Fig. 2.

Three stages in the evolution of division of labour in trematode colonies. The total number of rediae (flattened ellipses) per colony and their total biomass, represented by their combined surface area, remain unchanged. Similarly, the size of the competing colony of sporocysts of a different species (red clump) also remains constant. A redia’s potential production of cercariae (black shapes) is indicated by the shading, with darker greenish shades representing low reproductive potential but high defensive ability, and vice versa. a No functional or morphological differentiation among rediae. b Some rediae compromise their cercarial production to invest more in attacks against the competitor, whereas others have limited their defensive duties to focus on growth and cercarial output. c Full separation of functions between specialised soldiers and specialised reproductives. Note that colony success increases by 50%, from a total of 12 cercariae per unit time (a) to 18 cercariae (c)