Insects do not have a four-chambered heart or a closed circulatory system identical to mammals. However, they possess a functional equivalent designed for their unique physiology. This system efficiently distributes necessary substances throughout their small bodies, supporting their active lives without the complexity of vertebrate circulation.
The Dorsal Vessel
The primary pumping organ in most insects is a tube-like structure known as the dorsal vessel. This muscular tube extends along the insect’s back, running from the posterior abdomen forward into the head. The dorsal vessel is typically segmented, with a series of small, valved openings called ostia located along its length, particularly in the abdominal region. This vessel is often divided into two main parts: a posterior “heart” region, usually found in the abdomen, which is the main contractile area, and an anterior “aorta” that extends into the thorax and head. While not a chambered heart like a human’s, the dorsal vessel performs a similar function by rhythmically contracting to propel fluid, ensuring distribution throughout the body.
An Open System and Hemolymph
Insects operate with an open circulatory system, meaning their internal fluid, called hemolymph, does not remain confined within a network of blood vessels. Instead, hemolymph flows freely within the insect’s body cavity, known as the hemocoel, directly bathing all internal organs and tissues. This contrasts with the closed circulatory systems of vertebrates, where blood is always enclosed. Hemolymph is a complex fluid, similar to both blood and lymph in vertebrates, composed of a liquid plasma and various cells called hemocytes; its primary functions include transporting nutrients, hormones, and waste products throughout the insect’s body. Hemolymph does not typically carry oxygen, as insects have a separate respiratory system involving tracheae that delivers oxygen directly to tissues.
Circulation in Action
The dorsal vessel contracts in a wave-like, peristaltic motion, beginning from the posterior end and propelling hemolymph forward towards the head. As the hemolymph is pumped anteriorly through the aorta, it is discharged into the hemocoel in the head region. From the head, the hemolymph then circulates backward through the various sinuses of the hemocoel, bathing the organs. It re-enters the dorsal vessel through the ostia during the vessel’s relaxation phase, completing the circulatory loop. To assist hemolymph movement in appendages like antennae, wings, and legs, insects also possess accessory pulsatile organs, which are auxiliary pumps that ensure fluid reaches these more distant areas.
Efficiency for Their Scale
The open circulatory system and the dorsal vessel are effective for insects due to their relatively small size. This simpler system requires less energy to operate and maintain compared to a high-pressure closed system. Their small body mass means that tissues are close enough to the circulating hemolymph for efficient exchange of nutrients and waste. The insect tracheal respiratory system, which delivers oxygen directly to cells via a network of tubes, eliminates the need for hemolymph to transport oxygen. This specialization allows the circulatory system to focus on other transport functions, and the combination of their small scale and specialized respiration makes the open circulatory system an efficient and evolutionarily successful design for insects.