Castles in the air

29 November 2013 by Malcolm Campbell, posted in Biology

If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." from Walden by Henry David Thoreau (1817-1862)

The wild wind has wrested all but the most tenacious leaves from their branches.

Where once was a lush canopy, there now remains only a scaffold – boughs laid bare against the flinty grey of the late autumn sky.

But then, for all its efforts to leave only arboreal skeletons, the wind has been largely ineffective in loosening the grip of the hornets’ nests from their lofty heights. 

The hornet nests remain, firmly tethered to their branches; a testimony to insect industriousness.

The hornet nests persist, suspended from their stalks, like papery silver fruit, stubbornly swaying as they are buffeted by gust after gust. They are robust fortresses against the onslaught of autumn – a safe haven to ride out the impending winter. They are veritable, not metaphorical, castles in the air.

The nests seem such an effective way to contend with the challenging conditions. When, by chance, the wind does succeed in ripping a nest from a tree, it reveals a structure of amazing ingenuity, and cunning.

The nest is built of cells – elongated cylinders, perfectly crafted to contain a pupating larva. The cells are constructed adjacent to each other, and stacked like stories, according to an architecture that emerges from the hive mind of the collective. The nest emerges as an oblong ball – a teardrop – sheathed in layer-upon-layer of outer insulation.

The entire structure is built from paper. The hornets are a member of the wasp family, the vespids, whose members build their nests of paper, including the so-called “paper wasps”.

Every year, in the spring time, vespid queens construct a new nest. In keeping with their role in founding a new nest, they are called foundresses. Foundresses strip tiny pieces of wood from fallen logs and the like. They chew these pieces and mix it with their wood-degrading saliva. In doing so, they create a pulp – a mash of wood fibres in a porridge-like slurry.

The foundresses use their pulp to construct the nest. They spread the pulp and then, using their mandibles, the foundresses knead the liquid out of the pulp, creating a raised sheet of insect-made paper. Each foundress builds on the sheet, layering the pulp on top of the previous sheet, so that eventually the pieces of the nest – the cells – take shape. The foundress, or foundresses (sometimes a few will work together), will lay their eggs in the cells. When the eggs hatch, and workers emerge, they participate in both capturing food, and in building the nest. Over time, more cells are created, forming stories, and the outer insulating layers are laid down. The entire nest takes shape.

The resultant structure seems ideal for overwintering. Constructed in dense, layered stories, with its outer insulating layers, the nest seems optimised for the paper wasps to aggregate to ride out the winter weather. What’s more, the aerodynamic nature of the nest’s exterior enables the wind to whip about it, with minimal impact on those within.

In all, the nest appears to be a perfect solution for contending with the impending challenges of winter.

But this would be wrong minded.

At this time of the year, the nest is empty.

With the first frost, vespids abandon their nest. A subset of the sisterhood, who will function as next year’s foundresses, have made their way to a more secure location, like the inside of a tree trunk. This site enables them to escape from, and hibernate through, the impending winter. This is their hibernaculum – their hibernation home.

The existence of a hibernaculum poses an intriguing question. Why build a nest, particularly one built to last through the winter, if a hibernaculum is all that is needed?

The answer lies in the carnivorous nature of vespids like the hornets and the paper wasps.

Hornets and paper wasps are voracious, opportunistic carnivores. Their appetite is vast, and includes their own kind. The nest, it turns out, is an effective way to defend against predation, particularly against other paper wasps.

Even a small amount of paper can protect larva from predation in the newly founded nest. Foundresses adjust the amount of pulp they apply to pupal cocoon cells depending on the risk of predation. The greater the risk, the more pulp the foundress applied to the cells. Despite the fact that this is a costly activity for the foundress, the benefits in terms of hindering predation seems likely to outweigh the cost.

As the nest grows, it provides a more substantial barrier to predation. This said, there is still the risk that wasps that are not nestmates could enter the nest, to prey on the larva within. When the wasps are of the same species, this is a significant risk for the colony. How do paper wasps contend with this risk?

It so happens that, generally speaking, vespids are incredibly xenophobic. Hornets and paper wasps fear and have aggressive tendencies towards others that are not like themselves. Their xenophobia is not limited to other species – it extends to others of their own kind that are not nestmates.

Vespids identify others who are not nestmates by two means – scent and sight. Each nest appears to have a particular odour “scent print”, like a fingerprint, that distinguishes it. The scent is derived from complex hydrocarbon molecules from the wasps cuticle. Only wasps that have the same cuticle hydrocarbon scent are recognised as nestmates and not attacked. Some wasps have managed to evade this system of detection to parasitise other wasps by adopting, or mimicking, the odour of the cuticle of their prey.

