The function of haemoglobin in relation to filter feeding in leaf-mining chironomid larvae.

(With One Text-figure) Tendipes (= Chironomus) plumosus larvae are unique among species of Tendipes in employing a filter-feeding mechanism for obtaining their food (Walshe, 1947 a). The larva spins a sheet of salivary secretion across the lumen of its tube, after which, by rapid anteroposterior undulations of the body, it irrigates the tube. Plankton or other organic particles in the incoming water current are thereby caught in the salivary net and subsequently eaten. This filter feeding is an energetic process involving glandular activity in spinning the salivary sheet and muscular activity in maintaining a rapid current of water during the filtration of food particles. A study (Walshe, 1950) of the amount of filter feeding at different oxygen concentrations has shown that this is essentially an aerobic process, declining at low oxygen concentrations and absent under anaerobic conditions. Normal larvae are incapable of feeding below about 10% air saturation of the water, while larvae without functional haemoglobin (i.e. after carbon monoxide treatment) cease to feed at 26 % air saturation. Even above this value larvae with carboxyhaemoglobin feed less than normal animals. Thus the haemoglobin, in addition to its established significance as an oxygen carrier at low oxygen pressures (Harnisch, 1936; Ewer, 1942) and during recovery from oxygen lack (Harnisch, 1936; Walshe, 19476), also plays a part in enabling the larvae to continue an active aerobic feeding process even when little oxygen is available. Other chironomid larvae, namely, the leaf-mining genera, employ a filter mechanism for obtaining their phytoplanktonic food and, although the mechanism is slightly different from that of T. plumosus in that the larvae turn round in their tubes after spinning the net (Burtt, 1940), the principle is the same and also demands considerable activity on the part of the animal. Some of the species of these genera contain abundant haemoglobin, so a number of experiments was made to see whether in these larvae also the blood pigment is of functional value whilst filter feeding. Larvae of two filter-feeding genera, namely, Glyptotendipes and Endochironomus, were removed from the leaves of their host plants and put into straight glass tubes of the same dimensions as their natural burrows in or on the plants. The larvae lived well in these glass tubes, lining them with salivary secretion, filter-feeding normally and remaining in them until the end of pupation. Their feeding behaviour could be