Immersion of larvae in various external salinities indicates a sound ability to maintain the haemolymph ionic balance. Removal'of the pars intercerebralis region of the brain and ligature at the neck region do not provide any indication of involvement of the hormones of the brain and other endocrine organs of the cephalic region. Destruction of the thoracic ganglia apparently creates an imbalance in the concentrations of sodium and potassium in the haemolymph, suggesting the thoracid ganglia as a source of a hormonal factor controlling ionic balance. The regulation of internal ionic balance under hormonal influence of thoracic ganglia has become evident from the experiments performed during the present study.