sea water and hemolymph osmolality were measured whit a Wescol Model 1500 vapor pressure osmometer. approximately 100 ul of hemolymph was sampled by syringe though the arthrodail membrane at the base of a walking leg and quickly expelled onto a piece of plastic film. osmotic pressure was measured in an 8 ul of hemolymph placed on a filter paper disc in the osmometer. the total time that elapsed between expelling the osmometer was approximately 5 s.