Etude de la répartition du Grillon des bois Nemobius sylvestris (Bosc) dans un habitat naturel. - II. Les adultes

Abstract

Adult populations of Nemobius sylvestris were studied in the same two quadrats used for the study of the distribution patterns of their larval populations (Morvan, Campan and Thon, 1977). Adult wood-crickets were not found above ground in winter or even spring time in the Toulouse region. Their population increase in summer was apparently due to the imaginai molt of a new cohort in late spring. Spatial distribution of males associated with females was aggregative in the two quadrats, whereas the distribution of isolated males or females was clumped in quadrat B only. Adult distribution was also strongly influenced by the orien¬ tation of the forest border. On quadrat A, whose edge ran parallel to the daily course of the sun (E-W), the density of adult Nemobius sylvestris was greatest at distances between 5-8 and 15-18 metres from the border. On quadrat B, whose edge was oriented N.NE -S.SW, adult wood-crickets were particularly numerous in the first ten metres from the forest border. An analysis of variance shows that adult distribution depends both on sex and border orientation in quadrat A. Adult distribution was also influenced by the proximity of trees, but in a different way in males and females, and at different times of the year. Generally speaking, adult density was greater at a distance of 1 to 2 metres from the base of a tree. Individual marking of adult wood-crickets has shown that they do not generally wander for more than 10 metres from the area where there were first captured. There is a positive correlation between the distance travelled and the interval between captures. In quadrat A, adult wood-crickets apparently wandered at random, whereas they tended to move towards the forest edge on quadrat B.