Some unique cases of bicephality in Tegenaria atrica C. L. Koch are described. This phenomenon is known to be the result of broadening and then sagittal bifurcation of the anterior germ band. In consequence there develop two head tagmae, equivalent if the germ band divides symmetrically or non-equivalent if the division is asymmetrical. The bifurcation of the germ band can be shallow, which produces two heads, or deep, which may lead to the development of so-called “Siamese twins”. A less frequent cause of bicephality is bifurcation across the anterior end of the germ band in the horizontal plane. Then two head somites develop, the main head, usually with a normal structure, and a so-called accessory head, usually incomplete, situated one above the other. In the authors' opinion the study of the morphology of the anterior body in bicephalous individuals provides exceptional opportunities for studying the interrelations between the functioning of the particular parts of the brain and widely understood behaviour of spiders.
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Vol. 14 • No. 7