Ultrastructural and light/dark adaptational characteristics of the compound eyes in the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

Abstract

The fall armyworm, Spodoptera frugiperda, is a highly destructive agricultural pest native to the Americas, becoming a major invasive species worldwide over the past decade. In this study, the ultrastructure of the compound eyes and light/dark adaptational changes in S. frugiperda were investigated using light and transmission as well as scanning electron microscopy. The compound eyes of S. frugiperda are of the superposition type, featuring a clear zone. Each ommatidium contains eight retinula cells, seven of which extend through the clear zone to the basal lamina, while one cell is located near the basal lamina. The clear zone is longer in dark-adapted eyes than in light-adapted eyes. In dark-adapted eyes, the rhabdoms extend through the clear zone, with their distal ends connecting to the crystalline cones. In light-adapted eyes, however, the rhabdoms do not reach the distal region of the clear zone but are instead confined to the proximal level of the clear zone. Although the rhabdom occupation ratio to the retinula remains constant under both light and dark adaptation, the cross-sectional area of the rhabdoms and their associated retinulae is significantly larger under dark adaptation. These ultrastructural and adaptational characteristics were discussed in the context of the moth's activity preferences, particularly its nocturnal behavior.