Étude de la variation intra-stade au cours du développement larvaire de Enallagma vernale Gloyd (Zygoptera: Coenagrionidae): discussion sur le mécanisme de différentiation des types de développement

Data were obtained from a laboratory rearing which allowed the study of each larva individually, from hatching to emergence, in a controlled environment. These were used to analyse variation occurring in larval growth. Three different growth patterns in E. vernale were observed, each characterized by a particular number of instats (i.e. 12, 13 and 14 instars). Five variables were considered for this study: the head width and the lengths of the mentum, the metathoracic femora, the dorsal gill and the mesothoracic wingpads. By means of principal component analysis, size variation (first principal axis of each covariance matrix of larval dimensions) is compared to shape variation (described in the other principal axes); the correspondence of principal axes with identifiable trends of variation is stressed. The relative importance of size and shape variation during development is analysed for each growth pattern observed. A mechanism by which different growth patterns are initiated is suggested and allometry equations are discussed accordingly. The adaptative value of this mechanism is indicated.