The function of haemoglobin in relation to the maintenance of neutral buoyancy in Anisops pellucens (Notonectidae, Hemiptera).

Among insects, haemoglobin is known to occur only in certain chironomid larvae, in Gastrophilus larvae, and in the water bugs Macrocorixa geoffroyi and those belonging to the subfamily Anisopinae. Only in the chironomid (Walshe, 1950) and Gastrophilus larvae (Keilin & Wang, 1946) is much known of its function and properties. A useful summary of the characteristics of haemoglobin in these insects has been provided by Buck (1964). In Gastrophilus (Keilin, 1944) and in Anisops and Buenoa (Anisopinae) (Hungerford, 1922; Poisson, 1926; Bare, 1929) haemoglobin is found in certain richly tracheated cells in the abdomen; in the first it may also occur in other tissues in smaller amounts. Anisops and Buenoa are the only insects known to possess haemoglobin in both larval and adult stages, but its function and properties in these genera have remained unexplored. Both are remarkable for their habit of remaining poised in mid-water in more or less neutral density for a large part of each dive (Bare, 1926, 1929; Jaczewski, 1936; Kaiser, 1940), and the suggestion that this ability is due to the possession of haemoglobin has been made by Bare (1929), Hutchinson (1953) and by Hungerford (1958). Miller (1964a, b) described some preliminary experiments whose results were in agreement with this hypothesis, and further supporting evidence is presented here from recent work carried out at Makerere College on Anisops pellucens in Uganda. The haemoglobin in this species is shown to have a strikingly low affinity for oxygen and this property permits unloading of the pigment during a normal dive of about 5 min. duration even in well-aerated water. The rate of depletion of the external gas store is thereby diminished and the bug is thus enabled to remain in or near neutral buoyancy for several minutes. The haemoglobin may therefore be looked upon as an unusual type of store which is regularly called upon during each dive, and not one which is used only under conditions of severe oxygen deprivation. Its use enables the bug to exploit the mid-water zone, a habitat not readily available to aquatic insects other than the larvae of the nematocerans, Chaoborus and Monochlonyx (Damant, 1924).