Caterpillars, known for their transformative life cycle, have internal systems that differ significantly from mammals. Many wonder if these fascinating creatures possess a heart similar to our own. Their internal workings are complex, operating on principles distinct from what we typically assume about animal anatomy.
The Caterpillar’s Dorsal Vessel
Unlike humans, caterpillars have an open circulatory system centered around a single, tube-like organ called the dorsal vessel. This vessel runs along the caterpillar’s back, extending from its rear end towards its head. The dorsal vessel functions as the primary pump; its abdominal portion is often called the “heart” region, while the anterior part is the aorta.
The dorsal vessel’s “heart” region is segmented and contains small, valved openings called ostia. As the vessel relaxes, hemolymph enters these ostia from the surrounding body cavity. When the vessel contracts, often aided by alary muscles, the ostia valves close, forcing the hemolymph forward towards the head. This pumping action circulates hemolymph throughout the caterpillar’s body cavity, known as the hemocoel, before it re-enters the dorsal vessel.
Hemolymph: Caterpillar “Blood”
The fluid circulating within a caterpillar’s open circulatory system is called hemolymph. Comparable to vertebrate blood but with significant distinctions, hemolymph typically appears clear, yellowish, or sometimes greenish. It lacks the red color of human blood because it does not contain hemoglobin, and it fills the body cavity, directly bathing internal organs and tissues.
Hemolymph is primarily composed of water, inorganic salts, and various organic compounds like amino acids, proteins, lipids, and sugars such as trehalose, which serve as energy sources. Circulating immune cells, called hemocytes, are dispersed within the hemolymph, playing a role in defense and wound healing. A crucial difference from vertebrate blood is that hemolymph does not transport oxygen; its main functions include nutrient and hormone transport, waste removal, and maintaining internal pressure.
How Caterpillars Breathe
Since hemolymph does not carry oxygen, caterpillars have a separate, specialized system for respiration. They breathe through a network of internal tubes called tracheae, which extend throughout their body. These tracheal tubes open to the outside environment through small, paired openings along the caterpillar’s sides, known as spiracles. Air enters the tracheal system directly through these spiracles, bypassing the circulatory system entirely.
The tracheal tubes then branch into finer tubes, delivering oxygen directly to the caterpillar’s individual cells and tissues. Carbon dioxide is also expelled through this same tracheal network and out through the spiracles. While caterpillars do not actively “breathe” in the mammalian sense, their body movements, such as segment compression and relaxation, can facilitate air movement.