Structural Properties of the Intrinsic Muscles of the Malpighian Tubules of the Female Stable Fly, Stomoxys calcitrans L.

The four Malpighian tubules of the female stable fly, Stomoxys calcitrans L., were divided into an anterior dorsal and a posterior ventral pair. A marked structural divergence existed between the distal terminations of these two pairs. This divergence consisted primarily in the number and size of crystalline granules present in the respective distal regions. The terminal diameter of the dorsal tubules was at least twice that of the ventral tubules. Intrinsic muscles were found only in that short proximal section (ureter) of each pair of tubules that emptied into the midgut. Other regions of the tubules showed no evidence of a muscle sheath. The small intrinsic muscles (0.25 to 0.6 μm diam) of the ureter were visible under Nomarski optics at high magnification (600X). The muscle fibers were embedded in the basal regions of the large epithelial cells that line the lumen of the ureter. Occasionally, these muscles were arranged in a multilayered lattice. Myofibrils were separated into sarcomeres of irregular alignment with classical A (0.8 μm in length) and I bands. The Z disk consisted of discontinuous rows of dense bodies, and in partially contracted muscle some myofilaments of the A band passed between the dense bodies of the Z bands. T-system tubules and the sarcoplasmic reticulum were sparse to non-existent.