A quantification of the behaviourial dynamics of certain Lepidoptera in response to light
Many types of insect, in particular the nocturnal Lepidoptera, will fly towards artificial sources of illumination. Such animals are often described as being positively phototactic, but although little progress has been made towards a fundamental understanding of this phenomenon, its existence continues to be exploited with the use of light-traps. This thesis attempts to explain, in part, why certain British night-flying Lepidoptera are caught, or fly close to, light-traps. The experimentation and analysis has been structured into three separate but inter-related studies. The first is an investigation into the effects that weather factors exert on light-trap catch. Standard analytical procedures were extended to reveal that sensitivity to these factors is related to insect size and gross morphology. The second of these studies is an analysis of the types of moth flight pattern produced when these insects are exposed to various forms of illumination under field conditions. Moth tracks were recorded on video with the help of image intensification, and the frame-by-frame co-ordinates transferred to, and processed by, a microcomputer, which produced matrix maps of speeds, accelerations and time-surface densities around a light-trap. Instantaneous windspeeds were recorded. The dynamical analysis suggests that moth flight towards light arises primarily from a misinterpretation of the stimulus, competing at short distances with a strong escape response, thus evoking a profound state of disorientation. Furthermore, the data indicated that the types of pattern found were species linked. In the third study, a remotesensing technique was used to quantify moth aerial density, which was compared with simultaneous light-trap catches nearby, giving an estimate of absolute trap catching efficiency under various meteorological conditions. Because of their mode of operation, and their increasing loss of effectiveness in higher windspeeds, light-traps have only a limited capacity to reflect aerial density.