Honey Bee Senses

Posted at: July 3, 2002 09:39 AM | Comments (0) | Edit

Vision

The individual light sensors which make up the compound eyes of insects are called "ommatidia". Each eye of the honeybee has 4500 ommatidia, all of which collect light from slightly different directions. From this mosaic of light sensors a complete image is pieced together. 4500 light sensitive units is a relatively small number compared to, say, the number of photoreceptors in the human retina (1.5 million), or elements in a current state-of-the-art digital camera (3 million). Hence the image that the honeybee sees is of very low resolution.

Each ommatidium contains nine light sensitive cells. Two of these are receptive to green light, two to blue light, and two to ultra violet light. Another two are receptive to different colours depending on their position in the eye. If the ommatidium is looking downwards these receptors are sensitive to green light, if the ommatidium is looking upwards they are sensitive to UV. This pattern of sensitivities means that honeybees are blind to red light, but sensitive to UV. Under UV flowers look different to how they do under normal lighting. They take on the appearance of a target with an array of lines pointing towards a dark spot in the center. It is thought that this is the cue which bees use to recognise flowers.

The ninth photoreceptor cell in each ommatidium is sensitive to polarized UV light. The signals from this cell are processed separately to those of normal vision. Polarised UV light is used for navigation when the sun is obscured.

Although a bee's vision is of a much lower resolution than humans, it has a higher flicker-fusion frequency. Bees are able to detect changes in visual stimuli at a rate of over 100Hz. This is in comparison to the humans who can only see at 20Hz. This means that bees can see fast moving objects better than we can, e.g. other bees.

Olfaction:

The antennae, as well as being sensitive to touch, are also sensitive to odors. Covered with odor receptors, they are the bee's "nose". Most of these receptors are of a general sensitivity and can distinguish between different odors. They are particularly sensitive to floral scents. The bee is able to learn and distinguish the scents of hundreds, or maybe even thousands, of different flowers. In addition to the generalized receptors, there are also a few specialized ones. One chemical to which bees are particularly sensitive is oleaic acid. This is the odor given of by a dead bee as it begins to decay. Workers respond to this odor by carrying the dead bee out of the hive.

Pheromones, which are chemical signals, play an important role in communication within the hive. Workers will emit alarm pheromones to alert other bees to the presence of danger. There is also a recruitment pheromone which is released when good source of food, or suitable nesting site is found. Both of these pheromones will attract other workers. The Queen also emits pheromones. These act to suppress the ability of workers to lay eggs.

In addition to olfaction, the honeybee also has a sense of taste. The taste receptors are located on the proboscis which is used to test the suitability of food. The bee is sensitive to the four tastes of sweet, sour, acid and bitter.

Hearing

Surprising as it may seem, bees are able to hear. They don't hear sounds in the same way as mammals however. Instead they are sensitive to a few specific frequencies of vibrations which are transmitted through the air or the physical structure of the hive. They have three sets of organs for detecting these vibrations. One set is located in the legs. These organs listen for messages transmitted through the honeycomb by the Queen. The other two sets of hearing organs are in the antennae. These function like tuning forks and are only sensitive to certain frequencies. The lower part of the antennae are sensitive to about 20 beats per second. This is the frequency associated with the waggle dance which bees use to communicate the location of food. The higher parts are sensitive to between 250 to 300 beats a second. This is the frequency at which that the wing muscles vibrate when the wings are folded. The Queens use this method of communication to "speak" to the hive and control swarming.

Proprioception and Magnetism

Proprioception is the sense used by an animal to feel the position and orientation of its body. Honeybees have very sensitive receptors located in the neck joint which determine which way up the head is pointing. The bees use this information to determine which way is up and to enable them to always fly upright.

Perhaps the most enigmatic of the bee's senses is their ability to read the Earth's magnetic field. Magnetism is used by many animals, including dolphins and pigeons. The honeybee, however, is more sensitive than any other creature known. They use this sense not only for navigation, but also when building the honeycomb panels of a new hive. If a powerful magnet is placed close to a hive which is under construction, a strange looking cylindrical comb will be produced. The shape of this comb will look like nothing ever found in nature.

It is not known exactly how this sensory modality functions. What is known is that a bee's abdomen contains many millions of tiny magnetic crystals. These crystals, which are first formed during the pupal stage of development, are contained within cells called trophocytes. Each trophocyte is connected to the nervous system.