La Fecondité de Baetis rhodani Pictet

Résume La fécondité de Baetis rhodani Pictet a étéétudiée sur tout un cycle saisonnier dans le lissurage, ruisseau du Sud‐Ouest de la France. La variation de la fécondité suit celle de la taille; les femelless les plus grandes sont les plus fécondes et pondent les plus gros oeufs. Le maximum est atteint en hiver; le minimum estival résulte à la fois d'une diminution du nombre d'ovarioles et d'ovules produits par ovariole. Des lignes de régression ont étéétablies entre la fécondité et plusieurs mensurations larvaires et adults, elles ont permis de calculer chaque mois le potentiel de reproduction de la population d'après des prélèvements quantitatifs de larves au dernier stade. Une relation nombre d'oeufs pondus‐‐‐numbre de larves au dernier stade qui en sont issues a fourni a base d'une estimation de la production de la génération printanière en utilisant la méthode de la courbe d'Allen. La part prise par les jeunes stades non capturables dans la production de cette cohorte a été calculée è partir de cetteestimation. Summary The fecundity of Baetis rhodani throughout a seasonal cycle is measured and the production by the spring generation is calculated from the figures obtained. Females of B. rhodani cannot be identified with certainty and therefore they were not cauht wild but bred from mature nymphs in the laboratory. Various measurements were made(Fig. 1). Eggs range in number from 4500 in February and March to 200 per female in July. This decrease coincides with a diminution in size and a rise in temprature (Fig. 2). In summer there are about 200 fewer ovarioles per female and production per ovariole is about half what it is early in the year. Smaller females produce not only fewer eggs but also smaller eggs (Fig. 3). The relation between certain dimensions and number of eggs is found to remain close (Table 4), and it is therefore possible to predict how many eggs will be laid per square metre from a quantitative sample of full‐grown nymphs that have been measured (Table 6). Continuous sampling and measuring therefore make it possible to calculate the number of eggs at the beginning of a given generation and the number of adults that ultimately emerge. Calculation of production is based on the assumption that mortality follows the Allen curve (Fig. 10). It is shown that the very small nymphs, ignored in many other calculations of production could contribute up 18% to the total production of the spring generation.