Berekening van de maturatieperiode van de Zwarte heidelibel (Sympetrum danae)

The maturation period of Sympetrum danae was studied over a period of three years and calculated using the statistical model that Underhill & Zuchinni (1986) used to describe primary moult in birds. The development of the pterostigma colour was used as a measure for maturation. During the maturation period the pterostigma colour changes from white to intense black. To visualize this process the stages of the maturation were estimated in the field as follows: • stage "fresh": the colour of the pterostigma is practically white and the body is more or less pale; • stage "immature": the colour of the pterostigma is not white anymore but also not completely dark; • stage "mature”: the colour of the pterostigma is black. In this manner the number of dragonflies in each stage is counted. As a result of this model the duration τ in days of the maturation period (stage "immature”) can be calculated. In addition two other parameters are calculated i.e. the mean starting date μ and the standard deviation σ of the starting dates. In this model it is assumed that the starting dates have a Gaussian distribution N[μ, σ]. These parameters were calculated using a computer programme. The duration of the maturation period in the year 2007 stands out with respect to the year 2006. 2005 appeared to be a more average year. The duration of the maturation period of S. danae appears to depend strongly on weather conditions (Table 2). In 2005 the duration of the maturation period was seventeen days, which is in accordance with the duration mentioned in Nederlandse Vereniging voor Libellenstudie (2002). In years with more extreme weather conditions the duration of the maturation period may differ by as much as a factor three.