Life cycle evolution in mites and other groups of Arachnids
1973 - Volume: 15 Issue: 3 pages: 384-390
L evolution du cycle vital chez les Acariens et les autres groups d Arachnides. - In connection with the present conception of the subclass of the mites as consisting of two different groups (Actinotrichida and Anactinotrichida), a further investigation of the affinities of these with the other groups of Arachnidea appeared to be of essential importance. The present paper is the first result of such a comparative study; it deals with the evolution of the life cycle in mites and other groups of Arachnidea. Special attention has been paid to the number of stases, and to the occurrence of elattostases and calyptostases (of these terms an exact definition is given). Data on other groups of Arachnidea have been taken from literature; these data are not always easy to interpret because several authors did not distinguish moults between stases from growing-moults. Besides, such terms as stases, elattostases and calyptostases appear to be practically unknown outside acarology. Sorne confusion appears to be existing as to the terms prelarva, larva, prenymph and nymph; these have often been applied in connection with the degree of regressive evolution, and not exactly in connection with the level of postembryonic ontogeny (originally, they have been defined in the lastmentioned sense). In both groups of mites, the ancestral number of stases is six; this number is found also in several groups of Arachnidea. A larger number of stases is known with certainty from Scorpionida, Araneida and Amblypygi; in the case of Solifugae and Opilionida, it is still uncertain whether growing-moults have been included in the number of moults mentioned in literature. One phenomenon appears to be general in the evolution of the Arachnidean life cycle, viz., the elattostasic or calyptostasic regression of the first stase(s). A second phenomenon, viz., a tendency towards neoteny (in the original sense of this term), appears to be certain in the case of Scorpionida, Amblypygi and Araneida, but could be present in more cases (for instance in mites with less than three nymphs). An elattostasic or calyptostasic regression of other stases than the first(s), appears to be a phenomenon restricted to some groups of Actinotrichid mites (deutonymph in many Acaridida, protonymph and tritonymph in Trombidei). In this connection it is, however, interesting to cite the dysharmonic evolution of the trichobothria of the Amblypygid legs, a very important discovery described by Weygoldt (1970); this regression could be the very first beginning of a divergent phylogeny, the beginning of a separation of stase 3 from the nymphal phase.
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