The venom optimisation hypothesis: a spider injects large venom quantities only into difficult prey types.

The spider Cupiennius salei needs 0.01-10 microl venom to kill a prey item. Since its venom glands contain only 10 microl and regeneration requires 8-16 days C. salei should use its venom very economically. By a monoclonal antibody we measured, for the first time, the amounts of venom injected by a spider into different prey types. Crickets and stick insects, as victims without special defence mechanism, received only the minimum amount of venom which is not significantly different from the LD(50). Blowflies and ground beetles received considerably more venom because they are difficult to overwhelm or even endanger the spider by their defence behaviour. These results support our venom optimisation hypothesis which supposes that spiders use their venom as economically as possible.