Flagellum

Definition

A long, whip-like cellular appendage that generates propulsive movement through undulatory or rotational motion. In arthropods, flagella occur primarily on sperm cells (where they form the motile tail), on certain sensory neurons (as a modified cilium), and on some unicellular gut symbionts. The arthropod sperm flagellum typically shows a 9+9+2 microtubule arrangement (nine outer doublets, nine accessory tubules, and a central pair), contrasting with the 9+2 pattern of most other eukaryotes. Flagella are structurally and evolutionarily distinct from prokaryotic flagella and from the larger, jointed locomotor appendages of arthropods.

Etymology

Latin, diminutive of flagrum (whip)

Example

In Drosophila melanogaster, sperm flagella measure nearly 2 mm—approximately fifty times the body length—requiring coordinated assembly of axonemal components during spermatogenesis; defects in flagellar proteins cause male sterility.

Synonyms

• undulipodium (historical, broader)

Related Terms

• axoneme
• cilium
• spermatozoon
• microtubule
• acrosome
• sperm tail
• trichocyst

Usage Notes

In entomology, distinguish flagella from the multi-segmented flagellum of insect antennae (a homonymous but structurally unrelated structure). Eukaryotic flagella and cilia share the same axonemal architecture and are sometimes collectively termed undulipodia, though 'flagellum' conventionally refers to longer, fewer appendages with undulatory beats, while 'cilium' refers to shorter, more numerous structures with oar-like power strokes. Arthropod sperm flagella are internal and non-motile until ejaculation, unlike the external, metabolically active flagella of many protists.