Symely, a seldom occurring developmental anomaly in the spider<i>Tegenaria atrica</i>

This article presents the results of teratological studies on the spider Tegenaria atrica. Temperatures alternating between 14°C and 32°C every 12 h were applied as a teratogenic factor in the early stage of embryogenesis. This treatment resulted in specimens with various developmental malformations, including symely of the walking legs. Symely, which is one of the anomalies that results from disturbances during the formation of metameres to the germ band, usually affects the third and fourth pair of walking legs. Fusion of legs from the same pair entails changes in the symmetry of the whole body, particularly in the structure of prosoma and its position in relation to the opisthosoma. These changes are the cause of the exceptionally high mortality among embryos.