A comparative study is made of the life-cyle in Chelicerata, and its evolution. Various types of forms or instars, and various evolutionary phenomena are distinguished. They are arranged in a chronological diagram constituting a general model of the evolution of the chelicerate life-cycle. A glossary is added in which terminology is defined.
Although the postembryonic development has been studied in many groups of Chelicerata, comparative data on this development are rare, and our general knowledge of the chelicerate life-cycle is not homogeneous. This is partly caused by the great biological differences existing between the various groups. In some groups, such as Actinotrichid and Anactinotrichid mites, immature and adult forms can often be found throughout the year, and even together in one sample, whilst the various immature forms present distinct differences. In many groups of Chelicerata the life-cycle can, however, present seasonal aspects; in addition, species have often to be reared in order to study the complete development. The differences between the various forms of a lifecycle can, moreover, be rather indistinct. In some cryptozoic groups the life-cycle is not even completely known.
Until recently there was also no uniform theoretical base for the general study of the postembryonic development in Chelicerata: there were no general concepts for the various types of forms, and there was no general terminology based on comparative studies.
The aim of the present paper is to lay the theoretical foundations for a comparative study of the chelicerate life-cycle and its evolution. This study was started some years ago (van der Hammen, 1974a(van der Hammen, , 1974b(van der Hammen, , 1975)), when I tried to apply the terminology developed in acarologyto other groups of Chelicerata. It appeared that acarological terminology was not sufficient for this purpose *In the present paper, Chelicerata are classified according to van der Hammen (1977); the classification of Mites is based on van der Hammen (1972b).