Insects do have a heart, but its structure and function differ significantly from the human heart. The insect circulatory system is an open system, not relying on a complex network of closed vessels like arteries and veins. It centers around a single, dorsal pumping organ that moves a multi-purpose fluid throughout the body cavity. Oxygen delivery is handled by a separate respiratory system, allowing the circulatory system to focus primarily on transporting other materials. Understanding the insect heart requires shifting the perspective from a high-pressure pump to a simple, longitudinal tube.
An Open Circulatory System
The insect circulatory system is classified as open, meaning the circulatory fluid is not constantly contained within a closed circuit of vessels. Unlike the closed system found in vertebrates, the fluid flows freely throughout the main body cavity, a large space known as the hemocoel. Here, internal organs are directly bathed in the circulating fluid.
The hemocoel is divided into compartments, or sinuses, by thin, muscular diaphragms. These diaphragms help guide the fluid flow around the viscera and nerve cord, ensuring directed movement. An open system requires less energy to operate than a high-pressure closed system.
Hemolymph The Multi-Purpose Fluid
The fluid circulating within the insect hemocoel is called hemolymph, a substance that combines the functions of blood and lymph found in vertebrates. Hemolymph is primarily a watery plasma, often clear, yellowish, or greenish in color. This coloration is due to the lack of hemoglobin, the iron-based protein that gives vertebrate blood its red color.
Hemolymph does not play a significant role in oxygen transport, which is handled by the insect’s extensive tracheal system. Its main responsibilities include transporting nutrients like sugars, amino acids, and hormones to tissues. The fluid also collects metabolic waste products from the cells for removal by the Malpighian tubules.
Hemolymph contains specialized cells called hemocytes, which are suspended within the plasma. These cells are functionally similar to the white blood cells in vertebrates, providing immune defense by engulfing foreign microorganisms (phagocytosis). Hemolymph also assists in hydraulic functions, such as helping a newly emerged insect expand its wings or assisting in the molting process.
Anatomy of the Dorsal Vessel
The insect’s heart is a single, closed, muscular tube called the dorsal vessel, which runs longitudinally along the back beneath the upper body wall. This vessel is the primary organ generating circulatory movement and is functionally divided into two regions.
The posterior section, located in the abdomen, is the heart itself—a muscular pump divided into a series of chambers. The heart wall is perforated by small, valved openings called ostia, which permit the one-way inflow of hemolymph from the surrounding body cavity. Pairs of fan-shaped alary muscles anchor the heart and assist in its rhythmic, peristaltic contractions.
The anterior section of the dorsal vessel, extending forward through the thorax toward the head, is the aorta. The aorta is a simpler tube lacking the ostia and chambered structure of the heart. It acts mainly as a conduit, carrying the fluid pumped forward toward the front of the body where it opens into the head cavity.
How Hemolymph Circulates
Circulation begins when the heart muscle relaxes, causing the ostia to open and draw hemolymph from the surrounding pericardial sinus into the heart chambers. A wave of muscle contraction, usually starting at the posterior end of the abdomen, then forces the hemolymph forward. This peristaltic pumping action propels the fluid through the chambers and into the anterior aorta.
The hemolymph is discharged from the open-ended aorta into the head capsule, bathing the brain and other tissues. From the head, the fluid percolates backward through the hemocoel, moving around internal organs and body sinuses. The action of the dorsal and ventral diaphragms helps direct this flow toward the abdomen, allowing the fluid to re-enter the heart through the ostia.
Because the main dorsal vessel is often too weak to push fluid into long, narrow appendages, many insects possess specialized, supplementary pumps. These are known as accessory pulsatile organs (APOs). APOs are often located at the bases of antennae, wings, or legs and contract autonomously to ensure hemolymph circulates within these extremities.