Img 1 Adult cat fleas have a black, round, simple “eyespots”, one on each side of the head.
Adult cat fleas do have eyes. One is located on each side of the head. Instead of compound eyes, fleas posses simple eyes with a single biconvex lens. The eyes are black and round.
Location & Appearance
Fleas have two small eyes, one on either side of their heads Img 1 & 2. The eyes sit slightly beneath, and ahead of, the base of the antennae. They’re located in the same position where most insects have compound eyes.
Img 2 Magnified image of a cat flea, clearing showing its eye spot.
Flea eyes are round with a black pigment. They appear as dark spots on the head. The eyes closely resemble the dorsal ocelli of other insects Img 3.
The Structure of Simple Eyes
Fleas don’t have compound eyes. Instead, they posses simple eyes with a single biconvex lens. Long sense cells reach from the base of the eye to the lens, but they don’t form rhabdoms. There are a few small corneagenous cells. The entire inner eye is surrounded by a black pigmented sheath.
Img 3 Fleas have simple eyes that resemble the dorsal ocelli (highlighted) of other insects.
Flea Eye Evolution
It’s been theorized that fleas eyes were once dorsal ocelli, and they got displaced from the top of their heads to the sides. However, it’s now believed that they’re actually reduced and modified compound eyes. Through the course of evolution, the compound eyes were replaced with heavily sclerotized, ocelli-like “eyespots”.