Flowers and bees are a perfect match. What tools do they use to collect nectar?

A bee lands on a flower to collect nectar (Photo/Wei Jiangkun, Wu Jianing)

When you think of bees, what comes to mind? The buzzing sound and sweet honey?

So what “tools” do bees use to collect honey?

The bee's proboscis consists of a pair of outer mandibles and a pair of labial whiskers, forming a tubular structure, which contains a clever middle labial tongue. The middle labial tongue has thousands of bristles on it, which can be quickly ejected or retracted in the proboscis tube to dip into sticky nectar.

Bee mouthparts (Photo/Wei Jiangkun)

During a bee's lifetime, this tiny structure needs to retract and contract millions of times as it explores flowers for nectar, a liquid food full of energy and nutrients that is either stored in tiny tubular corollas or left open to the air.

The nectar of different plants has different sugar content and viscosity. The sweetest and most energy-rich nectar is often the most viscous and difficult for insects to collect. Bees can easily dip into high-viscosity nectar with their hairy mid-lip tongue. At the same time, they can also actively change the control method of their mouthparts, using their beaks to directly suck nectar deep in the corolla, which is more efficient for transporting low-viscosity distant liquids.

The specialized mouthparts and feeding strategies of bees have gradually matured through hundreds of millions of years of co-evolution with plants. Recently, a study published in PNAS revealed the secrets of how bees collect nectar.

Flowers of different shapes have different ways of collecting nectar. Image source: wikipedia

Pollinators and angiosperms: I give you nectar and help me pollinate!

Plants secrete nectar for their own reproductive needs.

For older plants (such as most ferns and gymnosperms), their reproduction mainly depends on wind. During the breeding season, the spores of ferns and the pollen of gymnosperms will be dispersed in the air and blown away by the wind.

Pine trees releasing pollen into the air Image source: Wikipedia

But angiosperms (flowering plants) have "developed" a more diverse way of pollination - through animals.

The nectaries of plant flowers can secrete nectar rich in energy and nutrients, thereby attracting various animals such as bees, butterflies, moths, beetles, flies, bats, etc. to come close to feed (flies are also pollinating insects). In the process of these animals taking nectar, they will come into contact with pollen attached to the plants, and spread to other flowers in subsequent movements, thus completing pollination. In order to obtain the most efficient pollination service, various species of angiosperms have evolved different flower structures and nectar with different physical and chemical properties over hundreds of millions of years.

For nectar-eating insects, nectar is a very valuable source of energy and nutrition, and is often fought over by different types of pollinators in nature. In order to improve the efficiency of nectar collection and avoid competition, the mouthparts of nectar-eating insects have also developed various specializations, allowing them to collect nectar from the corolla in different ways. For example, the mouthparts of butterflies are like long and thin straws, the mouthparts of nectar-eating ants are like short lips, and the mouthparts of some nectar-eating beetles are like hairy brushes.

Different nectar-eating insects have different mouthparts (Image source: Wikipedia)

The mystery of the bee-flower relationship

Pollinators are not only the key to the reproduction of angiosperms, but also play a vital role in our human life. One-third of the fruits and vegetables we eat rely on the pollination services of pollinating insects, and bees are the world's most important pollination service, with extremely important ecological and economic value.

Therefore, the widely domesticated western honey bee (Apis mellifera) in the family Apidae has become one of the most studied animals by humans besides humans, mice and fruit flies. Its interactive behavior and ecological relationship with plants have been extensively studied, for example, the relationship between the length of the bee's proboscis and the depth of the corolla corresponding to nectar ingestion.

The Western honey bee is currently the most widely raised bee by humans. Image source: Wikipedia

In addition to comparisons on length scales, researchers have begun to focus on the exquisite structure and dexterous manipulation of bee mouthparts in recent years. They have conducted in-depth research on the dynamic process and physical mechanism of the microscopic structure of bee mouthparts when collecting sticky nectar, and used this to explain the relationship between the specialized morphology of the mouthparts and the viscosity of nectar.

The microscopic process of bees collecting liquid food with their mouthparts (Photo/Wei Jiangkun)

In the bee-flower system, the nectar collection process of bees is affected by two factors: the viscosity of nectar and its depth in the corolla. However, previous studies have often discussed the two separately and have not considered them as a system. Recently, Wu Jianing's research group at Sun Yat-sen University carefully observed the microscopic dynamic process of bee mouthparts ingesting nectar deep in the corolla, and revealed its movement and behavior mechanism from a mechanical perspective.

How do bees collect honey? There are several techniques!

Research has found that bees have more than one way to manipulate their mouthparts.

When it ingests low-viscosity nectar (sugar concentration 10%), it tends to extend its beak to directly suck the nectar;
When taking high-viscosity nectar (sugar concentration 50%), they tend to use their hairy middle lips and tongue to quickly and continuously pop out and retract to dip and lick the nectar;

When bees ingest medium-viscosity nectar inside the corolla (sugar concentration of 30%), they lick the nectar quickly at first. As the nectar surface gradually moves away from the mouthparts, the bees significantly reduce the frequency of tongue popping out and retracting, and eventually switch to stably extending the mouthparts to directly suck the nectar.

The movement pattern of the bee's lips and tongue changes with the viscosity and depth of the nectar (Photo/Wei Jiangkun)

Because the internal structure of the bee's mouthparts is extremely complex and the driving muscles and structures of these two feeding methods are different, this behavioral specificity may be the result of natural optimization.

Through experimental and theoretical modeling analysis, the authors explained the two different viscous microfluidic collection methods involved in this process: sinus pumping and sucking, and licking with the middle lip and tongue (including the coordinated expansion of bristles and capillary-driven transport processes), and confirmed that the adjustment strategy of the bee's feeding behavior can ensure its feeding efficiency for nectar of different distances and viscosities. The authors did not find the coexistence of the two feeding methods in bumblebees (another genus under the family Apidae), and there is no record of multi-mode feeding methods in existing literature for other bees, or even other pollinating insects.

Sucking and licking have different efficiencies in different situations (Photo/Wei Jiangkun)

How can we make better use of the advantages of bees in collecting honey?

The unique liquid feeding method of bees adjusts their behavior, giving them greater ecological adaptability in complex environments and potentially making them the most efficient pollinators in the world.

This research allows us to understand the dynamic interactions between plants and pollinators at the microscopic level and deepens our understanding of the close connection and mutual influence between animals and plants.

In the future, researchers will continue to explore the way bees collect nectar in nature and try to design bionic viscous microfluidic collectors that can be used to collect and detect biological fluid samples or environmental fluid samples.

References:

[1].www.pnas.org/doi/10.1073/pnas.2305436120

[2].https://www.science.org/content/article/watch-honey-bee-tongues-act-either-spoons-straws-depending-flower

Author: Wei Jiangkun and Wu Jianing

Author's unit: Sun Yat-sen University

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