The mud dauber wasp Sceliphron laetum (F. Smith) lays a single egg in a mud chamber that is provisioned almost exclusively with orb-weaving spiders. In Madang, Papua New Guinea, the wasps provision their chambers with between three and nine spiders that weigh between 0.01 and 0.28 g and are from at least twelve species. The number of spiders placed in each chamber is negatively correlated with the mean mass of each spider. A field experiment revealed that females cease provisioning after capturing a certain mass of spiders, rather than simply filling each chamber to its volumetric capacity. Furthermore, the wasps select different spider species according to the provisioning sequence. In general, wasps avoid provisioning the early larval instar with species of Gasteracantha, perhaps because the newly emerged wasp larvae cannot penetrate the hard integuments of these spiders.