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TICK'S AFTER DARK ANTICS
kathryn{at}biologists.com
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For a small creature, ticks have a big reputation. Epidemiologists like Lise Gern know that their tiny bites are far from harmless, infecting innocent victims with one of several unpleasant diseases. However, Patrick Guerin is fascinated by the infamous arthropod's metabolic budgeting. Ticks only feed three times in the course of their lives. They can live for years in the laboratory on just a few drops of blood, but they must use their energy wisely. Virtually nothing was known about Ixodes ricinus' daily movements and the compromises they make to get the most from their three meals. Working with Jean-Luc Perret in Neuchâtel Switzerland, he simulated the tick's environment in the lab while recording their movements, and discovered that ticks naturally limit their activities until after dark (p.1809). And when they took a closer look at the tiny animals, they discovered that ticks have simple light receptor cells, which help coordinate their nocturnal movements.
After waiting to ambush a passing victim, perched on the top of a plant in the desiccating sun and wind, ticks need to rehydrate. But they can't drink from pools of water trapped in the undergrowth, in case they pick up a fatal infection. Instead, a tick descends from its dry plant-top perch and returns to the humid plant litter on the ground to rehydrate by water sorption from the air; but at an energetic price. Guerin and Perret wondered whether the arthropod had any strategies for minimising the physical impact of its search for water.
Perret went clambering through the forest, collecting the waiting creatures by dragged a towel through the undergrowth. Back in the lab he provided each tick with a vertical plastic tube, plugged at the bottom with damp cotton, so that they could perch at the top as if hunting for victims, and descend to the cotton plug when they needed to rehydrate. But if Perret were going to capture the tick's spontaneous behaviour, he would have to record their activity as he simulated day and night, while making sure that there were no tasty scientists around to distract the ticks. He adapted a webcam to detect infrared radiation and recorded the tick's antics as they roamed undisturbed for ten days.
Amazingly, the tiny arthropods only moved after dark. Perret explains that insects lose water through their spiracles when they are physically active and respiring. By restricting their movements to after dark, ticks could limit their water losses by moving when there are `less severe desiccating conditions in the field'.
But how did the ticks know when it was dark? Although some ticks have simple eye structures, I. ricinus didn't. He teamed up with Peter-Allan Diehl and Michèle Vlimant to take a closer look at the tiny arthropods, and discovered that they had two sets of light sensitive photoreceptors arranged down both sides. And the nerves from each light receptor connected directly to the tick's neural centre, the synganglion. Guerin adds that although it isn't clear whether the tick's newly discovered visual system contributes to its hunting tactics, it certainly helps it to decide when to conserve energy by only moving after dark. Which could prove to be vital intelligence in the epidemiologist's battle to protect us from the tiny arthropod's appetite for blood.
References
Perret, J.-L., Guerin, P. M., Diehl, P.-A., Vlimant, M. and
Gern, L. (2003). Darkness induces mobility, and saturation
deficit limits questing duration, in the tick Ixodes ricinus. J.
Exp. Biol. 206,1809
-1815.
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