The fine structure of the mature spermatozoon of the insect Thermobia domestica has been investigated. This flagellate spermatozoon is unique with respect to the relative positions of the centriole, nucleus, mitochondrial nebenkern derivatives, and acrosome along the length of the cell. The acrosome lies at the posterior end of the nucleus. Unlike spermatozoa of most animals, the Thermobia spermatozoon has a nucleus that reveals an unusual lamellar pattern of the material inside of it. This flagellate spermatozoon is also unusual in its tendency to intertwine with other spermatozoa, and during movement the intertwined portion of two (double) spermatozoa is always directed forwards. In the axial filament complex, one of the fibrils of each peripheral double fibril bears a hook-shaped process. An indistinct granular material is seen between the 9 outer coarse fibers. Cytoplasmic tubules, probably corresponding to the "microtubules" of other investigators (67), have been observed around the nucleus, mitochondrial nebenkern derivatives, axial filament complex, and acrosome. A description is given of a complex membrane system which surrounds and separates the main organelles of the cell from each other.

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