Associative learning and the proboscis extension reflex in honey bees

Some of the most interesting behaviour patterns amongst insects are found in the social hymenoptera, bees, wasps and ants. These animals are capable of memorising landmarks, which they use for navigation (Dyer, 1996; Judd & Collett, 1998), and they have elaborate systems for communication amongst individuals, such as the waggle dance of honey bees (Lindauer, 1967). The disposition of bees to learn to associate colours and shapes of flowers with a good source of food was recognised by one of the earliest experimental ethologists, Karl von Frisch, and he exploited this to investigate several features of the sensory capabilities of bees. Bees remain a favoured subject for behavioural experiments because they are quite easy to obtain and to train in laboratory conditions. It is also possible to make electrophysiological recordings from single neurons in a bee's brain during the performance of some simple behaviour patterns in order to reveal circuits that are important in learned behaviours.

One of the simplest behaviours that a honey bee (Apis mellifera) produces is to extend its proboscis, or tongue, in response to a drop of sucrose solution applied to chemosensory hairs on the proboscis or antenna (Fig. 9.3a). A bee will occasionally extend its proboscis without any obvious stimulation, or if a tiny puff of a particular odour such as the smell of carnation or orange is blown at an antenna. If the bee has recently tasted sucrose, the likelihood that it will extend its proboscis in response to a subsequent odour puff increases - stimulation with sucrose is said to sensitise the proboscis-extension response. A much greater enhancement of the response to a puff of odour occurs, however, after the odour puff has been paired with delivery of a drop of sucrose. For maximum effect, the odour puff must be delivered between 1 and 3 s before the drop of sucrose (Bitterman et al., 1983). The next time that the odour is directed at an antenna, there is a very high chance that the bee will extend its proboscis (Fig. 9.3b). In the bee's brain, an association has been made between the odour and the likely presence of sucrose. The order in which the two

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Figure 9.3 Conditioning of the proboscis extension reflex in honey bees (Apis) to odours. (a) A diagram to illustrate the conditioning procedure. A puff of air laden with a chosen odour is blown across an antenna, and sucrose delivered to the proboscis when it is extended acts as the reward. (b) A graph to show the acquisition of the conditioned response. Groups of bees were trained by pairing a puff of odour with a sucrose reward 2 s later. After this pairing procedure, about 80 per cent of bees tested responded to the odour with proboscis extension. In comparison, few untrained bees responded to the odour. (b redrawn after Menzel, 1990.)

Figure 9.3 Conditioning of the proboscis extension reflex in honey bees (Apis) to odours. (a) A diagram to illustrate the conditioning procedure. A puff of air laden with a chosen odour is blown across an antenna, and sucrose delivered to the proboscis when it is extended acts as the reward. (b) A graph to show the acquisition of the conditioned response. Groups of bees were trained by pairing a puff of odour with a sucrose reward 2 s later. After this pairing procedure, about 80 per cent of bees tested responded to the odour with proboscis extension. In comparison, few untrained bees responded to the odour. (b redrawn after Menzel, 1990.)

different stimuli are delivered is vital for the formation of memory. If the sucrose is applied to the proboscis just before the odour is blown over an antenna, a second puff of the odour delivered a minute later is unlikely to cause proboscis extension.

The proboscis-extension reflex is a good example of conditioning, a type of behavioural change often studied in vertebrates. The odour stimulus becomes conditioned so that after it has been paired with a sucrose reward, it reliably evokes a stimulus that was previously only rarely linked with this stimulus. A single pairing of a puff of carnation with a taste of sucrose is sufficient to enhance the new coupling between carnation and proboscis extension for many hours. However, the coupling will weaken and become extinguished if several odour puffs are delivered with no sucrose as a reward, and can be replaced by a new association between another odour, such as orange, and proboscis extension. The memory of the association between an odour and sucrose goes through different phases in time, so that the initial short-term memory is transferred into longer-lasting medium-term and long-term memories (Hammer & Menzel, 1995).

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