Giant spermatozoa and a huge spermatheca: A case of coevolution of male and female reproductive organs in the ground louse Zorotypus impolitus (Insecta, Zoraptera)

• Male and female apparatus have a large seminal vesicle and huge spermatheca.
• Sperm have the greatest size so far described in Hexapoda.
• Small spermatophores are attached externally to the female abdomen.
• Each spermatophore contains only one sperm.
• Sexual selection was involved in determining the huge sperm and large spermatheca.

The male and female genital apparatus of the recently discovered ground louse Zorotypus impolitus were examined using light and electron microscopy. The rounded testes and a large seminal vesicle are connected with a complex of four accessory glands by a long tapering ejaculatory duct. Two accessory glands have the same whitish coloration, whereas the third one is pale blue, and the elongated and cylindrical fourth one translucent. The sperm are the largest known in Hexapoda, 3 mm long and 3 μm wide, with a volume of ca. 21,000 μm3; the ratio between the diameter of the axoneme and the width of the main body of the sperm ranges between 1:10 and 1:13. The exceptional width of the spermatozoa is due to an extreme enlargement of the mitochondrial derivatives and accessory bodies. A single sperm is contained in a small globular spermatophore (100 μm). The highly unusual external transfer correlates with an atypical mating behavior. The male produces several to many spermatophores during the mating process. As in other zorapterans the ovaries are panoistic and the eggs bear two micropyles. An exceptionally large apical spermathecal receptacle is present; it is connected with the vagina by a long spermathecal duct, which varies structurally along its course. A correlation between the sperm size and the size of the spermatheca is likely. Ultrastructural features of different species support two strikingly different models of male and female reproductive apparatus in the small order Zoraptera. This is in stark contrast to the extreme uniformity of their external morphology. It is likely that sexual selection played a decisive role in the evolution of the reproductive system.

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