One-two poison: scorpion starts with a cheap shot.

A South African scorpion economizes as it stings, injecting a simple mix first, followed by a venom that's more complicated to produce.

The first droplet from the stinger of the Parabuthus transvaalicus scorpion consists mostly of a strong, toxic solution of potassium, says Bruce Hammock of the University of California, Davis. Only afterward does the scorpion release a cocktail of proteins and some 100 peptides, Hammock, Bora Inceoglu, and their UC-Davis colleagues report in an upcoming Proceedings of the National Academy of Sciences.

All the world's 1,250 scorpions carry some kind of venom, but only 25 to 50 can pose a threat to people. The P. transvaalicus venom can kill a person. Hammock says he started analyzing this toxin in hopes of furthering the development of antivenins.

Scientists had noted that during a sting, scorpions expel a clear liquid followed by a cloudy one. Because of laboratory technologies that can now cope with tiny volumes, Hammock and his colleagues could chemically analyze sting droplets, he says.

Inceoglu collected venom--carefully--by permitting scorpions to sting vials covered with a film. An attacking P. transvaalicus typically released 1.2 microliters of a clear liquid that the researchers call a prevenom. Hammock recalls that at first his team was vexed because something in this droplet interfered with attempts to analyze the prevenom's proteins. Then the researchers realized that this interference was actually the message, a toxic concentration of potassium ions that was 16 times as high as in the venom released later. From their studies of cells in laboratory dishes, the researchers suggest that a protein component of the prevenom jams the mechanism that would normally counter a flood of potassium across a sting victim's cell membranes.

When the researchers looked at the later venom, they found far less potassium but six times as much protein.

Standard tests proved the prevenom toxic to both insects and mammals, two major targets of scorpions. The prevenom causes more pain to mammals than the later venom does, the researchers report. Both the prevenom and venom paralyze insects, but the venom is five times as toxic to mammals as the prevenom is. Therefore, the prevenom could quickly subdue insect prey or tell an attacking mammal to back off.

Another scorpion specialist, Philip Brownell of Oregon State University in Corvallis, says that he's not too surprised that the scorpions release a prevenom. Most secretory tissues in animals produce a series of products. "What's newsy," he says, "is that there are very distinctive and reasonable functions for the two fluids in the venom."