SummaryThis study represents an enquiry into the significance of the nocturnal habit inThe validity of the generic name Ciniflo Blackwall is established.Autograph experiments indicate that all three species are nocturnal in habit and show a composite rhythm of movements and rest which is controlled primarily by alternating light and darkness, in the absence of which a rhythm can be set up by regular 24‐hour cycles of temperature fluctuations. At the same time 18‐hour cycles of light and darkness set up a weaker locomotory rhythm than that cawed by ′314‐hour cycles. The intensity of nocturnalism is the same in all three species, over 90 per cent of their activity taking place during the hours of darkness.When the water‐relations of the three species are considered it is found that there Ciniflo ferox (Walck.), C. similis BI and P. fenestalis (Stroem). is a critical temperature a t approximately 35°C. above which all three species quickly lose water by evaporation in dry air, but below this C. ferox and C. fenestralis lose water more rapidly through their lung‐books than does C. sirnilis. As regards rateof water‐loss through the lung‐books, the species can be arranged in a series from C. eirnilis through C. ferox to C. fenestralis, which shows the highest rate both in air of 50 per cent relative humidity and in dry air. The rate is almost doubled when 10 per cent of carbon dioxide is present, as this keeps the lung‐books open. Conversely the time of survival in air of 50 per cent relative humidity and in dry air is longest in C'. sirnilis and shortest in C. fenestralis, death ensuing when from 20 to 25 per cent of total weight has been lost by evaporation.C. sirnilis ‘tires’ rapidly when forced to run at full speed without stopping, C. ferox less quickly, while C. fewtralis can maintain full speed for much longer. All three can run for long periods however when supplied with oxygen. C. ferox becomes anaesthetized most quickly in ether vapour, then C. fenestralis, while C. similis remains conscious longest. Finally C. ferox has the largest number of leaves in its lung‐books, C. few trdis least. The greater ‘stamina’ of C'. ferox when compared with C. similis depends therefore upon a proportionately larger respiratory surface acquired at the expense of greater dependence upon environmental humidity. That of C'. fenestralis is correlated with its smaller size.It is shown that the respiratory pigment of the blood which acts as an oxygen store as well as an oxygen carrier is a typical haemocyanin, the absorption curve of which has been plotted. The histology of the integument is discussed: Sudan staining indicates the presence of an epicuticular lipoid layer. Removal of this with chloroform greatly increas