Rapid and step-wise eye growth in molting diving beetle larvae.

However complex a visual system is, the size (and growth rate) of all its components-lens, retina and nervous system-must be precisely tuned to each other for the system to be functional. As organisms grow, their eyes must be able to achieve and maintain emmetropia, a state in which photoreceptors receive sharp images of objects that are at infinity. While there has been ample research into how vertebrates coordinate eyes growth, this has never been addressed in arthropods with camera eyes, which tend to grow dramatically and typically in a step-wise manner with each molt (ecdysis). Here, we used histological and optical methods to measure how the larval eyes of Sunburst Diving Beetles (Thermonectus marmoratus, Coleoptera, Dytiscidae) grow, and how well optical and morphological parameters match, during the dramatic growth that occurs between two consecutive larval stages. We find that the eye tubes of the principal eyes of T. marmoratus grow substantially around molt, with the vitreous-like crystalline cone contributing the most to the overall growth. Lenses also reform relatively quickly, undergoing a period of dysfunction and then regaining the ability to project sharp images onto the retina around 8 h post-molt.