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SUMMARY The nature of the food and the feeding mechanisms of various chironomid larvae with different modes of life have been studied by observing their feeding behaviour in the laboratory and their gut contents in nature and under experimental conditions. Of Tendipes (=Chironomus) larvae, only T. plumosus employs a filter‐feeding mechanism with a salivary net spun across the lumen of its tube anterior to the body. Other larvae of the plumosus‐group (e. g. T. dorsalis), although identical in larval morphological characters, feed only on organic matter in the mud. The possession of the unique and complicated feeding behaviour should be regarded as grounds for retaining T. plumosus as a species distinct from the others resembling it in larval characters, some of which are doubtfully valid. Tendipes larvae of the thummi and bathophilus types have no filter‐feeding mechanism but also ingest the mud; this is true too of mud‐living Tanytarsini, e. g. Micropsectra, Tanytarsus (Tanytarsus), Sergentia. Larvae of Tanytarsus (Rheotanytarsus) are filter‐feeders making use of the natural flow of a stream to bring them suspended particles which form their food. The filtratory salivary net is slung between spreading arms built by the larva at the entrance to its tube. Larvae with portable cases (e. g. Stempellina, Lauterborniella) feed on particles of algae or detritus in the vicinity. Chironomid larvae mining in aquatic plants employ a filter mechanism for obtaining their phytoplanktonic food. Like Tendipes plumosus larvae, they spin a salivary net across the lumen of their burrows but, unlike them, turn round after having spun the net so that, while driving a water current through the burrow and thereby catching phytoplankton, the net is posterior in the. The net is also a much deeper cone than that of T. plamousus. Three genera of filter‐feeding leaf‐miners were studied: Endochironomus and Pentapedilum. The genera show certain constand differences in the details of their feeding behaviour for instance in the shape of the net, method of its construction, defaecation behaviour, and extent to which filter‐feeding is obligatory. These differences are described. The rhythm of the feeding behaviour is not conditioned by the amount of plankton being caught in the net. The length of time spent in irrigating during each feeding cycle is, however, negatively correlated with the rate of undulatory body movement during irrigation. The larvae only defaccate whilst feeding: the length of interval between successive defaecations is proportional to the amount of food eaten. The possible origins of the various filter‐feeding mechanisms in Tendipedinae are discussed.

1. The behaviour of final-instar larvae of Chironomus plumosus housed in U-shaped glass tubes was observed at various concentrations of dissolved oxygen and carbon dioxide. 2. Respiratory behaviour, consisting of intermittent irrigation of the tube, alternates with periods of filter-feeding or complete immobility. In well-aerated water about 50% of the time is occupied by respiratory behaviour, 35% by filter-feeding and the remainder by periods of rest. As the oxygen concentration in the water drops, progressively less time is occupied by filter-feeding and immobility and more by respiratory irrigation. Below 10% air saturation of the water larvae no longer feed. When placed in completely anaerobic conditions larvae at first irrigate intermittently but subsequently relapse into immobility. 3. During respiratory behaviour the amount of irrigation and the length of pauses between periods of irrigation change at different oxygen and carbon dioxide contents of the water in such a way as to suggest that the respiratory irrigation is controlled by internal pH changes in the larvae. 4. A spectroscopic examination of the haemoglobin in living larvae showed that the blood pigment holds an approximately 9-min. store of oxygen for the resting animal. In addition to this it acts in the transport of oxygen from the tube water to the larval tissues when the larva pauses between periods of irrigation. It thus decreases the amount of anaerobiosis to be endured during short periods of inactivity. Nevertheless, larvae without a functional haemoglobin (i.e. with carboxyhaemoglobin) still continue to pause during their respiratory behaviour, and the pauses are not strikingly curtailed in length. 5. At very low oxygen concentrations (7.5-9.0% air saturation), when the larva irrigates the tube almost unceasingly, the haemoglobin remains in a state of partial oxygenation, during which time it is functioning continuously in oxygen transport. At these oxygen concentrations larvae with carboxyhaemoglobin do not show respiratory activity but assume the immobility characteristic of anaerobic conditions. 6. Larvae with carboxyhaemoglobin tend to be less active than normal animals, except in well-aerated water, the decreased activity being largely due to a reduction in the amount of filter-feeding. Such larvae have not been observed to filter-feed at oxygen concentrations below 26% air saturation, whereas the limiting concentration for normal larvae is 10%. 7. After a prolonged period of anaerobiosis larvae show evidence of the repayment of an oxygen debt by prolonged irrigation of the tube when oxygen is once more available. A return to a normal irrigation rate is rapid and is usually followed by a period of filter-feeding The rate of recovery is proportional to the oxygen content of the incoming water, but normal larvae can recover even in water only 7 % air saturated. Larvae with carboxyhaemoglobin, on the other hand, show a considerably retarded rate of recovery from anaerobic conditions, and cannot recover in water less than 15% air saturated. 8. The main significance of haemoglobin in the life of a full-grown Chironomus larva would thus seem to be threefold: (a) haemoglobin enables the larva to maintain the active process of filter-feeding when relatively little oxygen is present; (b) it acts in oxygen transport at very low oxygen concentrations, thereby enabling continued respiratory irrigation; and (c) it greatly increases the rate of recovery from periods of oxygen lack, making such recovery possible even under adverse respiratory conditions.