Flight Activity in the Blowfly Calliphora Erythrocephala, in Relation to Light and Radiant Heat, with Special Reference to Adaptation

Many insects depend for survival on wide dispersal by flight. Small insects are carried about passively by air currents (Hardy & Milne, 1937, 1938; Freeman, 1945; Gisl6n, 1948; Johnson, 1954) and the displacement of even such a large and powerful form as a locust depends largely on the wind direction during flight (Waloff, 1946; Rainey & Waloff, 1948). Spontaneous flight activity is on the whole most obvious in fine sunny weather, but we have little information on the way in which various weather factors might affect it. The subject is difficult to study in the field because most of the factors commonly change together. Laboratory studies by Davies (1935, 1936) and Broadbent (1949) have concerned the effect of temperature, humidity, wind speed and light intensity on aphids, and Nicholson (1934) has described the effect of temperature on the flight activity of various species of blowflies. There appears to be, however, no detailed account of the way in which light and radiant heat may operate at intensities comparable to those found in sunshine, and little attention has so far been given to the effects of adaptation. The time course of adaptation is of considerable ecological significance, for the conditions to which insects are exposed in sunny weather consists of a continuous series of changes, arising both from changes in the weather and from movements of the insects themselves. The change in activity in response to any particular change of conditions is therefore closely dependent on the rate and extent of adaptation. The experiments described in this paper were carried out to clarify these matters. The blowfly, Calliphora erythrocephala Mg., proved a good subject for these experiments. Observations were made upon a small population of these insects confined in a transparent chamber in a small wind tunnel, and subjected to changes in the experimental factors over a range of conditions comparable to those normally encountered in the field.