Walking by Ixodes ricinus ticks: intrinsic and extrinsic factors determine the attraction of moisture or host odour.

Spontaneous walking by Ixodes ricinus nymphs in the horizontal plane was investigated within choice arenas to test whether these ticks move preferentially up a humidity gradient, and whether they are attracted towards host odour, specifically volatile substances from the back of a dog's ear. Responses by ticks with high and low fat contents and in different states of hydration were compared. Ticks with higher energy reserves were more likely to walk horizontally over short distances. Only if they were slightly dehydrated were they more likely to walk towards fully saturated air than drier air, and only if the atmosphere was sufficiently wet were they likely to walk towards odour secreted by host skin. It seems that, under certain circumstances, ticks of this ambushing species will move short distances towards host odour.