Non-Newtonian Harvestmen

Harvestman (though not the Mitostoma chrysomelas of this post)

Photo: Dschwen (Wikimedia Commons) 

One of the most common cases of mistaken identity in the natural world is the colloquial classification of harvestmen as spiders. Fact: harvestmen are not spiders at all; arachnids, yes, but spiders, no. Harvestmen are a group unto themselves (order Opiliones), and are far more closely related to scorpions and pseudoscorpions than to spiders (despite their superficial resemblance). However, this kind of taxonomic milieu is nothing more than pedantic babble if you happen to be a springtail cornered by a harvestmen in the leaf litter. You see, to the average springtail, a harvestmen is death on stilts. Sticky death on silts.

Some interesting research has used high-speed videos to capture the harvestman Mitostoma chrysomelas hunting springtails. Mitostoma ensnare their extremely fast-moving prey using sticky setae (hair-like structures) on their pedipalps. Each setae bears a droplet of super sticky glue that has just the right chemistry and physics for capturing springtails. It has long been suspected that these droplets were used by harvestmen to hunt, but this study is the first to actually document it.

Springtails are one of those groups of hexapods that go more or less unseen. Most species are tiny, like the snowfleas (Hypogastrura nivicola) you may come across on a warm winter's day. Some of the larger southern hemisphere springtails approach a centimetre in length, but they, like their diminutive cousins usually remain out of sight, in the soil and leaf litter. They have some marvellous adaptations, not the least of which is the ability to launch themselves extraordinary distances using their spring-like furcula (the springtails' eponymous jumping organ). Most springtails can also shed liquids like a rubber raincoat, an important adaptation for living in wet soils. Both of these features, it would seem, are just the sort of adaptations that would keep springtails safe from predators like harvestmen, but that's not the case when it comes to Mitostoma.

 

Mitostoma glue is one liquid that springtails can't shed. Rather, the glue can stick to springtails without any trouble at all, establishing a firm hold that becomes ever more firm the longer and harder a springtail struggles. A springtail, with it's power-packed furcula, can put up a good fight, but for all the effort it exerts, the springtail only ensures itself a more certain doom. That's because Mitostoma glue becomes more viscous - it gets thicker, becoming a more effective trap - the faster it is pulled away from the setae by struggling prey. The pressure exerted by the distressed springtail causes the harvestman's glue to, in a sense, solidify. Liquids that posses this property are called non-Newtonian fluids: their viscosity changes when forces are applied to them. Some non-Newtonian fluids become more viscous, like when you stir gravy, while other liquids become less viscous, like when you shake ketchup out of a bottle. In both of these cases, the application of pressure to the fluid changes its viscosity. (Newtonian fluids, by the way, typically don't respond to pressure in these ways; think about splashing your feet in a puddle - there's no change in the water's viscosity.)

The non-Newtonian properties of Mitostoma glue make escape for springtails a highly unlikely outcome, should they be captured in the menacing pedipalps of a harvestman. It's all part of the incredible predator-prey relationships going on at the macro level around us all the time. I have often told students that if they are interested in predator-prey interactions they needn't spend fruitless weeks and months following Polar Bears (Ursus maritimus) on ice flows to see a rare dispatching of a Ringed Seal (Pusa hispida), or trek over endless kilometres of mountain wilderness in hopes of seeing a Cougar (Puma concolor) take a Bighorn Sheep (Ovis canadensis), but rather to begin by watching the countless invertebrate battles for life and death that surround them in their own backyards. Though small, the non-Newtonian harvestmen prove that the macro world is well worth watching.