It's not easy being a blood-sucking insect – you've got to find a host and quickly drink its blood before the unwilling victim realises and retaliates. With these challenges, it's easy to understand why bloodthirsty insects choose to snack on some hosts and not others, choosing less defensive prey for mealtimes. How they learn to discriminate remains unknown and so Claudio Lazzari, from the Université François-Rabelais, France, decided to investigate learning in the bloodsucker Rhodnius prolixus (p. 892).

Learning and memory have been well characterised in insects with more appetising diets, such as honeybees, but we know little about the cognitive abilities of blood-sucking critters. Studies investigating learning in non-blood-sucking insects take advantage of the proboscis extension response – a characteristic behaviour where the proboscis (the insect's food-sucking tube) extends when the insect is presented with something tasty. Lazzari realised that he could also use this same behaviour in R. prolixus to study learning, explaining that: ‘We know that they have this very well-characterised response to heat, and we can use this stimulus to train them to associate this information [with specific consequences]’.

To begin the study, two of his students, Clément Vinauger and Hélène Lallement, set out to first establish whether R. prolixus has the ability to learn a simple task: not to extend their proboscis when tempted. They starved the insects for 2 weeks to make sure that they were famished before placing them in front of a heated plate. Then they ramped the temperature up to 35°C for 10 s, hoping to fool the insect into thinking that it was close to a warm-blooded animal. The cooperative bloodsucker extended its proboscis, only to be disappointed when it touched the plate instead of the tasty treat it had expected. After allowing the dejected insects to recover, the team tested them again repeatedly and after 26 training sessions the insects finally began to realise that they'd been duped. And when the team dropped the hotplate's temperature to 30°C, to check the insects weren't just getting tired, most of the bugs started responding again. So, the insects were capable of learning not to extend their proboscis.

Next, the team tested how the insects learnt to react to an unpleasant stimulus. They punished the insects after they had extended their proboscis, by whacking the temperature up to 50°C. The bugs learnt very rapidly, and after five trials half of the insects had already stopped extending their proboscis altogether. The team then asked whether the educated insects could remember their lesson. After an hour's break the insects were again placed in front of the heating element and this time they stopped responding to the warm plate after just three trials. They obviously remembered their previous lesson. In addition, by increasing the time between experiments, the team were able to establish that this memory persisted for at least 72 h.

‘We didn't expect that they would be able to learn so quickly, or that they would remember it for so long – that was a great discovery’, recalls Vinauger. But perhaps the most important aspect of the study, says Lazzari, is that it provides experimental tools to further investigate this bugs' cognitive ability and ask what other things affects the insect's host choices and memory. As R. prolixus transmits the Chagas disease-causing parasite, how it chooses and remembers hosts has important consequences for how the disease spreads.

C. R.
Learning and memory in Rhodnius prolixus: habituation and aversive operant condition of the proboscis extension response (PER)
J. Exp. Biol.