How do bumblebees fly?
If you spend some time outside in the UK during the summer months, you will inevitably notice bumblebees diligently flitting between flowers to collect nectar, before heading home to their hives or nests. A bumblebee’s wings seem awfully small for their large body, particularly when compared to a bird, or even to other insects. From appearances alone, you may wonder how a bumblebee is able to fly at all! So how do they manage to generate enough force to lift their body and fly?
It's all about wing speed
The key is the speed at which bees (and other insects, such as flies) are able to move their wings. Bees are able to beat their wings extremely fast – around 200 times a second! This allows their wings to move the same amount of air as a pair of larger, slowly beating wings, like those of birds and bats.
An extra benefit to this speed is that, combined with the small size of insects, the air effectively feels ‘thicker’ to an insect wing. Consequently, when the wings beat, they create complex patterns of vortices around the wing. This amplifies the amount of force the wing creates when it moves through the air. This rapid wing movement and resulting aerodynamic trickery explains why insects with such small wings can fly so well.
How do they beat their wings so fast?
The next logical question to ask is how do bees manage to move their wings so quickly in the first place? To understand this, we need to take a look at the how their muscles are laid out in the thorax.
Crucially, unlike birds and bats, bees (and flies) have no muscles that attach directly to the wings (other insects, like dragonflies, do have muscles attached to the wings, but that’s a story for another blog post!). The main wing movement is driven by two large pairs of muscles in the thorax. Figure 1 shows these muscles: the DVMs (dorsoventral muscles) run from the top to the bottom of the thorax and the DLM (the dorsal-longitudinal muscle) runs from the front to the back of the thorax. When the DVMs contract, the whole body gets squeezed top-to-bottom, and this causes the wings to flap upwards. In contrast, when the DLM contracts, the body gets squeezed front-to-back, and the wings flap downwards. This is shown in figure 2 below. By contracting at different times, these muscles make the wings flap.
A key feature of these muscles is that they show something called stretch activation. This means that the muscles become active shortly after they are stretched to a certain degree. The DVMs and DLM are antagonistic muscles, meaning that they act in opposite directions, so that when one contracts, the other is stretched – like the triceps and biceps muscles in your arm. So, the fact that the DVMs and DLM are antagonistic and show stretch activation means that when one contracts and moves the wings, the other is stretched and becomes active and so starts to contract itself, moving the wings in the opposite direction, which stretches the other muscles again so that they become active and contract, and so forth. This sequence of events occurs without the need for the brain to send messages through nerves to the muscles, which is slow when compared to the muscles being able to act autonomously; and it is this that enables bees and flies to beat their wings so rapidly.
How does this compare to other flying animals?
Birds and bats share the same body architecture as us, as shown in figure 3. To flap their wings, the chest muscles pull the wings forward while the shoulder muscles pull the wings backwards. While these muscles are antagonistic, they do not show stretch activation and so this muscular activity must to be dictated by the nervous system which, as previously explained, is much slower. Bird and bat wings also require more force to move when compared to bumblebee wings as they are much heavier and generate more drag. The lack of stretch activation and the forces required for movement explains why these animals can’t beat their wings nearly as fast as insects do.
Insects are a remarkably successful group partly because of this biomechanical wizardry that allows them to move their wings so fast, and thereby generate enough force to lift large bodies with comparably small wings. It allows them to be incredibly manoeuvrable, and also fly for hours at a time on very limited resources. So, it’s precisely this ability that makes insects so fascinating for us at Animal Dynamics, and such a useful source of inspiration when it comes to designing our vehicles.
Written by: Jonny Page
Edited by: Zoe Griffiths