Which insect group may be the ancestors of fleas?

The Siphonaptera (fleas) are believed to be descended from an ancestor of the Mecoptera, commonly known as scorpionflies.1

Fleas and scorpionflies share several distinct biological characteristics, including a spined gizzard (proventriculus), six rectal glands, and similarities in their reproductive systems, such as the structure of the male spermatozoon and the type of ovary. Furthermore, fleas are classified within the panorpoid complex, a group that also includes Diptera (true flies), Trichoptera (caddisflies), and Lepidoptera (butterflies and moths), all of which are thought to have evolved from a common ancestor that lived in moss.12