air sacs
From ZooTerms (Dictionary of Invertebrate Zoology)
air sacs: 1. (Arthropoda: Insecta) Pouch-like enlargements of tracheal tubes in winged forms, usually lacking taenidia, capable of inflation and thought to function as an aid in flight and to lessen specific gravity.
2. (Cnidaria: Hydrozoa) In Siphonophora, that portion of a pneumatophore that contains gas.
