The muscles of three different arthropods, a mite, a fly, and an ostracod, show variations in the length of the A region within a given individual. There is no indication that the observed differences in A band length are related to the functional state of the muscle since little, if any, decrease in the length of the A bands was noted when sarcomeres shortened. The length of the A region was determined by polarized light microscopy and in the case of the mite and the ostracod this measurement was made on intact muscles. It is concluded that the size of the A filaments in an individual can vary in a manner unrelated to immediate functional changes. The I filaments may vary in size, but this could not be clearly demonstrated.

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