principles are produced and stored. In animals that discharge their secretion as a liquid, gland capacity may be estimated directly by weighing the amount of fluid ejected. But when the defensive substance is emitted as a gas, a more refined assay is in order. In two previous papers of this series, we dealt with the production of gaseous hydrogen cyanide by certain species of millipedes, and described the structure and mode of operation of their cyanogenetic glandular apparatus (H. E. Eisner et al., 1963; T. Eisner et al., 1963). We have now developed a quick and highly sensitive assay for hydrogen cyanide, and it is our purpose here to report on the measurement of cyanogenesis in two species of millipedes, Apheloria corrugata and Pseudopolydesmus serratus (Text-fig. 1). Both are members of the order Polydesmida, the only one in which cyanogenetic species are known to occur. Polydesmoid millipedes generate hydrogen cyanide only in response to disturbance. The poison emanates as a gas from droplets of secre-
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Eisner et al. (1967) studied this question.