AbstractAxonemal doublets of some insect spermatozoa were fixed in a mixture of glutaraldehyde and tannic acid, post‐fixed in uranyl acetate, and examined by electron microscopy, in order better to characterize the protofilament pattern. Most species had outer and inner dynein arms; others had only the inner one or none. Electron micrographs show the individual protofilaments to be well resolved and to be separated by an electron dense material. A certain “noise” inherent in the electron‐microscopical technique was found and is believed to be due to irregularities in fixation, embedding, and section staining, and to beam damage. The noise level was reduced by using a computer program in which similar picture elements are averaged. The resulting averaged images of the axonemal doublets show a few widened “gaps” in the wall of protofilaments. These widened gaps coincide with the location of dynein arms, spokes, or intertubular material. There were, on the other hand, no widened gaps at the level of attachement of the accessory tubules. We tentatively conclude that at least some of the proteins that associate with microtubules are inserted deep inside the microtubular wall rather than having a superficial attachement. The internal structure of the A‐subtubule is rather constant in species where both sets of dynein arms are present, whereas that of the B‐tubule i
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