Etude electrophysiologique de la croissance de l’oeil composé de Enallagma cyathigerum (Charpentier) et E. clausum Morse (Zygoptera: Coenagrionidae)

The eleclroretinogram resulting from the stimulation of the eye of larva and adult Enallagma is a slow potential characterised by a negative, monophasic wave form (peak followed by a plateau). The peak represents photoreceptor response while the plateau reflects the optic lobe responses. Recordings during the life-cycle of Enallagma demonstrate a progressive development from the optic lobe to the ommatidia. Intensity-response functions of the larva and the adult reflect their mode of life and represent physical modifications of the rhabdom during the life-cycle. About the time of emergence the intensity-response function of the dorsal and ventral regions of the eye shows desynchronisation. The ventral has characteristics of the adult while the dorsal has characteristics found at a specific maturation period. Sensitivity curves of the larva and the adult are clearly distinguishable. In the former, the principal peak is around 525-540 nm while in the latter it is around 366 nm. In addition, the adult possesses a second minor peak around 466-490 nm or 490-540 nm, depending on the species. Light and dark adaptations of larvae and adults are similar i.e. they are intensity dependent. Complete adaptation is achieved within 40 seconds; however, while the larva shows a gain of 1.0-1.5 log units, the adult shows only a gain of 0,1 log unit. Nevertheless, adults show an ’’off’response when the light stimulus is terminated. This response, which probably is neural in origin, maintains a constant level of sensitivity. — This study shows .that organisation of the compound eye in odonates is based on a "primitive” model as opposed to the "evolved" system found in Corduliidae. There are also other physiological differences. The primary role of vision, which in these insects is the capture of prey, necessitates a system well adapted for survival in water and air. Enallagma not only has a specialised compound eye but also one whose full potential can be realised rapidily.