Although it has been known since the time of Aristotle that spiders can make airborne excursions, the first accurate observations were published by the pioneer British arachnologist, John Blackwall (1827). It was not until some years later that McCook (1877, 1878, 1889-93) made the suggestion that such behaviour might be a means of dispersal for the species concerned and during the last fifty years a number of arachnologists have published observations on this subject (Nielsen 1932, Braendegaard 1937, 1938, 1946, Bristowe 1929, 1939, Berland 1932, Smith 1904, Emerton 1908, 1919, McKeown 1952). In addition to the work of these observers, the contribution made by spiders to aerial plankton has been demonstrated by collections from aircraft towing nets (Collins & Baker 1934, Crosby & Bishop 1936, Glick 1939) and from nets on wireless masts (Freeman 1946) and the masts of ships (Hardy & Milne 1937). Generally speaking only small numbers were taken, though Glick was able to collect about 1500 specimens, including 4 at 5000 ft. Freeman used three nets 1 yd in diameter, and during 1934 and from March to April 1935 he took 134 specimens, representing twenty species. On the other hand, Owen & Le Gros (1954) found that seventy meals taken by swifts in July and August 1952, July 1953 and July 1944 contained 1425 spiders, representing about twenty-two species. Bristowe (1939) gives an excellent account of dispersal behaviour and summarizes much of the published work. The following general points emerge: