From an apposition-like stage to a superposition eye: developmental assembly of the nocturnal visual system in the diamondback moth Plutella xylostella.

Superposition compound eyes are a hallmark of many nocturnal beetles and moths and represent a major evolutionary innovation for dim-light. Although their optical organization has been extensively documented, the developmental processes underlying their assembly remain poorly understood. Here, we traced the morphogenesis of the compound eyes in the diamondback moth Plutella xylostella from the prepupal stage to adult emergence using transmission electron microscopy. Early ommatidia adopt an apposition-like arrangement, with crystalline cones in direct contact with distal retinulae. The retinulae subsequently undergo coordinated proximal displacement, while secondary pigment cells expand into the intervening space, forming a pigment-free clear zone. The tracheal tapetum develops via distal extension of numerous tracheoles between adjacent retinulae, developing in close synchrony with the formation of the clear zone. The rhabdom elongation proceeds in a distinct distal-to-proximal sequence, and corneal nipples appear prior to deposition of fibrous corneal lamellae. These observations demonstrate that the superposition eyes of P. xylostella arise through stepwise modification of an apposition-like developmental framework rather than through the formation of entirely novel structures, supporting the view that evolutionary innovation in insect compound eyes primarily results from modification of conserved developmental programs.