Like mites, they eat specks of floating detritus, algal cells and fungal spores. Some have mouthparts specially adapted for eating the lipoproteins of the surface film itself. Others specialise in the bacteria that proliferate just under the surface. The Britannica tells me that fertilizers increase the number of collembolans in soil; if this is a direct effect, they may have some way of absorbing nitrates. Aphid-like woolly adelgids have this ability, I know, and recently a nitrogen-fixing strain of spirochaete bacteria has been isolated in the guts of certain termites, I've read in the NY Times, in Aug 2001. So, do elevated nitrate levels in our aquaria encourage collembolans, I wonder?

The most important predators of collembolans in our aquaria are mites and pulmonate snails, which prey on the eggs. Perhaps a very small surface-feeding fish might have the patience to stalk and eat them. I'm not completely sold on the practicality of raising springtails to feed fish, but The Bug Farm at High Prairie Farms will supply you a culture of terrestrial springtails; see Jim Atchison's collembolan article at their website. One caution he suggests: keep your springtail cultures well separated from your Grindal worms; the springtails can infest the worm culture and outcompete the enchytraeids.

If you're in the habit of drawing a paper towel across the surface from time to time, to take up surface scum, you'll be decimating the springtail population. And the surface-dwelling mites too. You might want to get rid of them anyway, but you should recognize collembolans for a harmless part of your captive ecosystem.

Biologist Frans Janssens, University of Antwerp, maintains an excellent collembolan website. The site includes full tips of culturing collembolans (though not aquatic ones), mostly as spider food. Besides a film of water and a suitable substrate, the easily-reared species he is working with need only some baker's yeast. Don't neglect the image gallery at this site, including your familiar visitor Podura aquatica, and the scanning electron micrographs that detail collembolan structure at astounding scale.

Janssens figures that, with an estimated 50,000 species of collembola, at the present rate of publication, biologists will have them all scientifically described in about 526 years.

In Britain, the Postal Microscopical Society (the PMS, by an unhappy chance) includes a Springtail Study Group, which maintains a site with information on collecting, culturing and observing springtails.