The Insect "Heart"

diagram of blood flow in the insect circulatoy system

The dorsal vessel is a flexible tube that runs longitudinally through the thorax and abdomen, along the inside of the dorsal body wall. The dorsal vessel is closed at the posterior end and open at the anterior end. In the abdomen, the dorsal vessel is called the heart, which is divided into chambers separated by small valve-like openings called ostia, through which blood enters the heart. Each chamber has a pair of alary muscles which expand and contract to facilitate the flow of haemolymph through the heart. The section of the dorsal vessel after the heart does not possess valves or musculature but is instead simple tube called the aorta which facilitates transport of the haemolymph to the head (anterior end), where it empties into the body cavity.

The circulatory path:

Haemolymph from the body cavities enters the ostia when the alary muscles of the heart chambers relax. The alary muscles then contract to close the ostia and their valve-like structure prevents the haemolymph from returning to the body cavities. The haemolymph moves through the dorsal vessel by continual peristaltic contractions of the alary muscles. The contractions begin at the posterior chamber of the heart and continue forward, pushing the haemolymph anteriorly, toward the aorta.

The continual pumping pushes the haemolymph through the aorta and into the head, where it bathes the organs and muscles, and then flows back down the body via a series of cavities until it reaches the abdomen and re-enters the heart.