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

The tissues of insects are bathed freely in haemolymph, and the relationship between the ionic composition of the blood and the tissues has proved a fruitful field of research. In particular, much work has been done on the mechanisms of nerve-impulse conduction (see Narahashi, 1963) and osmoregulation (see Shaw & Stobbart, 1963) in relation to ion levels in the blood. Insect haemolymph is physiologically different from vertebrate blood in many respects, the most obvious being that it performs no respiratory function and contains no equivalent of the erythrocytes. A result of this is that whereas human blood contains about 5,000,000 cells/^l. and these occupy nearly 50% of the total blood volume, insect blood usually contains less than 100,000 cells//il. and in many species less than 10,000 cells/^l. (Tauber & Yeager, 1935, 1936). Presumably because of these comparatively low blood-cell densities most insect physiologists have treated whole-blood measurements of the concentration of ions in insect blood as if they were identical with the physiologically significant concentrations in the plasma.-f Some authors, indeed, have explicitly stated that this must be the case. For instance, van Asperen & van Esch (1956) say that only small differences could be expected between the whole haemolymph and plasma of their cockroaches because the blood appeared clear and only contained 67,000 cells//il. Tobias (1948 a) appears to be the only author to have attempted a direct investigation of this problem. The work of Ramsay (1955) on Carausius 'serum' is not strictly relevant since he prepared this by centrifuging blood previously heated to ioo° C. At this temperature it must be assumed that the cell membranes were broken down and that all soluble cell contents released. Tobias, on the other hand, measured sodium, potassium and magnesium in Periplaneta in both whole blood and in plasma prepared by spinning off the haemocytes in micro-centrifuge tubes. On a molar basis the plasma contained about 40 % less of each ion than did the whole blood, but he states that the differences were not statistically significant, and apparently pursued the matter no further. Van Asperen & van Esch (1956) considered that these results were 'quite inexplicable', a view reiterated by Pichon (1963). However, haemocytes can occupy up to 10% of the total blood volume of heat-fixed cockroaches (Wheeler, 1963), and tissue concentrations of potassium, at least, can be more than ten times that of the