Bees that see at night

Rock bees, also called the Asian giant honey bees (Apis dorsata), have colour vision in mesopic light intensities or twilights, similar to how humans experience colour in similar light conditions, finds a new study. This characteristic, despite them being largely diurnal (active in the daytime), gives the species an advantage over other pollinators. 

This discovery also debunks the earlier belief that nocturnal vision was mostly achromatic and nocturnal pollination was led by smell since most night-blooming flowers are highly fragrant. The more popular “model” European honeybee, A. mellifera, is colour blind in dim lights.  

Most insects including bees are diurnal. An earlier study on a carpenter bee (Xylocopa tranquebarica) has proved that many insects have made evolutionary shifts to nocturnality despite having eye structures that are unsuitable for night vision. 

A.dorsata, like all bees, has apposition compound eyes and a trichromatic colour vision system, which enables them to see in the ultraviolet (UV), blue and green wavelength regions of the electromagnetic spectrum, explained G.S. Balamurali, a researcher at the School of Biology, Indian Institute of Science Education and Research (IISER) Thiruvananthapuram Centre for Research and Education in Ecology and Evolution (ICREEE) and one of the authors of the study. “Many flowers in nature reflect UV, forming ecologically relevant patterns to guide bee behaviour,” he said. 

Apposition compound eyes, characteristic of diurnal insects, allows the entering light to reach the retina of each ommatidium (a cluster of photoreceptor cells surrounded by support cells and pigment cells) as a single spot. A composite of all the spots creates the image. Apposition compound eyes make the vision of these bees less detailed, said Hema Somanathan, professor of biology at IISER, Thiruvananthapuram. Balamurali said that the trichromatic visual system of bees predates the evolution of flowers and the colour diversity in flowers is believed to have evolved to be better detected and discriminated by the bee visual system.

Nocturnality and colour vision in bees could be evolutionary

Nocturnality has evolved in bees multiple times and is believed to be due to certain pressures ranging from predation, competition and resource utilisation. Some bees are seen to tweak their temporal activity window to avoid competition and over exploitation of food resources. Another way they share resources is by leaving their footprint using a gland on their feet to inform other bees that a particular flower has been visited, said Somanathan. The tongue lengths in bees differ and is a morphological trait used to avoid competition. While the species with longer tongues can exploit deeper parts of flowers, those with shorter tongues cannot. 

Bees also exhibit interesting behaviours to avoid predation. One such behaviour, according to Balamurali, is the spectacular defensive display known as shimmering. Shimmering is a collective behaviour in which individual bees coordinate the flipping of their abdomens, creating a traveling wave-like display on the colony’s surface. This collective visual display is used to startle and deter predators.

Typically diurnal, rock bee can see in bright moon lights

The paper states that while A. dorsata has larger eyes than other honeybees such as A. cerana and A. florea, there wasn’t prior information on their physiological and neural adaptation to dim lights and in processing color information in low lights. The researchers said that while the carpenter bees are nocturnal and were known to have the ability to discriminate colour at starlight levels, they expected colour vision in A. dorsata to worsen in dim light given that the species is typically diurnal with nocturnal foraging restricted only to bright moonlit nights and has much smaller eyes than the nocturnal carpenter bees. But much to the surprise of the researchers, A. dorsata’s colour vision in dim light was found to be similar to humans. A. dorsata is, hence the second insect after carpenter bee to have dim light night vision despite having apposition compound eyes. However, they can’t see under starlight conditions on moonless nights, said Somanathan.

Nocturnal colour vision in A. dorsata is good news for biodiversity since rock bees are considered the number one pollinators of multiple plants. However, the role of colour vision, and, most importantly, how the integration of colour and odour affects foraging in nocturnal bee pollinators, remains understudied, said Balamurali. 

Honey bees have an advantage over carpenter bees in pollination since carpenter bees exhibit lower flower constancy than honey bees, which means that they visit multiple species of flowers simultaneously within foraging trips, which reduces pollination efficiency. “Whether A. dorsata exhibits flower constancy and efficiently pollinates plants remains to be explored,” he said, adding that their foraging ecology and pollination services during nocturnal periods need more studies. 

Somanathan said this discovery poses two questions — how do the bees manage to see colour in low light conditions at night and why did they make this transition from day to night activity? While the former is a question about the mechanisms and adaptations involved that is explained through a study of their vision, the latter is an evolutionary question. “Reconstructing past evolutionary events to find an answer is challenging. What we can do instead is to look at contemporary behavioral patterns to see how this bee species is using this particular morphological trait to its advantage — are they using it to avoid competition or predation or what kind of flowers they are exploiting in dim light and how are they locating them,” she said. 

Read more: [Explainer] How are fireflies affected by light pollution?

Banner image: Apis dorsata. Photo by Rison Thumboor/Wikimedia Commons.