The other way that vespids recognise “strangers” is by sight. Paper wasps differ in their facial markings to such an extent that it is not difficult for humans to distinguish one paper wasp individual from another. Paper wasps possess this same ability – facial recognition. Paper wasps are able to recognise nestmates from non-nestmates.

In some species of paper wasp, the facial recognition discriminatory capacity is incredibly sensitive. Polistes fuscatus is a paper wasp species where several foundresses work together to found a new nest. Working together enhances the chances of success for the new nest, as the foundresses can collectively build, defend, and provide for their nest. This comes with a cost though, as nestmates will have more varied appearances arising from the multiple mothers.

Nests founded by a single foundress, like those of the species Polistes metricus, have a more difficult time in establishing a nest, but nestmates are easier to identify, given their single mother original. Remarkably, the greater diversity of facial appearances is not a challenge for Polistes fuscatus. Polistes fuscatus paper wasps have a better capacity to discriminate between individuals than do Polistes metricus paper wasps. That is, the more complex social structure in Polistes fuscatus nests is associated with a greater capacity for facial discrimination.

Based on scent and visual cues, paper wasps are able to identify invaders and expel them with incredible viciousness. This xenophobic behaviour has advantages during the hot summer months when food sources are abundant, and the nest peaks in its productivity. On the other hand, as resources become limiting, and the need to identify scarce overwintering hibernaculum sites is pressing, being hostile to one’s own kind becomes problematic.

There are times when within-species tolerance is necessary for the long-term survival of the species. So it is with the vespids like hornets and paper wasps.

While mid-summer vespids are generally xenophobic, tolerance for non-nestmates can increase in the autumn. It is thought that relaxing xenophobia enables vespids of the same species to work collectively to gather scarce resources, and, perhaps more importantly, for foundresses to share a hibernaculum.

Intriguingly, in the spring following hibernation, the foundresses will frequently return to the nest of their birth. Here, the foundresses are re-exposed to nest hydrocarbons that seem to re-invoke xenophobia, and the cycle of xenophobia and tolerance begins again when the foundresses establish their new nests. It almost as though the return to the birth nest reminds the foundress who she is. As such, the nest’s persistence through the winter is not important as a residence, but rather as a reminder, a reinforcer, of the xenophobia necessary to carry the wasp through the next season. In some ways, the nest is a physical manifestation of a memory – a built, three-dimensional aide memoire.

The hornets' nest is also a potent reminder for we humans. It is a potent reminder of the power of place. The nest’s persistence from one year to the next triggers a social response in the wasp that is appropriate to the hornet’s survival at that time. All of us are susceptible to similar triggers – the return to a particular place causes us to behave in a manner consistent with our past experience in that place.

Sometimes this power of place is beneficial – we conform with the norms of the location so as to get along better with others there. Sometimes this power of place extends beyond beneficial – creating a strong positive undercurrent in our lives – we are reminded of wonderful times past, and this elevates our thoughts and our interactions. On the other hand, the power of place can be detrimental – promoting patterns of behaviour that are unhelpful at best, and destructive at worst.

Fortunately, we are not vespids, like hornets and paper wasps. We can choose to embrace the positive aspects of power of place, or reconfigure our behaviour in a negative space so that it carries positive connotations in the future. Our behaviours need not be captive to the space we find ourselves, we can change our behaviours so as to “redefine” the space – turning a negative into a positive. And that’s no castle in the air.

Images: All photographs by Malcolm M. Campbell.


Cervo R (2006) Polistes wasps and their social parasites: an overview. Annales Zoologici Fennici 43: 531-549

Cini, A., Gioli, L., & Cervo, R. (2009) A quantitative threshold for nest-mate recognition in a paper social wasp. Biology Letters 5: 459-461

Dapporto, L., & Palagi, E. (2006) Wasps in the shadow: looking at the pre-hibernating clusters of Polistes dominulus. Annales Zoologici Fennici 43: 583-594

Furuichi, S., & Kasuya, E. (2013). Costs, benefits, and plasticity of construction of nest defensive structures in paper wasps. Behavioral Ecology and Sociobiology (early online)

Sheehan, M. J., & Tibbetts, E. A. (2011). Specialized face learning is associated with individual recognition in paper wasps. Science 334: 1272-1275

Tibbetts, E. A., & Reeve, H. K. (2003). Benefits of foundress associations in the paper wasp Polistes dominulus: increased productivity and survival, but no assurance of fitness returns. Behavioral Ecology 14: 510-514


4 Responses to “Castles in the air”

  1. johnbrooks Reply | Permalink

    Enjoyed your article, thanks!

  2. MikeSr Reply | Permalink

    Thank you for an informative and enjoyable read. Cheers;

Leave a Reply

− one = 3