On the face of it, parasites use a variety of similar strategies. Some make cat urine suicidally attractive to mice, which are promptly eaten so that the parasites can go through the next phase of their life cycle in the cat. Others prompt ants to expose themselves on high tree branches, the better to be eaten by birds. And still others cause snails to hang out in open spaces, with swollen eyestalks pulsing like neon signs, for apparently the same reason.

At the level of biochemistry, it can be difficult to determine which changes in a host result from its immune system trying to fight off the parasite, and which are the parasite bringing the host under its sway. Mr. Herbison and his colleagues suggested that if researchers could identify similar kinds of molecular changes in different hosts with different parasites, they could possibly see the strings the parasite is pulling.

Using 12 earwigs and 12 sandhoppers, the team investigated how infected insects differed from the uninfected. They took an inventory of the proteins being manufactured by the earwigs and sandhoppers, noting which were being made in greater numbers and which had declined.

They found that while very few proteins were changed in the same way in infected individuals of both species, the jobs the proteins had were often similar. For instance, 23 percent of the proteins that changed in sandhoppers were involved in energy regulation and metabolism, and 39 percent in earwigs. This fits with the higher levels of activity observed in infected insects.