The neotropical moth U. fulgens
migrates diurnally over Lake Gatun on the Panama Canal (a
). Despite suggestions of 4–6-year migration cycles driven by host plant toxicity (Smith 1983
), migration patterns in this species are very unpredictable. Experiments reported here were conducted from 4 to 22 June 2006. Solitary U. fulgens
individuals migrating over Lake Gatun could be readily tracked because they flew at low heights (less than 2 m above the surface) with relatively straight paths. Details of field measurements of U. fulgens
moths appear elsewhere (Dudley & DeVries 1990
; Dudley et al. 2002
). Briefly, we pursued migrating moths in a 14 feet aluminium dinghy driven such that it remained aligned parallel to the moth's trajectory. We used an anemometer (TSI Model 8347, St Paul, MN, USA) held laterally from the boat and behind the free-flying moth to measure its airspeed. After measuring airspeed, we captured the moth with an insect net (see film in the electronic supplementary material), stopped the boat and measured the moth's track with a hand-held compass.
Figure 1. (a) Rectangle delineates release sites for U. fulgens moths on Lake Gatun, Panama. The lake shore and islands near the site of release are outlined. Migratory airspeeds for (b) sham-treated versus (c) flagella-amputated moths before and after treatment. (more ...)
Captured moths were subjected to either flagella amputation or sham treatments. In the former, moths were restrained and both antennal flagella were cut using microscissors, but leaving the basal mechanosensory apparatus intact. In the latter treatment, only the wispy tips of both flagella were cut. Following either treatment, moths were allowed to walk on a dry surface until they voluntarily initiated flight. After moths settled into a regular flight pattern, we again measured their airspeed and track as described above. If moths did not resume normal flight, we tried to induce them to fly by gently blowing on their heads. Typically such moths, if they did indeed take off, flew less than a few metres vertically before rapidly losing height and were deemed to be ‘flight-impaired’.
It was not always possible to measure both direction and airspeed pre- and post-antennectomy in every moth. Thus, although 12/13 sham (12/25 experimental) moths were flight-capable, we recorded airspeeds of only nine sham (seven experimental) moths. If moths flew in a certain direction but could not sustain flight long enough for airspeeds to be measured, we noted only the direction data (13 shams, 16 experimental). Owing to motion of the boat on water, experimental reattachment of cut antennae was not possible (e.g. Sane et al. 2007