Accessory Pulsatile Organs

Because the circulation of hemolymph is vital to all insect tissues, several intricate structures ensure circulation of hemolymph through the appendages. Collectively, these are termed the accessory pulsatile organs (APOs), but modifications to ensure circulation in the appendages also include diaphragms and directed channels. When present, APOs occur at the bases of wings, antennae, legs, and cercal appendages at the back of the abdomen.

Early studies of the neuromusculature of the locust leg revealed a proximal bundle of muscles in the extensor tibia (jumping muscle) of the hind leg that exhibited rhythmic contraction. Amputating the hind leg of the grasshopper or locust very near the connection with the thorax, and attaching the femur to a convenient substrate with the tibia pointing straight up, demonstrates this rhythmic activity. After a delay of several minutes, the tibia will move back and forth spontaneously.

A small patch of muscles (called a "leg heart") near the coxal—trochanter—femur joint generates rhythmic pulsations thought to assist in the movement of hemolymph in the large femur of the jumping leg. To ensure hemolymph supply to the entire leg, there is a delivery route out and a collecting route back. Movement of hemolymph in an open circulatory system may be assisted by gross movement of internal organs, such as contractile activity of the Malpighian tubules and of the mid- and hindgut.

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