1. After controlling for phylogenetic correlation, clutch size in spid
ers (suborder Araneomorphae) increases with body mass. 2. Clutch mass
scales isometrically with adult female body mass while individual egg
mass scales with negative allometry. Clutch size therefore is a result
of scaling constraints on egg size and total clutch mass. 3. There is
no empirical evidence for a size-number trade-off in allocation of re
sources within clutches. Theory predicts that spiders are selected to
maximize clutch size, with no cost of decreased egg size. Spiders are
constrained from achieving an optimal clutch size.