Readers’ wildlife photos

Today we have another photo-and-text biology story from Athayde Tonhasca Júnior. Today’s subject is insect sleep and nocturnal behavior.  Athayde’s narrative is indented, and you should click on the photos to enlarge them.

When the night has come and the land is dark

We humans spend between 25 to 35% of our lifespan sleeping. It’s a significant chunk of time in the land of Nod, so sleep must be vital. But surprisingly, doctors and scientists are not sure why we sleep so much – or at all, for that matter. A period of unconsciousness may be required so that our body can repair itself and restore some physiological or chemical functions; it could help us conserve energy, especially at night, when our ancestral search for food was less efficient.

Two girls on the stove bench. Art by Albert Anker, 1895. Wikimedia Commons:

Whatever the reason, we need our resting. And so do other animals, including insects. In the case of bees and wasps, females have the obvious option of a cosy nest to tuck into for the night. But males do not: they are not welcome back once they mature and leave the nest, so they are destined for a vagabond life. When the night comes and the temperature drops, they have to rough it by entering a state of torpor until the morning sun heats them up again.

Flowers are the bedding choice for males of many species: on a chilly morning, you may find sluggish bees curled up among petals. These sleeping arrangements offer some protection against roaming predators, are convenient spots for meeting females during daytime, and also make males of some species exceptional pollinators.

Night, night, sleep tight © Todd Esque, U.S. Geological Survey:

But protection doesn’t seem to be crucial, since many bees just sleep in the open. If you visit a garden during early morning or early evening, you may spot bumble bees slumbering on top of a flower, or even hanging upside-down from a blossom or under a leaf. Most of them are males, but the odd female may join them if she’s caught outside when temperature drops quickly at sunset. Watch a sleeping bee clinging to a plant and refusing to be woken up.

Sleeping bumble bees clinging to a Welsh onion (Allium fistulosum) inflorescence:

Some bees and wasps seem to deliberately make themselves uncomfortable for the night. They hang from the end of a twig, a dried flower head or a thin stem, clamped on with their mandibles. We don’t know why they sleep like this; perhaps to make them less visible to predators, who may mistake them for plant extensions.

A Nomada sp. cuckoo bee sleeping anchored by its mandibles © Giles Gonthier, Wikimedia Commons:

Male bees are not much into socialising: they tend to avoid each other, when not being downright aggressive. But after sunset, males of some species spend the night close together, forming clusters of bees clinging from branches or flowers. Not only that, these males tend to return again and again to the same spot every evening for their sleep. Gregarious sleeping seems like a dangerous life choice: such concentrations of lethargic bees must tempt many a predator. One possible explanation for this behaviour is the dilution effect: the larger the group of prey, the smaller the chance of any individual being the victim of a raider. Some fish, penguins and mammals that live in the open resort to this type of defence.

A long-horned bee (Melissodes sp.) slumber party © Todd Sonfileth, Oregon Public Broadcasting:

We don’t have much information about the consequences of sleeping in the open for male bees. But when you tuck in tonight, remember that a peaceful, uneventful kip is not a given for everybody.

When the night comes to an end in a Central American forest, its nocturnal denizens – bats, owls, rodents and cats – begin to retreat to their shelters. At this time, before the crack of dawn, light levels are up to 100 million times dimmer than during the day, so it’s still too dark for the day-shift inhabitants.

A tropical forest understory, which is never bright even in daytime © Bruno Henning, Wikimedia Commons:

But one creature is already busily going about in the dense tangle of vegetation: the halictid (aka sweat bee) Megalopta genalis is flying from flower to flower collecting pollen and nectar, then navigating safely back to its nest.  This is one of the 250 bee species, about 1% of the total bee fauna, known to be nocturnal (active at night) or crepuscular (active during twilight, dusk or dawn). This is a surprisingly large number, considering that bees are essentially adapted for bright sunlight.

A Megalopta genalis from Panama © USGS Bee Inventory and Monitoring Lab, Wikimedia Commons:

Bees, like other insects and crustaceans, have compound eyes, which comprise thousands of independent light receptors known as ommatidia (sing. ommatidium). Images are formed by combining the input from the ommatidia, which are oriented in different directions – up, down, sideways and forwards. As a result of this configuration, image resolution is not very good. When a bumble bee hovers near you, it probably was attracted by the colour of your hat or your scent: it just wants to check whether you are a giant flower. Insects may not see clearly, but many of them have a large angle of view – a feature that makes flies and dragonflies so hard to catch. Some insects also see polarised light, which we can’t.

Drawing of the compound eye of a drone fly from Robert Hooke’s Micrographia, 1664:

 

a: The single-chamber camera eye of humans: spatial resolution is achieved through the lens and a concave retina. b: An insect compound eye: spatial resolution is achieved through a series of small lenses and a convex retina. Striped areas indicate regions of photoreception © Buschbeck & Friedrich, 2008

 

In addition to their compound eyes, bees and most other insects have simple eyes (ocelli). These are single lens organs to detect movement and light, and they help the insect navigate during flight. Daytime bees find their way by identifying landmarks around their nests and along foraging routes; directions and distances are also determined visually, with the help of polarised light.

Compound eyes and ocelli of the orchid bee Euglossa hansoni © Insects Unlocked, Wikimedia Commons.

But how does Megalopta genalis cope with very low light? We know that nocturnal bees can react to faint movements, see polarised light and orientate from landmarks just like diurnal bees, but how they do these things is not completely clear. Night-flying bees have larger compound eyes and ocelli when compared to diurnal bees, but there must be other factors at play such as neurological adaptations; eyes alone don’t explain their visual performance.

The ability to fly at low light evolved independently in bees from the families Andrenidae, Apidae, Colletidae and Halictidae, so there must be selective advantages to being nocturnal or crepuscular. They may be less vulnerable to parasites and predators, or there could be less competition for food. The night fliers may be responding to food availability: many plants bloom only at dawn or at night to save water, while others accumulate nectar through the night, offering rich rewards to early risers.

If you are wondering whether these night-time comings and goings have any consequences for plants, the answer is yes. Nocturnal bees pollinate a range of plants, many of economic importance. Several of these bees are good at buzz pollination, so they may be particularly valuable for plants with poricidal anthers such as Solanum spp. (e.g., aubergines, peppers and tomatoes). But we know little about nocturnal bees: collecting data on pollination ecology is hard enough during day-time, and it becomes a real challenge when the lights are out.

Night-flying bees are representatives of a vast number of nocturnal flower visitors such as moths, whose role as pollinators are becoming increasingly recognised. We need more night owl ecologists to help us understand better their role in our ecosystems.

Nocturnal bees that visit flowers of Campomanesia phaea, whose fruits are eaten raw or prepared into jellies, sherbets or juices. A: Ptiloglossa latecalcarata; B: Ptiloglossa sp.; C: Megalopta sodalis; D: Megommation insigne © Liporoni et al., 2020: