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Formica Pupae: Cocoons or Not?

Tipping over rocks can lead to interesting discoveries.

On a recent trip to western New York, I tipped over a rock.


I found a colony of ants in the Formica fusca group. (They looked silvery, so possibly Formica subsericea, but my eyes are getting older.)

The worker ants scurried to remove the brood to safety.

Something else caught my attention. Do you see it?

Maybe it will be clearer here. Notice the workers are rushing to hide the pupae.

The cocoon on the left is probably a reproductive pupa (queen or male). It is larger than the workers and the cocoon is made of darker, coarser silk.

The cocoons on the right are worker pupae. They are the same size or smaller than the workers and made of lighter silk.

What's up with the white, naked pupa in the center?

It is the size of a worker. Conventional wisdom says whether or not a larva spins a cocoon before pupating is species specific. Why doesn't this one have a cocoon like the others?

Some suggestions:

1. Wallis (1960) and others have proposed that Formica fusca larvae must be covered with bits of debris before they can spin a cocoon. Perhaps this one wasn't buried properly and failed to spin?

2. Perhaps it isn't a Formica fusca pupa.

Colonies in the Formica fusca group are frequently parasitized or enslaved by other species. This video shows a mixed colony of Formica sanguinea and Formica fusca with a bare pupa.

I didn't see any evidence of Formica sanguinea workers in the colony I photographed, though.

Have you ever seen Formica pupae without cocoons? If so, why don't they all have cocoons? What might be the advantages and disadvantages of spinning a cocoon versus not?

References:

I. Wallis, D. (1960). Spinning movements in the larvae of the ant, Formica fusca. Insectes Sociaux. 7: 187-199. 10.1007/BF02224080.

See previous post about Formica fusca group ants.

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