During the spring and summer of 2010 I spent several days watching and photographing ants and visitors on a nest of the Allegheny mound-building ant. On June second I saw what I thought was the most remarkable event I have witnessed on the mound of Formica exsectoides.
When I first saw the mound it was evident that the ants were in a state of excitement. They were running around much more rapidly than they had in the previous times I had watched them. There were also a number of ants with their jaws wide open in an obvious defensive pose.
These two workers are in a such a posture. Their jaws are ready to snap down on any threat.
All of the ants except the one at about the ten-o-clock position were in the defensive posture.
The other ants seemed to pay an inordinate amount of attention to this particular ant, which was moving in an abnormal manner. It was twitching more than anything else.
Close inspection of one of the photographs shows that this ant has a gash in the front of its head, and a strange, hairy creature on its neck.
Hollywood could probably use this guy as a model for some kind of demonic monster. It is hairy and seems to have horns and is probably feasting on a still-living creature.
Later photographs in the sequence showed that the creature is some type of springtail. Springtails belong to a group of creatures (the Collembola) which formerly were classified as insects but now are generally called “non-insect hexapods.”
Photographs later in the sequence showed the springtail leaving the head of this ant (possibly in response to disturbance by other ants), but in a later photograph it went back and could be seen on the other side of this ant’s head.
The other worker ants removed this damaged ant from its location to another on the mound that was far removed from any visible entrances. I had not seen the springtail while I was taking these photos so I did not get photos that showed where it ended up.
So here is what I think happened: The ant was damaged in some way, probably while it was inside the mound. It may have just had some kind of physical accident, but there are insects that are capable of parasitizing ants (particularly flies and wasps) and this ant may have been a victim of one of them. The springtail probably lives inside the mound, and it was an opportunistic predator–drinking the newly available fluids of the damaged worker. Other ant workers have a survival response to remove damaged workers from the mound, so they did.
If I witness something like this again I might be tempted to put the damaged ant in a jar to see if some internal parasite hatches out of it.