The organization of the indirect flight muscle of an aphid (Hemiptera-Homoptera) is described. The fibers of this muscle contain an extensive though irregularly disposed complement of T system tubules, derived as open invaginations from the cell surface and from the plasma membrane sheaths accompanying the tracheoles within the fiber. The sarcoplasmic reticulum is reduced to small vesicles applied to the T system surfaces, the intermembrane gap being traversed by blocks of electron-opaque material resembling that of septate desmosomes. The form and distribution of the T system and sarcoplasmic reticulum membranes in flight muscles of representatives of the major insect orders is described, and the extreme reduction of the reticulum cisternae in all asynchronous fibers (to which group the aphid flight muscle probably belongs), and the high degree of their development in synchronous fibers is documented and discussed in terms of the contraction physiology of these muscle cells.

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