Genital mechanics and sexual selection in the spider Nesticus cellulanus (Araneae: Nesticidae)

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Abstract
The spider Nesticus cellulanus (Clerck, 1758) was investigated with respect to the functional morphology of its genital organs, using for the first time the method of freeze-fixation of the spiders in copula by liquid nitrogen and subsequent preparation of serial sections of the copulatory organs in functional contact. Use of this method provides new and detailed insights into the function of complex genitalia. Previous ideas, based on artificial expansion of the male copulatory bulbs and simulation experiments, must be rejected or doubted. The role of female choice as a universal cause of species specificity and complexity in genitalia, proposed by Eberhard, is discussed. The total absence of sensilla in the contact zones of the female copulatory organs indicates that the female is not stimulated by the copulatory organs of the male, nor can she evaluate their mechanical fit. "Female choice" can therefore act only indirectly, by influencing the number of offspring, which is assumed to be correlated with the fitting of the male genitalia, thus determining the number of sperm transferred. This postmating "choice" is absolutely independent of the female nervous system and is seen to be at least an important additional mechanism of sexual selection.

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