In Rhodnius most of the haemoglobin ingested is broken down in the lumen of the gut to protohaematin which is excreted unchanged. But a small amount is absorbed and circulates in the haemolymph as kathaemoglobin (parahaematin). This is taken up by the salivary glands where it appears as a cherry-red pigment with properties similar to haemalbumin. Blood pigment is also transferred to the yolk of the eggs and becomes concentrated in the stomach of the newly hatched nymph as a bright red fluid (parahaematin). In the next few days most of this is digested in the gut to give protohaematin; some is transferred to the salivary glands to give rise to their usual pigment. Blood pigment in the haemolymph of Rhodnius is taken up also by the pericardial nephrocytes and by the epithelial cells of the stomach and intestine. Here it is converted to a brown pigment (a modified haem pigment), to a green pigment (probably of the verdohaem type and resembling choleglobin), and finally to biliverdin. Biliverdin accumulates throughout life in the pericardial cells, which become bright green. In the gut it is discharged to the lumen and appears in the faeces. The free iron accumulates throughout life in the cells of the stomach and intestine; in old insects these are heavily laden with iron deposits. After the injection of haemoglobin into the haemolymph all the above processes are exaggerated. In addition, some breakdown of blood pigment takes place in the Malpighian tubes; and the lumen of the tubes may become charged with massed droplets of biliverdin displaced from the pericardial cells. The process of excretion of this pigment by the cells is described. Comparative studies have been made on some other blood-sucking arthropods. Hemiptera: Triatoma infestans, T. brasiliensis, Eutriatoma sordida, Cimex lectularius. Anoplura: Pediculus humanus. Siphonaptera: Nosopsyllus (Ceratophyllus) fasciatus. Diptera: Anopheles maculipennis, Aedes aegypti. Acarina: Ornithodorus moubata, O. delanoei, Ixodes ricinus. In all these haemoglobin is digested in the gut more or less completely to protohaematin. It is demonstrable in the haemolymph in Ornithodorus, probably as alkaline haematin, and in Ixodes, in a form resembling methaemalbumin. It appears in modified form in the salivary glands in Cimex only. It is transferred to the eggs in Cimex, Ornithodorus and Ixodes as alkaline haematin; in Pediculus apparently as unchanged oxyhaemoglobin. It is broken down to biliverdin in the Malpighian tubes of Triatoma infestans, T. brasiliensis and Eutriatoma; in the pericardial nephrocytes of Triatoma brasiliensis, Eutriatoma (probably associated with bilirubin), and Pediculus; and in the lumen of the gut in Pediculus. An altered haematin and perhaps a verdohaem pigment similar to choleglobin have been demonstrated as intermediates in Triatoma brasiliensis.