Development of the sensory systems in the larval and metamorphosing European grass frog (Rana temporaria L.).

It was investigated when and how the four examined sensory systems and the behaviour performances depending on them are affected by metamorphosis and whether they develop at an uniform rate from hatching up to metamorphosis. The following conclusions were reached on the basis of the results: -- The central nervous system starts to metamorphose earlier than the habitus and the viscera. -- Motor reactions based on the new capacities of the metamorphosed sensory systems only set in after metamorphosis. -- During metamorphosis the threshold of general sensitivity for the release of motor reactions is drastically raised. -- The sense of taste does not begin to function until after metamorphosis. -- The olfactory sense is the only one of the four tested systems that functions immediately after hatching. -- The level of development of the sensory systems and motor reactions is low in the early larva. The lack of taxis is characteristic of this. -- There is a phase of accelerated development of the sensory systems and motor reactions between the early and late larval stages. -- By comparison with the situation after metamorphosis the sensory systems of the old larva are fairly simple in their structure, but the motor reactions they permit are sufficient for the larva to cope with its aquatic biotope. -- Not until after the morphological structures have attained a new level of development do the relevant motor reactions show a corresponding advance.