Up to now, no intercalary males of Cylindroiulus nitidus (Verhoeff) had been found, neither in Britain, nor in S.W. Germany. Hand sampling led the author to the discovery of three intercalary males in 1983 at Milldale (Derbyshire, Great Britain) on the one hand and one at Mettlach (Saarland, W. Germany) on the other. British adult males have been reared in field plots at Milldale and Baulme-la-Roche (France), Baulme-la-Roche males at Milldale and German ones at St. Ingbert (Saarland). The results of these experiences are given. It seems that the frequency of intercalaries depends on the years. It is suggested that in C. nitidus intercalaries are a resistance form which may have appeared during evolution. The adult to adult succession could be possible only under good environmental conditions, intercalaries becoming then useless. It is stressed that periodomorphosis and the adult to adult succession are both of great adaptive value compensating for some disadvantages of neoteny.