28. D. Moazed and H. F. Noller, Nature 327, 389 (1987 ); Biochimie 69, 879 (1987 ). 29. J. Wower, S. S. Hixson, R. A. Zimmerman, Proc. Natl. Acad. Sci. U.S.A. 86, 5232 (1989); P. Mitchell, K. Stade, M. Osswald, R. Brimacombe, Nucleic Ac- ids Res. 15, 8783 (1993); C. C. Hall, D. Johnson, B. S. Cooperman, Biochemistry 27, 3983 (1988); G. Steiner, E. Kuechler, A. Barta, EMBO J. 7, 3949 (1988). 30. E. V. Puglisi, R. Green, H. F. Noller, J. D. Puglisi, Nature Struct. Biol. 4, 775 (1997 ). 31. We thank B. Cormack, G. Culver, L. Holmberg, K. Fredrick, K. Lieberman, and E. Strauss for helpful com- ments on the manuscript and S. Joseph and C. Merry- man for helpful discussions. Supported by grants from the National Institutes of Health, the National Science Foundation, the Lucille P. Markey Charitable Trust to the Center for Molecular Biology of RNA, and a Burroughs- Wellcome Career Award to R.G. 21 November 1997; accepted 10 February 1998 Visual Input to the Efferent Control System of a Fly’s “Gyroscope” Wai Pang Chan, Frederick Prete, Michael H. Dickinson* Dipterous insects (the true flies) have a sophisticated pair of equilibrium organs called halteres that evolved from hind wings. The halteres are sensitive to Coriolis forces that result from angular rotations of the body and mediate corrective reflexes during flight. Like the aerodynamically functional fore wings, the halteres beat during flight and are equipped with their own set of control muscles. It is shown that motoneurons innervating muscles of the haltere receive strong excitatory input from directionally sensitive visual interneurons. Visually guided flight maneuvers of flies may be mediated in part by efferent modulation of hard-wired equilibrium reflexes. Flies are among the most maneuverable of all flying animals and generate elaborate flight behaviors under visual control (1). For example, a male housefly initiates a correc- tive tracking maneuver within 30 ms of de- tecting a deviation in the flight trajectory of a female that it is chasing (2). Flies have several unique specializations that enable them to detect and respond to moving tar- gets with such rapidity. These specializations include a visual system with a flicker fusion rate of 300 Hz (3) and wings capable of achieving an aerodynamic performance that is two to three times greater than that gen- erated by conventional steady-state mecha- nisms (4). Perhaps the most remarkable spe- cialization of the flight system of flies is the evolutionary transformation of the hind wings into equilibrium organs called hal- teres, tiny club-shaped organs that beat an- tiphase to the wings during flight (5). Al- though the halteres have lost their aerody- namic role through evolution, the sensory fields at their base have hypertrophied rela- tive to their homologs at the base of the wings (6). In the blow fly Calliphora vicina, the haltere is equipped with about 335 strain-sensitive campaniform sensilla orga- nized in five distinct fields on the haltere base (7). Sensory cells innervating a sub- population of these sensilla encode Coriolis forces that result from the cross product of the haltere’s linear velocity with the angular velocity of the fly’s body around the yaw, pitch, or roll axes (8). Through their strong connections with steering motoneurons of the wing, the haltere afferents mediate sta- bilizing flight control reflexes (9, 10). With their halteres removed, flies are unstable and quickly crash to the ground (11). In many animals, efferent regulation modulates the sensitivity of sensory systems. In mammals, mechanical feedback mediated through efferent control of the outer hair cells within the cochlea is responsible for the sharp tuning of the primary auditory recep- tors (12). In vertebrate muscle spindles, fusi- motor efferents can adjust the length of in- trafusial fibers to set the sensitivity of the spindle sensory afferents (13). Similarly, in W. P. Chan and M. H. Dickinson, Department of Integra- tive Biology, University of California, Berkeley, CA 94720, USA. F. Prete, Department of Psychology, DePaul University, 1036 West Belden Avenue, Chicago, IL 60614, USA. * To whom correspondence should be addressed. Fig. 1. (A) Lateral view of a blow fly show- ing the haltere (in black) and the anterior (right) and posterior (left) spiracles. The arrows in this and subsequent figures in- dicate the haltere hinge. (B) Internal view of the right metapleural region showing the origin and insertion of the haltere con- trol muscles. The mesothoracic phragma was trimmed to expose the muscle inser- tion sites at the base of the haltere. (C) Close-up view of the haltere base show- ing the insertion sites of the direct control muscles. The first pterale (PT1) lines the dorsal posterior margin of the haltere base. The second pterale (PT2) is located medially at the haltere base and bridges the dorsal and ventral margin of the haltere’s articulation with the metathorax. The third pterale (PT3) forms the posterior corner of the haltere base. The largest sclerite, the basalare, is fused to the pleural process and sits just anterior and ventral to the haltere. These four sclerites are surrounded ante- riorly and dorsally by the anterior notal process, posteriorly by the posterior notal process, and ventrally by the pleural process. REPORTS www.sciencemag.org  SCIENCE  VOL. 280  10 APRIL 1998 289