Ostia

Ostia are small, slit-like, paired openings in the dorsal vessel that allow hemolymph to enter or leave the vessel. Incurrent ostia allow hemolymph to enter during diastole and excurrent ones permit hemolymph to exit. Some Orthoptera have 12 pairs of incurrent ostia, nine in the abdomen and three in the thorax, but most insects have fewer, with 2, 3, or 5 pairs of ostia being common. Ostia more commonly occur in the heart, but may also occur in the aorta. Pairs of ostia are usually located laterally, with one on each side of the heart, but some are ventrally and dorsally located.

Ostial openings tend to occur at the base of shallow pockets or at deeper, funnel-shaped invaginations in the wall of the dorsal vessel (Fig. 24), which often give the heart a chambered appearance. It can be difficult to distinguish incurrent and excurrent ostia. Excurrent ostia more often occur in the thoracic portion of the vessel, but some also occur in the abdomen of some insects. Some ostia do not have...