Roosting insects: avoiding predators, or just after a cosy nap?


On Sunday, as I walked a load of washing to the Hills Hoist, an odd pattern caught my eye. It’s autumn here in Armidale (which is very cool climate by Australian standards) and temps have been cooling off for a while. But Sunday was our first proper cold snap, a lot earlier than usual and much colder than the March average minimum (about 11°C). We got down to about 3°C overnight and there was frost in some parts. I was shivering before I’d finished hanging the washing out.

Our vegie garden is effectively dead – a few late-ripening tomatoes and the flowering borders are hanging on. But as I walked past a dead plant full of dill seeds, something caught my eye.

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I’ve walked past this plant every day for months, but this time some umbellets of dill seeds looked odd. On closer inspection, I noticed blowflies (I counted five) tucked into them, almost completely camouflaged.

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Roosting behaviour is common in insects for various reasons. It’s generally thought to be for survival, e.g. to avoid dehydration during hibernation, or to avoid predators at night. Insects are really sensitive to cold temperatures. So at night, when temps rapidly cool off, they can’t do much except loll about on vegetation. This makes them more susceptible to being eaten.

But do insects sleep? It’s unclear how insect ‘sleep’ compares to human sleep, but recent evidence suggests that social animals may be more likely to sleep than solitary species. Most insect species are solitary, but we still don’t know anything about the lifestyles of most of the millions of insect species on Earth, so we don’t anything about their sleep habits. However, we do know that the activity of most insects, especially outside warmer tropical environments, quiets down at night, either in response to light or to temperature. Sidenote: we still don’t a lot about how many insects respond to light, and this is why light pollution is such a big deal.

This study from 1916 is an excellent observational report of insect ‘sleeping’ (or roosting) behaviour in a range of species in the USA, including bees, wasps, beetles, and butterflies. But, over 100 years later, we still don’t know when, why, and under what conditions different species exhibit this behaviour. It has mostly been documented in charismatic species, or for obvious aggregations. We all know about the hibernation roosting of Bogong moths or monarch butterflies. Roosting aggregations of other insects, including damselflies and owlflies, have been documented too. Roosting bees gets us all pretty excited…because bees.

But what about flies. I couldn’t find many studies on roosting flies, but the few authors who do document this behaviour either assume that it’s normal, or explain that they aren’t quite sure why it’s happening.

I’ve found lots of day-flying insects roosting at night or in the early morning; sometimes on their own, sometimes in groups. Sometimes they are neatly posed next to plant parts that play tricks with the eye (predator avoidance?); sometimes they’re very visible (??).

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Sometimes they die where they roost.

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The blowflies I found on the dill were definitely camouflaged…but were they seeking protection from predators, or just warmth from a microclimate during an unseasonal cold snap? Just another natural history mystery that we might have to wait decades to solve, because we don’t invest enough in understanding insects.

© Manu Saunders 2019