Abstract 1. 1. Published data on haemolymph were examined for patterns in its interspecific variation. 2. 2. Na and Cl dominate osmolarity in scorpions and some spiders, but in other spiders half the osmolarity is due to organic solutes. 3. 3. In the latter spiders the concentrations of most measured organic and inorganic solutes are strongly intercorrelated. 4. 4. In spiders and scorpions the concentration of Ca representative of each species seems to depend on Na, K and Mg.