The axoneme of the spider spermatozoon

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Abstract
The axoneme of the spider spermatozoon (Pholcus sp.) has been examined after fixation with a fixative that permits the visualization of the individual protofilaments. The axoneme was found to consist of nine microtubular doublets in a circle around three microtubular singlets, thus a 9 + 3 pattern. Each of the three central microtubules contains 13 protofilaments and is flanked by two short filaments, maybe the equivalent of the Central sheath. Distally the central microtubules are slightly flattened. The nine doublets have inner and outer dynein arms and have the same anatomy as have the doublets in regular 9 + 2 axonemes. Whereas the nine doublets in 9 + 2 flagella have unequal positions in relation to the central microtubules and hence unequal tasks in flagellation, the same is not true for the spider sperm axoneme.