Moths, like all insects, do not possess blood in the familiar sense, as seen in vertebrates. Instead, they circulate a distinct fluid called hemolymph, which performs many similar functions but fundamentally differs in composition and its role in oxygen transport.
Moths Have Hemolymph, Not Blood
Moths circulate a fluid called hemolymph, the insect equivalent of blood, which is distinct from vertebrate blood because it does not carry oxygen. Unlike the red color of vertebrate blood, which comes from hemoglobin, hemolymph is typically clear, yellowish, or sometimes greenish.
Hemolymph circulates within an open circulatory system, meaning it flows freely within the insect’s main body cavity, known as the hemocoel. This allows the hemolymph to directly bathe all internal organs and tissues. The fluid portion of hemolymph, called plasma, is mostly water, making up about 90% of its volume.
The plasma contains various dissolved substances, including inorganic salts like sodium, chloride, potassium, magnesium, and calcium. It also carries organic compounds such as carbohydrates (like trehalose, a primary insect sugar), proteins, and lipids. Suspended within this plasma are specialized cells called hemocytes, which contribute to several functions within the insect’s body.
The Role of Hemolymph
Hemolymph performs various functions that are necessary for a moth’s survival and physiological processes. One primary function is the transport of nutrients, including sugars, amino acids, and lipids, which are absorbed from the digestive system and distributed to cells throughout the body for energy, growth, and repair. It also transports metabolic waste products from tissues to excretory organs, such as the Malpighian tubules, for removal.
The hemolymph also plays a role in the moth’s internal defense mechanisms. It contains hemocytes, which are immune cells capable of engulfing foreign particles through a process called phagocytosis. These cells can also encapsulate larger invaders, isolating them from the rest of the body, and produce antimicrobial compounds.
Beyond transport and immunity, hemolymph contributes to maintaining hydrostatic pressure within the moth’s body. This internal pressure is used to facilitate movements, such as the expansion of wings after a moth emerges from its pupal stage. It also supports processes like molting and general body movements, especially in softer-bodied larval stages. Hemolymph can also aid in wound healing by clotting quickly to prevent fluid loss and the entry of pathogens.
How Moth Circulation Differs
The circulatory system of a moth operates as an open system, differing from the closed circulatory systems found in many vertebrates. In an open system, hemolymph is not continuously enclosed within a network of blood vessels. Instead, it flows directly through the hemocoel, bathing the organs within this large body cavity.
The primary pumping organ in a moth’s circulatory system is the dorsal vessel, a long, muscular tube located along the insect’s back. This vessel is divided into two regions: a posterior “heart” in the abdomen, and an anterior “aorta” that extends forward into the head. The heart part of the dorsal vessel has small, valved openings called ostia, through which hemolymph enters.
Muscular contractions of the dorsal vessel pump the hemolymph forward, discharging it into the head region. From there, the hemolymph percolates back through the hemocoel, surrounding internal tissues before re-entering the dorsal vessel through the ostia. Accessory pulsatile organs, or auxiliary hearts, may also assist in circulating hemolymph to appendages like wings, antennae, and legs.
Moth hemolymph does not carry oxygen for respiration. Instead, moths and other insects utilize a separate and highly efficient tracheal system for direct oxygen delivery to their cells. This system consists of external openings called spiracles, which lead into a branching network of internal tubes called tracheae and smaller tracheoles that extend to nearly every cell in the body. Oxygen diffuses directly from these tubes into the tissues, making the hemolymph’s role solely focused on other transport and physiological functions.