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?


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.

Ant of the Week: Formica fusca Group

One of the first things you notice about the Formica fusca group ants are their mounds.

(These photographs were taken in the Colorado Rockies).

Bare mounds of soil and small pebbles stand out amongst the vegetation.

As you get closer, you can see that the mound does have some brown bits of vegetation scattered about, such as dried conifer needles. There are also multiple entrance holes, something you don’t see as much in the mound-building harvester ants.

Closer still and you can see the black ants with their large eyes. These ants are called the “Formica fusca group” because there are a number of ant species that can be hard to distinguish from one another. (See the resources at the bottom for more information.) My best guess about these are that they are part of the Formica subsericea complex, possibly Formica podzolica,, although they look a bit small.

Formica fusca ants get their common name “wood ants” because they live in forested areas or woods, not necessarily because they live in wood. Wood ants, or thatching ants as they are sometimes also called, are easily confused with carpenter ants. Typically the Formica ants are smaller than Camponotus, and the back of their alitrunk (midsection) has a valley or depressed area, rather than evenly rounded like the carpenter ants.

Many species build their mounds in the cooler forests of North America, Europe and Asia. Some of the wood ants cover their mounds with pine needles and bits of twigs, so they look like they have thatched roofs, even more than we saw in the photograph above. The piled up homes catch the sunlight and create a warm interior that allows these ants to live in colder places than most types of ants.

Wood ants feed on aphid honeydew as well as scavenge dead arthropods. They are also predators of common forest pests and are considered to be beneficial to the forest.

Wood ants lack a stinger. They defend themselves by spraying droplets of formic acid when disturbed, which causes most enemies to retreat. They also can bite.

At a nearby mound I noticed an interesting behavior that may have been related to defense. The workers were wrestling with a green aspen leaf that had fallen on the mound.

As you can see, the workers are chewing on and biting off pieces of the leaf.

In this video, you can see the same behavior. At the same time, the ants are apparently leaving the brown twigs alone.

Do you think it is sanitation? If they wanted to get rid of the leaf, wouldn’t they simply cart it to the side and dump it? Do you think they using the bits of leaves for something? Or perhaps the moister leaves are more likely to mold and cause sanitation issues than the drier twigs and needles.

In any case, ants of the Formica fusca group are fascinating ants.


Formica fusca group at

Navajo Ant Project