Alary Muscles

The alary muscles, so named because of their general wing or delta shape in many insects, lie immediately on top of the dorsal diaphragm. The muscles probably aid the dorsal diaphragm in providing support for the heart, the part of the dorsal vessel in the abdomen. The muscle fibers fan out from a small point of origin on the lateral wall of the dorsum to a broad insertion on the heart in many insects, presenting the typical delta appearance. In some insects, however, the delta shape is not so evident. Some alary muscle fibers pass beneath the heart and extend laterally from side to side, and thus help to support the heart. In places, the fibers may also run parallel to the long axis of the heart for a short distance. The pairs of alary muscles tend to agree with the number of pairs of ostia, the (usually) lateral openings in the dorsal vessel that allow hemolymph to flow into the heart in the abdomen. Alary muscles generally do not occur in the thorax, but in some insects, a few ostia open outward in the thorax, allowing hemolymph to flow outward.

In addition to support, the alary muscle may assist in the expansion (diastole) of the heart after each contractile wave passes a given point, and thus aid in pulling hemolymph into the incurrent ostia. They are not necessary for diastole, however, as evidenced by severing them with little or no apparent effect on the heart beat.