Hemolymph

Definition

The circulating fluid of arthropods and some other invertebrates, analogous to vertebrate blood but functioning within an open circulatory system. Hemolymph fills the body cavity (hemocoel) and directly bathes tissues; it lacks enclosed vessels except for the dorsal heart and anterior aorta. In insects, it transports nutrients, hormones, and metabolic wastes, with oxygen typically carried by tracheal systems rather than respiratory pigments (though some aquatic and parasitic insects possess hemoglobin or hemocyanin). Hemolymph contains suspended immune cells called hemocytes and diverse plasma proteins including phenoloxidases, antimicrobial peptides, and clotting factors that mediate wound sealing and pathogen defense.

Etymology

From Greek haima (blood) + Latin lympha (water, clear fluid)

Example

When a caterpillar is wounded, hemolymph rapidly coagulates at the injury site through hemocyte aggregation and phenoloxidase cascade activation, forming a melanized plug that prevents desiccation and microbial invasion.

Synonyms

• haemolymph

Related Terms

• hemocyte
• hemocoel
• open circulatory system
• tracheal system
• phenoloxidase
• melanization
• ecdysteroid
• Juvenile hormone

Usage Notes

Do not confuse with 'blood'—hemolymph does not consistently carry respiratory gases in terrestrial insects, and its protein composition differs markedly from vertebrate plasma. The spelling 'haemolymph' predominates in British English. Hemolymph sampling (hemolymph collection) is a standard technique in insect physiology and immunology research.