Can bumblebees use optic flow to measure distance?

Abstract

Many animals use visual signals to navigate through their environment, and among the top navigators in the animal world we find insects. Even though their brains are incredibly small, insects are able to find their way using, for example, a strategy known as path integration. By updating the distance and direction travelled the insect is able to find the way back to its point of origin along a straight line, however winding the outbound path. Many species of insects possess a celestial compass using information such as the sun's position or the polarised light pattern surrounding the sun to estimate direction. However, how insects measure distance is only known from very few species, namely honeybees and desert ants. In this thesis I investigate whether bumblebees use translational optic flow, the image motion created on the insect retina as it moves, to measure distance during path integration. Bumblebees were trained to find a food source at a fixed distance inside a flight tunnel lined with a pattern generating optic flow.
In experiment one, I showed that bumblebees can find a target location in an environment where optic flow is the only available reference. In experiment two, I let the bees fly in a wider tunnel lined with the same optic flow pattern. Since the perceived optic flow rate depends on the distance from which the environment is viewed, the wider tunnel will create a slower optic flow rate in the bumblebees eye, and I predicted that the bees would perform a search behaviour further from the tunnel entrance than the position they were trained to. In this experiment, I show that bumblebees appear to change their behaviour in a qualitative way when flying in the wide tunnel. The bumblebees perform both less and shorter searches in the wide tunnel, indicating that they experience a difference between the narrow and wide tunnel. This behaviour has to my knowledge not been demonstrated in similar studies on other hymenopterans.
In experiment three I investigate the influence of a landmark on bumblebee search position. When a landmark is placed far away from the target area it is ignored by the bumblebees, but when the landmark is placed within the expected search range the landmark is taken into account. These findings are in line with what has previously been demonstrated in other species and is a first step towards unraveling how bumblebees measure distance.

Popular Abstract

How do bumblebees measure distance?

Many animals use visual signals to navigate through their environment, and among the top navigators in the animal world we find insects. Even though their brains are incredibly small, insects are able to find their way using, for example, a strategy known as path integration. By updating the distance and direction travelled the insect is able to find its way back to its point of origin along a straight line, however winding the outbound path. Many species of insects possess a celestial compass using information such as the sun's position or the polarised light pattern surrounding the sun to estimate direction. However, how insects measure distance is only known from very few species, namely desert ants and honeybees. Ants have the ability to measure distance by counting steps, while honeybees have been shown to use optic flow to measure distance. Imagine looking out a train window, the vegetation close to the tracks will appear to move passed the window at a much higher speed than the mountains in the far distance. This phenomenon is called optic flow and can be used by insects to accomplish several tasks. Bumblebees are able to use optic flow both to alter height, speed and to center between objects. But are they also able to use optic flow to measure distance?

My experiments are a first step towards understanding how bumblebees measure distance, and as a first step i showed that bumblebees are able to locate a known position in an environment where the only visual cue present is only optic flow. My second question was whether the bumblebees were in fact using optic flow to measure distance. In an experiment trained bees to find a feeder inside a flight tunnel lined with a pattern which generates optic flow. I then let the bees fly in a wider tunnel, which generated less optic flow than the training tunnel because of the distance between the walls. I expected the bees to target a position further into the tunnel than the position they were trained to. In this experiment, I show that bumblebees appear to change their behaviour in a qualitative way when flying in the wide tunnel. The bumblebees perform both less and shorter searches in the wide tunnel, indicating that they experience a difference between the narrow and wide tunnel. This behaviour has to my knowledge not been demonstrated in similar studies on other insects.

In experiment three I investigate the influence of a landmark on bumblebee search position. When a landmark is placed far away from the target area it is ignored by the bumblebees, but when the landmark is placed within the expected search range the landmark is taken into account. These findings are in line with what has previously been seen in other species and is a first step towards unraveling how bumblebees measure distance.

My research concludes that bumblebees are able to find a target position in an environment where the only visual cue available is optic flow. My experiments also conclude that bumblebees ignore landmarks when they are positioned outside their main search range. It is the first study to my knowledge demonstrating how bumblebees change their search behaviour in an environment where they do not expect to find food. Further research is needed to conclude whether bumblebees can use optic flow to measure distance.



Masters degree project in Biology (60cr), 2020-2021

Department of Biology, Lund University

Supervisor: Stanley Heinze, Lund vision group - Department of Biology, Lund University

• author: O’Reilly, Lina
• supervisor: Stanley Heinze
• organization: Degree Projects in Biology
• course: BION03 20192
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9041663
• date added to LUP: 2021-03-11 11:15:46
• date last changed: 2021-03-11 11:15:46