Migration has been defined by Schneider (1962) as a prolonged escape movement in which there is a tendency to maintain a constant direction and which results in the permanent or periodical abandonment of a habitat. Migration can be classified as dispersive, contractive, or collective according to the spatial effect. Many insects migrate in this sense, and numerous accounts of their migrations can be found in the literature. Recent reviews of insect migration have been published by Schneider (1962) and Williams (1957, 1958). In most of the migratory insects which have been carefully studied, the migratory direction is determined largely by the prevailing wind. Swarms of the desert locust (Schis tocerca gregaria Forsk.) are carried downwind, and since the winds in the lower few thousand feet of the atmosphere may be regarded ultimately as blowing from areas of high pressure to areas of low pressure, the downwind displacement results in movement into areas of low pressure where abundant rainfall produces conditions favorable for the reproduction of the locust (Rainey, 1951). The migratory direction of the coccinellid Hippodaniia convergens Guerin-Meneville in California is determined by the prevailing winds at its flight level (Hagen, 1962). In the summer, .the prevailing winds at this level are from the low lands toward the mountains where aggregation occurs, and in the winter, they are from the mountains to the low lands. The mosquito Aedes taeniorhynchus Wiedemann migrates downwind (Provost, 1952, 1957). In contrast to this, many migrating butterflies maintain, over long distances, a constant direction which is influenced little by wind, topography, or time of day (Williams, 1958). The determination of migratory direction appears to be under the control of the insect itself, but the nature of the underlying orientation mechanism has remained obscure. Only two migratory butterflies have been studied in great detail (Nielsen and Nielsen, 1950; Nielsen, 1961; Urquhart, 1960), and these studies did not include an investigation of the orientation mechanism. Recent work on the orientation of other animals has suggested new approaches to this problem. (For references, see Long Island Biological Association, 1960.) The purpose of the research reported here, was to study the migratory behavior of the gulf fritillary, Agraulis vanillae (L.), against the background of its general biology and to investigate the nature of the orientation mechanism involved in maintaining the migratory direction. The gulf fritillary is a member of the essentially neotropical nymphalid subfamily Heliconiiae. Michener (1942) recognizes eight subspecies as follows : Agraulis vanillae vanillae (Linnaeus) occurs in northern South America, Panama, and the southernmost of the Lesser Antilles Agraulis vanillae insularis Maynard ranges throughout the Bahamas and the Antilles except for the southernmost Lesser y\ntilles. Agraulis vanillae maculosa (Stichel) is found in northern Argentina, Paraguay, southern Brazil, and Chile. Agraulis vanillae forbesi Michener occurs in the coastal region of Peru. Agraulis vanillae galapagensis Holland is restricted to the Galapagos Islands. Agraulis vanillae lucinia C. and R. Felder occurs on the eastern side of the Andes in Ecuador, Peru, and Brazil. Agraulis vanillae incarnata (Riley) is occasionally found as far north as British Columbia but more commonly occurs in southwestern United States, Mexico, and Central America. Agraulis vanillae nigrior Michener is the subspecies of southeastern United States but is occasionally found as far north as New York. The migratory habit has evolved in at least two of these subspecies. Definite migrations of A. v. nigrior to the north in the spring and to the south in the fall have been observed in Florida (Williams, 1958). Hayward (1962) reported an eastward migration of A. v. maculosa at Tucaman, Argentina, on January 7, 1961. The research reported here was restricted to A. v. nigrior. This form intergrades with A. v. incarnata in Texas and thence southward but is distinct from A. v. insularis. 4 The early stages of A. v. nigrior were described long ago (Edwards, 1880; Scudder, 1889), but no detailed studies of its biology have been made. More recently, detailed descriptions of the early stages of A. v. vanillae have been published (Beebe, Crane, and Fleming, 1960) and certain details of the adult morphology have been described by Emsley (1963). The biology and behavior of the larvae, pupae, and emerging adults of A.V. vanillae were discussed by Alexander (1961a, 1961b) and compared with those of other members of the subfamily occurring in Trinidad.