20 Internal Anatomy – Circulatory System

Internal Anatomy – Circulatory System

The main organ of the circulatory system, the heart, appears as a clear tube running the length of the dorsal midline (our circulatory system is primarily ventral). There is a heart chamber associated with each body segment, and muscles that expand each chamber.

Insects have an open circulatory system. The “blood”, called hemolymph, is not always contained within vessels. In most cases, insect hemolymph is not associated with the respiratory system, and does not transport respiratory gases.

Connected together into a long tube, the chambers contract in sequence from the tip of the abdomen towards the head. These contractions force hemolymph towards the brain, where it spills out into the body cavity, bathing the tissues directly. The hemolymph is then drawn back into the heart chambers through tiny openings called ostia.

See https://www.youtube.com/watch?v=LLPmKydCudY (R. Gibson) for a video of a lining insect’s heart in action. The caterpillar in this video is resting, and likely a bit cold, so the heart pauses on occasion (the video also skips a little).