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Springtails and harvestmen, a new predator/prey story

By JW Dowey - 02 Oct 2014 8:47:0 GMT
 Springtails and harvestmen, a new predator/prey story

; Harvestman image; Credit: © JEB

The harvestman (Order Opiliones) is that floppy, long-legged so-called “spider” that fumbles its way over grasses, looking for all the world like a lost alien. I’ve never seen it eat anything and it’s been a mystery as to how it could. A century ago, the suggestion among the learned naturalist community was that its pedipalps secreted a sticky substance - hence the paper from Jonas Wolff of Kiel University and his colleagues in the Journal of Experimental Biology, entitled Harvestmen use glue to catch their dinner.

Insects have been known to glue their prey and of course spider silk can be very sticky. The harvestman technique was studied using high speed videography. The prey was jumping springtails (Order Collembola), so the slight application of glue had to be precise. The viscoelastic properties of the secretion held the insect more strongly the more it struggled. This is non-Newtonian physics! The only defence seemed to be the springtails’ removable scales. These helped the animal to escape in a few instances, by leaving the body clear of glue, with the scales embedded in it. Roughly half of the attacks succeeded in finding a meal for the harvestmen.

So the arachnid is quite deadly. The springtails removable scales are normally non-wettable, too, so the evolution of this glue has been precisely to catch them. The scanning electron microscope reveals that the springtail’s non-wettable cuticle becomes completely wetted. The glue can hold springtails bigger than the predator with high velocity jumps using their furca or spring. Even the wobbly legs come into play as they make the harvestman’s body into a shock absorber for those inevitable collisions.

So I was wrong. The innocent wobbler is actually an efficient predator on animals that we ourselves rarely see. Using the same technique as insectivorous plants and spider’s webs, even the most active prey can be simply captured. That accounts for the great numbers of harvestmen we see in the northern hemisphere at this time of year, feeding on a population of tiny insects that outweigh most other species. We hardly notice this enormous food source, but this is the important part of the food web we neglect at our peril.

The smallest vertebrate,Paedophryne amanuensis, eats springtailsin the awesome New Guinea rainforest, but we probably need to study these, the tiniest of animals, much more closely as we discover more of them in Two new species of frog discovered.