The Weta 28: 47-48 (2004) 47 Tracking of migrant Blue Moon butterfly, Hypolimnas bolina nerina, using web-based software Brendon Christensen Department of Conservation, P.O. Box 1146, Rotorua (bchristensen@doc.govt.nz) Introduction Unusual sightings of insects, especially migratory insects, can raise the question of where approximately they came from (Tomlinson 1973)? Such deduction can be made easier, and in some cases almost instantaneously, using the relatively new tools of free-access, web-based computer programmes. This short note outlines such a process following a recent sighting of the Blue moon butterfly, Hypolimnas bolina nerina, and a subsequent query on its likely origin, using an interactive web-based programme, utilising archived atmospheric and climatic records. Observation and tracking of Blue Moon Crowe (2002) states that Blue moon butterflies have been observed on or near the west coast of New Zealand, from March to May in most years, having been blown here with favourable winds from Australia (see also Tomlinson 1973). On 08 May 2004 I observed a male Blue moon making feeding attempts on camellia flowers, and resting on sun-exposed walls and fences, in the Lynmore suburb of Rotorua, Bay of Plenty, New Zealand. To work out the approximate origin of this particular butterfly I used the archived records from the Air Resources Laboratory (ARL) of the National Oceanic and Atmospheric Administration (NOAA), a section of the USA Government (http:// www.arl.noaa.gov/), to produce a backwards trajectory of the path that the butterfly most likely had taken to get to New Zealand. This website has a computer programme (NOAA HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) MODEL) that produces tracking and forecasting trajectories for particles in the air. With a simple input procedure (see below), starting at the ARL trajectory model website (http://www.arl.noaa.gov/ready/open/traj.html) trajectory figures can be produced showing the path of the particles due to atmospheric conditions, including the prevailing winds. • select FNL as the archived meteorological dataset. • select an archived file; e.g. the southern hemisphere set for early May 2004 (fnl.sh.may04.001). • Enter location co-ordinates for the observation site (e.g. Rotorua: -38.16, 176.25).