Subject: swarm of caterpillars
Location: Fairfax, VA
July 21, 2015 11:40 am
This has got to be the most unusual behavior I’ve ever seen – a swarm of small (5 – 8 mm in length), translucent caterpillars, slowly moving across a sidewalk (to get to the other side, of course!) in a stream nearly the width of the sidewalk, with a depth of several individuals. I did not see it from the beginning, but I estimate it took about 15 minutes for them to reach the other side. It was most similar to watching schooling fish, but in slow motion. I assume this is adaptive behavior for the same reason as schooling and herds? This was at 8 AM on a very humid day, if relevant. Any idea as to species? Have you heard of this before? Is swarm the right term?
This fascinating phenomenon is an aggregation of Fungus Gnat Larvae in the family Sciaridae. We generally reserve the term swarm for winged species while in flight.