Antlions: invisible predators of the soil

One thing that any nature enthusiast can attest to is that in nature, the closer you look, the more you see.  You can find life in the most seemingly inhospitable places, colors and patterns in the blandest-looking creatures, and—in the dry forest of Palo Verde—movement just below the soil surface by invisible insect architects known as antlions.

Antlions are not actually ants (and they’re certainly not lions); rather, they are insects in the order Neuroptera, related to lacewings and dobsonflies.  This morning, we set out to study the antlion larvae which burrow underground and turn ordinary soil into a sudden death trap.  While you very rarely get a look at the antlion itself, you can clearly see evidence of where it’s hiding.

antlion larvae

Antlion (larvae);  photo source: http://www.ipm.iastate.edu/ipm/hortnews/2008/7-30/antlion.html

 

Look for small (~1.5 in. diameter) funnel-shaped holes in the ground, then stick your nose right up close, wait a beat or two, and see tiny bits of dirt or rock launch themselves out from the base of the funnel.  These silent predators sense vibrations of termites, ants, and other prey near their hole, and then knock them down into the funnel by bombing them with clumps of soil.  Once the prey is deep enough down the hole, the antlion uses its mouthparts to quickly grab its meal and pull it under the soil.  Viewed from above, it looks like the victim is being dragged against its every effort into the underworld.

antlion pit

Antlion pit, ~24cm diameter and 15cm depth

 

Our assignment was to test how the antlions respond to disturbance—in this case, using your finger to sweep soil over the hole to cover it.  The action could mimic natural disturbance, such as a tapir trampling over the trap, or anthropogenic machinery plowing over the land.  With our study, we were looking to find out how the size (diameter and depth) of the newly made hole changes after repeated instances of disturbance, how soon after the disturbance the antlion recovers, and whether popping the antlion a termite snack ahead of time had any effect on its ability to recover.  In practice, what this experiment entailed was sweeping your finger over the ground and then staring at the same spot of dirt for 30 minutes, 45, or sometimes two full hours before the antlion started to reconstruct its lair.  As I’m sitting there under the shade of a tree, waiting, I’m reminded of a saying my mother used to tell me about a watched pot…

One upside to the often long recovery times is that when the antlion finally does start digging again, it comes as an exciting surprise.  There, in the midst of completely still soil, a grain of sediment suddenly wiggles.  Then the 3-square millimeters around it start to pulse, as if something is about to burst from the ground.  Instead, that tiny bit of ground starts to sink, and you see the tracks made by the antlion’s tunneling.  The antlion digs in spirals to form its funnel, flinging dirt in a firework-like display.

The results from our short study were interesting: antlions tended to build smaller traps than their original after the first disturbance, but repeated disturbances had no further effect on their rebuilt trap sizes.  Furthermore, feeding termites to the antlions had no effect on their ability to recover.  In terms of ecology, this implies a trade-off between energy expended and potential food gain—larger traps take more time and energy, but the larger size increases your range for catching ants.  It also seems like a single termite meal isn’t enough to override the stress of a disturbance and convince the antlion that this location is worth investing in a large trap.

Above all, this little experiment opened our eyes to the hidden activities of the soil, showing that you can find action, suspense, and intrigue right under your toes.

One comment to Antlions: invisible predators of the soil

  1. David says:

    Interesting that the ant lion you observed didn’t pay much attention to a single termite. I remember when I was a kid I used to place an ant in an ant lion pit and watch the ant lion get their prey. It always reminded me of the put in Star Wars.

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