The relationship between blood ions and blood-cell density in insects.

1. Pairs of blood samples were taken from individual Periplaneta americana , the first sample being used for a live-blood haemocyte count and the second (collected 10 sec. later) for potassium and sodium determinations. 2. Analysis of these matched data on cell count and whole-blood ion concentration reveals a positive correlation between the haemocyte density and the potassium concentration, the mean regression coefficient being +0·83 mM K+/l. of blood for 10,000 cells/µl. 3. Similar analysis of the sodium data gives a negative correlation, with a mean regression coefficient of -1·69 mM N+/l. for 10,000 cells/µl. 4. Haematocrit estimations on heat-fixed blood, whose haemocyte density was simultaneously determined, indicate a mean haemocyte volume of 720 µ 3 . 5. If the correlations relate entirely to the contents of the haemocytes, comparison with the haemocyte volume would indicate that the cell sap contains 115 mM/l. of potassium and probably less than about 60 mM/l. of sodium; these levels are very similar to those reported for other tissues of the cockroach. 6. Since cockroach blood contains relatively little potassium these figures suggest that a significant proportion of the whole-blood potassium may be contained within the haemocytes and so not be immediately available physiologically.