Fleas do have blood in them. Engorged fleas will carry around 5 µl of blood in their abdomens. Even unfed fleas have small amounts of blood, carried over from the larval stage. Fleas also have hemolymph in them, which is the circulating fluid (“blood”) of insects.
Blood (Hemoglobin) in Fleas
Blood in Unfed Fleas
Insects typically don’t have blood in them. However, fleas do, even prior to feeding. This is because larvae consume the blood feces from adult fleas. As a result, some hemoglobin is retained when the larvae pupate. Newly emerged adults have the equivalent of 0.066 µl of whole blood.
This trait is rare for insects, even among other parasites. For example, the larvae of mosquitoes and bedbugs don’t consume adult feces. Additionally, in most blood-feeding insects, blood meals are only taken by adult females.
Blood in Fed Fleas
An adult flea’s gut holds around 5 µl of blood. Males are smaller than females, with smaller abdomens. Cat fleas feed to repletion within 48 hours. Once engorged, a flea will be nearly double its unfed weight and size. Fleas ingest much more blood than they can utilize. The excess blood gets excreted as flea dirt, which supplies larvae with a source of nutrition.
Squishing a flea will spill out red vertebrate blood, as well as blueish hemolymph.
Hemolymph is Insect “Blood”
Fleas, like all insects, have hemolymph in their bodies. Hemolymph is the circulating fluid or “blood” of insects. It makes up to 20-50% of the total water in insects, with larval stages having a higher percentage.
Hemolymph is a blue-green color. It contains hemocyanin, which is a copper-based protein that turns blue when oxygenated. However, it quickly loses its color and appears grey. This is opposed to vertebrate blood, which contains iron-based hemoglobin. Hemoglobin turns a red color when exposed to oxygen.
Insects have open circulatory systems. The hemolypmh directly bathes internal organs and tissues, delivering nutrients and carrying away waste products. Hemolypmh serves as a pathway for hormones, which control biological processes. Hemolypmh transports heat away from heat-generating organs and towards areas where it’s needed, such as thoracic muscles. It also functions as a carbon dioxide sink.
Hemolymph can function as a hydraulic fluid. For example, the hydraulic pressure will unfurl the wings of a newly molted butterfly. Hemolymph is propelled to all internal surfaces with specialized pumps, pressure pulses, and body movements. The fluid passes through vessels, tubes, and diaphragms.
Hemocytes are the circulating cells or “blood cells” within hemolymph. They attack invading organisms and foreign substances.