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For eyes to maintain optimal focus, precise coordination is required between lens optics and retina position, a mechanism that in vertebrates is governed by genetics, visual feedback, and possibly intraocular pressure (IOP). While the underlying processes have been intensely studied in vertebrates, they remain elusive in arthropods, though visual feedback may be unimportant. How do arthropod eyes remain functional while undergoing substantial growth? Here, we test whether a common physiological process, osmoregulation, could regulate growth in the sophisticated camera-type eyes of the predatory larvae of Thermonectus marmoratus diving beetles. Upon molting, their eye-tubes elongate in less than an hour, and osmotic pressure measurements reveal that this growth is preceded by a transient increase in hemolymph osmotic pressure. Histological evaluation of support cells that determine the lens-to-retina spacing, reveals swelling rather than the addition of new cells. In addition, treating larvae with hyperosmotic media post-molt leads to far-sighted (hyperopic) eyes as expected from a failure of proper lengthening of the eye tube, and results in impaired hunting success. This study suggests that osmoregulation could be of ubiquitous importance for properly focused eyes.