Heart structure and beat in the stable fly, Stomoxys calcitrans

ABSTRACT. The heart of the adult stable fly, Stomoxys calcitrans (L.), is suspended from the dorsal sclerites of the abdomen by strands of connective tissue, and supported from below by alary muscles that insert into a central band of longitudinal muscle just beneath the aorta. Valved openings occur in three of the heart segments. The central band of muscle beneath the heart is innervated but there is no well-defined lateral cardiac nervous system. The myocardium consists of a single layer of circular muscle composed of a series of muscle fibres that are joined dorsally and ventrally by intercalated discs in the midline of the insect. T-system tubules are closely associated with the sarcoplasmic reticulum, forming dyads. The heart rate of intact stable flies varied from short intervals of almost no activity to periods with a very rapid beat (126–294 pulses/min), and when the connections to the central nervous system were severed the heart beat became very regular (258 pulses/min). Slight pressure applied to the dorsal septum stopped myocardial contractions in that segment. The myocardium was insensitive to perfusion with 10^-3M acetylcholine, l -aspartic acid, l -glutamic acid, Λ-aminobutyric acid, 5-hydroxytryptamine, octopamine, tyramine and proctolin 10^-5 m. However, Mn⁺⁺ caused either an intermittent beat at lower concentrations (0.5 min) or near arrest at higher concentrations (2 mM).