letters to nature NATURE | VOL 399 | 10 JUNE 1999 | www.nature.com 579 between 270 and 4,000 ms after target onset) and to ignore changes in the distractor. Failure to respond within a reaction-time window, responding to a change in the distractor or deviating the gaze (monitored with a scleral search coil) by more than 1Њ from the fixation point caused the trial to be aborted without reward. The change in the target and distractors was selected so as to be challenging for the animal. In experiments 1 and 2 the animal correctly completed, on average, 79% of the trials, broke fixation in 11%, might have responded to the distractor stimulus in 6% and responded too early or not at all in 5% of the trials. In Experiment 3 the corresponding values are 78, 13%, 8% and 2%. In none of the three experiments was there a difference between the performances for the two possible targets. Differences between average eye positions during trials where one or the other stimulus was the target were very small, with only an average shift of 0.02Њ in the direction of the shift of position between the stimuli. Only correctly completed trials were considered. Firing rates were determined by computing the average neuronal response across trials for 1,000 ms starting 200 ms after the beginning of the target stimulus movement. Tuning curves. Tuning curves were derived by fitting the responses to the 12 directions presented with gaussian functions: r null þ dirGain ϫ exp ð Ϫ 0:5‫ء‬ðdir Ϫ prefdirÞ 2 =width 2 Þ . The four parameters of a gaussian curve capture the four features of a direction-selective cell: preferred direction ( prefdir), response to the anti-preferred direction (r null ), the directional gain (dirGain; the maximal response modulation) and the selectivity or tuning width (width; the range of directions the neuron responds to).

We investigate the niche separation in space and time between the Palearctic sister species Leptidea sinapis and L. reali (Lepidoptera, Pieridae) in central Sweden. Using field sampling, we show that L. reali is a habitat specialist confined to meadows, whereas L. sinapis is a habitat generalist also inhabiting forests. This difference in habitat utilization was corroborated by experimental release of laboratory-reared L. sinapis and L. reali in two adjacent forest and meadow habitats during their natural flight period; virtually all recaptured L. reali that were released in the forest were later caught in the meadow, whereas L. sinapis shifted equally often from meadow to forest as in the opposite direction. In the field, both species fly in May-June, but L. reali appears on average a week earlier in spring and has a substantial second generation in July, whereas L. sinapis is practically univoltine. When overwintered pupae were incubated under identical conditions in the laboratory, females did, however, not differ in phenology, and L. sinapis males actually emerged earlier than L. reali males. When larvae were reared at 23°C on the host plant Lotus corniculatus at a range of daylengths, both species produced a substantial proportion of directly developing individuals at an 18.5 h daylength or longer. When reared at 23°C and a 22 h daylength, L. reali showed an overall higher propensity to develop directly than L. sinapis on plant species originating from both the meadow and the forest habitat. Both Leptidea species showed a lower propensity to enter direct development on forest associated plants than on meadow associated plants. Hence, we suggest that the difference in phenology and voltinism between

The annotated list of 155 species of Lepidoptera, in addition to five records on unidentified species with known generic affinity, from the Taymyr national district is the first comprehensive account from this area. It is based on the results of the collecting trip of 2002, and on old materials collected by different researchers, and all published records collected by intensive bibliographic search and through consultations with colleagues. Four species erroneously reported from Taymyr are excluded from the list; 65 species are new for this district, including Clepsis mehli that is new to Russia and East Palaearctic, and Argyroploce mengelana reported from Asia for the first time. Records of 32 species are based exclusively on earlier publications; occurrence of other species is confirmed by the investigated material (listed in the paper). We expect that some hundreds of species are still yet to be discovered in Taymyr. The fauna of moths and butterflies of Taymyr is clearly more similar to that of East Palaearctic or Polar Ural than to that of northwestern Europe. In an European perspective, this fauna looks quite exotic and may resemble one that existed in Europe during the ice ages.

The fauna of moths and butterflies of the Western part of the Kola Peninsula is reasonably well documented, while data from Eastern part are scarce, and only five species have so far been recorded from the White Sea shore between Kuzomen and Ponoi. Here we report the results of sampling conducted on June 29–July 2, 2004, in 14 sites along a 150 km stretch between Kuzomen and Pulonga. Among 146 collected species of Lepidoptera, 12 are recorded for the first time from the Kola Peninsula, which increased the regional fauna to 688 species. For Udea nebulalis, which is for the first time reported from Russia, we provide additional records from Karelia.