Compound eyes of the dragonfly Pachidyplax longipenni. Image credit Samuel Fabian (CC BY 4.0)
Animal eyes come in many shapes and sizes, from simple eye pits lined with light receptors to complex structures with bent lenses and corneas, as well as compound eyes. Nonetheless, the underlying principle is the same: an optical system forms an image that is captured by a photoreceptor array.
Over millions of years, eyes have adapted to an organism’s lifestyle and habitat and can therefore vary not only between species but also between sexes and developmental stages. For example, fast-flying predators such as dragonflies (with compound eyes) and hawks (with simple eyes) have larger eyes with larger lenses, enabling them to resolve finer spatial detail. However, although larger eyes generally perform better, most eyes remain relatively small because they are energetically expensive organs. Thus, eye design reflects a balance between costs and benefits.
Previous studies of eye design have largely considered costs and benefits separately, overlooking an important interaction: optics and photoreceptors compete for limited resources. Greater investment in optics improves image quality and information projected onto photoreceptors, while greater investment in photoreceptors enhances the system’s ability to capture that information. This raises a key question: is this competition a significant factor in eye design?
Heras and Laughlin addressed this question by introducing a mathematical framework that links investment directly to performance. Their approach allows modelling of how information capture changes as resources shift between optics and photoreceptors.
Using mathematical models of two types of compound eyes and one simple eye, they showed that all eye types studied possess an optimal allocation of resources that maximizes information capture. They further demonstrated that the energy costs of photoreceptors play a critical role in shaping an optimized eye. Applying their model to insect compound eyes, they estimated how resources are distributed between optics and photoreceptors and found that, as predicted, insects invest efficiently by allocating more resources to photoreceptors than to optics. Such an alignment of investments to maximize efficiency thus represents a new principle of eye design.
Research on how sensory systems are adapted to an organism’s lifestyle and habitat has largely focused on benefits rather than costs. The method developed by Heras and Laughlin provides a powerful tool for understanding how cost-benefit relationships have shaped the evolution of eyes, brains and behavior. Further progress will depend on improved measurements of costs. Their work also suggests that the remarkable energy efficiency of brains may arise from a similar balancing of cost–benefit functions across their component systems.
