Laboratory-reared larvae of the spider crab, H. araneus L., were studied with regard to their fresh weight (FW), dry weight (DW), carbon (C), nitrogen (N), hydrogen (H), and energy content (J; estimated from C). FW remains fairly constant in each larval stage, regardless of feeding or starving conditions. This is due to regular changes in water content as opposed to those in organic constituents. There is a considerable gain (by a factor of 2 to 3) within each of these two instars. In the magalopa also a high amount of C, N, H, and energy is accumulated, but most of this gain is lost again during the last third of its stage duration. In all larval stages, weight-specific energy (J/mg DW) follows rather a cyclic pattern with decreases before and after molts, and increases during intermolt periods. It shows a decreasing trend during larval development. During starvation, biomass declines in an exponential pattern. Larvae of all stages die, when ca. 40 to 60% of their living substance and energy is lost.