Various species of insects differ greatly in their specific resistance to the same insecticide. To cite an extreme case, it has been shown experimentally by Carpenter and Moore (1938) that a concentration of approximately 0-5 mg. per litre hydrogen cyanide kills 99 per cent, of the flour beetles, Tribolium confusum Duv., in a 10 minutes' exposure period, whereas a concentration of about 270 mg. per litre is required to kill 99 per cent, of the rice weevils, Sitophilus oryzae (L.), under the same conditions. This means a difference in dosage of about 500 times. Even within an insect species, a new resistant strain may develop as a result of constant application •of the same insecticide. Attention to such a phenomenon was first drawn by Melander (1914) who observed strains of San Jose scale resistant to lime sulphur sprays. Subsequently, a strain of the California red scale of citrus, Aonidiella aurantii (Mask.), was noticed to be resistant to HCN fumigation in 1914 and that of Saissetia oleae (Bern.) in 1915 (Quayle, 1938). Recently Lindquist and Wilson (1948), and Schmitt (1948, 1949) in U.S. A., and Keiding and van Deurs (1949) in Denmark have shown experimentally the development of resistance to DDT in house-flies. Thus the question of resistance of insects to insecticides has an important bearing on their use.The difference in the resistance of insects to a fumigant may be due to a bewildering •complex of factors. For a systematic analysis of these factors, the determination of the amount of poisonous gas actually taken in and retained by the insect is very important. Carpenter and Moore (1938) were the first to study this intake or sorption of HCN by a few species of insects using a manometric method. Sinclair (1944, 1945) estimated the sorption of HCN by recovering the amount of HCN sorbed by the insect, and estimating this amount colorimetrically. These investigators found that sorption of a fumigant by insects is correlated with their specific susceptibility. The present contribution partially corroborates the above finding and attempts, further, to analyse " Total Effective Resistance " into two components, viz., " Surface Resistance " and " Internal Resistance ".
Material and Apparatus.The insects used in these investigations were (1) adults of Tribolium castaneum (Hbst.) (Col., Tenebrionidae), (2) the seventh-stage caterpillars oiCorcyra cephalonica (Staint.) (Lep., Pyralidae), (3) first-and second-instar nymphs of Drosicha sp. (Horn., Coccidae), (4) third-and fourth-instar nymphs of Drosicha sp. and (5) the adult females of Drosicha sp. T. castaneum and C. cephalonica were reared on crushed jowar (Andropogon sorghum) seeds in a room with the temperature controlled between 80° and 85°F. and the humidity between 80 per cent, and 95 per cent. The adult females of Drosicha sp. were collected from 'Grevillea robusta (Proteaceae) and the nymphs from Prosopis juliflora (Mimosaceae). Samples were taken at random in order to minimise the extent of variation in age and sex. All the insects were preconditio...
