A Mermithid Nematode (Roundworm)  Parasite of Ants

I was up in See Canyon, along the base of the Arizona's Mogollon Rim, late in August of 2002. The area is beautiful, with big Ponderosa Pine and Douglas Fir. I looked under a rock next to a creek that was flowing nearby and found a colony of Formica ants (Formica occulta?). I noticed that some of the ants had distended abdomens and moved fairly sluggishly. I collected a few of the ants with distended abdomens, along with some of the non-distended abdomen ants.

When I got home and opened one of the distended ants, I found a > 2 cm roundworm very nearly filling the abdominal cavity! No wonder the abdomen was distended!

Dr. Ant of the Ant Farm's Message Board suggested in this thread that the worm is a nematode in the family Mermithidae. Roundworms in this family parasitize many different species of insects, including ants, beetles, grasshoppers and many species of flies. They have been considered as a biocontrol agent for mosquitoes and black flies. Dr. Wheeler (1910) also saw mermithid infections of ants. He named ants whose morphology had been changed by the worms as "Mermithergates".


Here is a Formica that has a relatively normal appearance...

...and this is one of the ants with a distended abdomen - a "mermithergate" ant

Both "normal" (right) and mermithergate ants on a millimeter ruler.

Ventral view of mermithergate ant

Lateral view of infected ant

I opened the abdomen and found the worm nearly completely filling the abdomen. You can compare the size of the worm to the size of the ant (background, left).

The worm was pretty fragile, and broke into pieces when I attempted to straighten it out.

Here is the worm on a centimeter ruler. Counting the broken fragments, it was probably 2.5 - 2.75 cm long.

Closeup of one end of the worm. You can clearly see the worm's cuticle in this photo.

Another shot of the same end of the worm.

And a closeup of the other end of the worm.

Close up taken near the middle of the worm, at a break in the integument. The large circular object is an air bubble. I don't know what the smaller circles within the worm are - cells, perhaps? I don't *think* they are eggs...

Earlier this year, I was watching a column of Messor pergandei. I saw a very strange M. pergandei in the column...in retrospect, I suspect it was also infected with a Mermithid worm.

One of the healthy Messors picked the infected ant up and carried it back to the nest. Note that Myrmicine ants usually don't carry each other this way.

Sadly, I did not collect this ant, so I am not sure whether or not she was really infected...but she certainly seemed so.

The general life cycle of Mermithid nematodes is:

  • Worm grows in host's body cavity, absorbing nutrients from host's hemolymph (blood)

  • Host insect reaches maturity.

  • Worm exits host, usually killing the host in the process

  • After a few days to weeks in the environment, the larva molts, becoming an adult

  • Adult worm mates with another worm lays eggs

  • In some species, eggs are eaten by the host, then hatch and burrow through gut wall

  • In other species, larval worms wander through the soil, looking for host.

  • The host is typically a larval insect.

  • Repeat cycle

I have also seen Messor pergandei showing a similarly distended abdomen, and moving in a sluggish manner. Oh, how I wish I had collected that ant!