Insects, unlike vertebrates, do not possess a four-chambered heart that pumps blood through a closed network of vessels. Instead, they feature a functional equivalent: a specialized pulsatile organ that circulates a fluid known as hemolymph. This unique arrangement is part of an open circulatory system, where hemolymph flows freely within the body cavity, directly bathing organs and tissues.
The Insect “Heart”
The primary circulatory organ in insects is the dorsal vessel, a long, tubular structure positioned along the insect’s back. This vessel is divided into two main sections. The posterior part, located in the abdomen, functions as the heart, exhibiting muscular and pulsatile characteristics. Moving forward, the anterior portion extends into the thorax and head, serving as a conducting tube called the aorta.
Along the heart section of the dorsal vessel, small, valve-like openings called ostia are present. These ostia allow the hemolymph to enter the heart from the surrounding body cavity. This structural design is different from mammalian hearts, which confine blood within a closed system.
How Circulation Works
Hemolymph circulation in insects begins with the rhythmic contractions of the dorsal vessel. These peristaltic contractions push hemolymph forward through the aorta, directing it towards the head. Once discharged from the anterior end of the aorta, the hemolymph flows openly into the insect’s main body cavity, known as the hemocoel.
Within the hemocoel, the hemolymph directly bathes all internal organs and tissues, facilitating the exchange of nutrients, hormones, and waste products. After circulating through the body, the hemolymph re-enters the dorsal vessel through the ostia, completing the cycle. In addition to the main dorsal vessel, many insects possess accessory pulsatile organs located in appendages like antennae, legs, and wings. These auxiliary “hearts” generate localized pressure, ensuring that hemolymph reaches and circulates efficiently within these distant and often narrow structures.
Why This System Suits Insects
The open circulatory system with its dorsal vessel is well-suited to the physiology and lifestyle of insects. Insect hemolymph does not primarily transport oxygen. Unlike vertebrate blood, which relies on hemoglobin to carry oxygen, insect hemolymph mainly distributes nutrients, hormones, and removes metabolic waste products.
Oxygen delivery in insects is handled by a separate, highly efficient tracheal system, a network of air-filled tubes that extends throughout the body, delivering oxygen directly to tissues and cells. This direct oxygen supply eliminates the need for a high-pressure, oxygen-carrying circulatory system.
The simpler, lower-pressure open system is sufficient for their small body size and lower metabolic demands compared to larger vertebrates. The hydrostatic pressure generated by the hemolymph within this open system also plays a role in processes such as molting, helping the insect shed its old exoskeleton and expand its new one.