METAMERIC MALFORMATIONS OF OPISTHOSOMA IN TEGENARIA ATRICA (ARANEAE, AGELENIDAE)

The opisthosoma of Tegenaria atrica is characterized by a high mor- phological stability, as it is exceptionally resistant to the teratogenic effect of most abiotic factors except temperature. The least frequent opisthosomal anomalies found in our experimental material were disturbances in the colouring of opisthosoma. Another rare anomaly was the elongation of the postspinneret zone of the opisthosoma, resem- bling the postabdomen of other arachnids. The occurrence of a much narrower elonga- tion, sometimes with traces of segmentation, resembling the postabdomen the advanced spider, can be interpreted as a specific atavism. An equally rare anomaly was the presence of two independent anal orifices in the postspinneret zone. All the above- described anomalies appear extremely rarely, and some of them are related to the atavistic characteristics observed in the spiders' ontogeny