Monday, January 2, 2012

Living Raft

I found this raft of living arthropods floating in the overflow pool of the water garden.  Riding the surface tension of the water, they have been drawn together into a colorful wriggling mass.
At first glance, the living raft appeared to be just another collection of debris floating across the water.  There’s always something floating in the water.  Usually it’s just dust or accumulated pollen or debris from the dwarf pine that hangs over one edge of the pool.  On rare occasions there will be something strange trapped by the water, so I stop and take a look every time I go past.

The sun reflecting from the little bodies caused the whole cluster to sparkle.  Besides those creatures already forming large aggregations, there were many floating as pairs or single individuals.  As the wind moved the large group around the pool, it collected the single drifters into its congregation.

It was hard to get a good look at the raft with the wind blowing it constantly around the pool, so I scooped it up in a shallow bowl and took it to a sheltered place for observation.  The raft seemed primarily composed of various species of Springtails and Mites.  These are common creatures of the soil ecosystem that live near the surface of the soil or in decomposing plant material covering the soil.  Besides being active creatures, they can exist by the thousands in a square foot of soil surface area.  It’s no wonder that a few of them would fall into a pool of water set flush in the ground.

The various species of Springtails and Mites definitely dominated the mix, but there were other soil arthropods present.  This small spider stood free of the water and ran unencumbered over the bodies of its companions.

A few of these denizens of the soil look quite large compared to the others, but the largest was just at the one millimeter mark.  This is what’s underfoot when you walk through the fields.  They play a vital role in the decomposition of organic materials, release of plant nutrients and control of soil born diseases.  It’s not often that they come out to display themselves in such an easy to view array.  After the photo shoot, I released all back onto dry land.  I also quickly washed the cereal bowl and put it back in the cupboard, before my wife saw what I was doing.


  1. Hmm, interesting. Thought you might be interested in this blogger's post:

  2. Hi Katie. Thanks for the reference. I'm afraid I don't have the resources to key Springtails to species, but the little gray ones do look like the photos I've seen of P. aquatica. The raft I found contains a mix that is representative of what I've found in pitfall traps. Since I don't intend to keep any of my victims, I use water in my trap and end up with a raft like this floating in the trap cup. I'll have to find a Collembola expert to educate me on the aquatic species. It seems quite awkward for an aquatic species to be so easily trapped by the surface tension.

  3. Wow - That is the most colorful soil arthropod raft I've ever seen! At the linked sight, someone wondered why they aggregate on the water surface. Though these critters live in moist, often saturated, soil they are not really aquatic, and they stay dry in those damp habitats by having hydrophobic outer surfaces. I believe they aggregate not of their own volition, but are spontaneously pushed together by the physical "polarity" and resulting surface tension of the water (like drops of oil).

  4. I agree James. These all look like species I'm used to encountering in soil samples. They have no control over their movement on the water and just keep drawing in more individuals as the raft is blown around the pool.

  5. waow a raft that breaths and bites too. hahahaha