Comparison of Fine Structure of the Compound Eyes in Eucryptorrhynchus scrobiculatus and Eucryptorrhynchus brandti Adults

SIMPLE SUMMARY: Eucryptorrhynchus scrobiculatus and its related species Eucryptorrhynchus brandti together damage Ailanthus altissima and Ailanthus altissima ‘Qiantou’. E. scrobiculatus possesses a large compound eye area and a higher number of ommatidia than E. brandti. Each ommatidium of E. scrobiculatus and E. brandti consists of a cornea, a crystalline cone, eight retinal cells, and its semi-fused rhabdom. The internal structure, including the cornea and rhabdom, of E. scrobiculatus is larger than that of E. brandti. Light/dark adaptational changes affect cone length, the position of pigment grains, and the cross-sectional area of the rhabdoms. ABSTRACT: Eucryptorrhynchus scrobiculatus and E. brandti are the main borers of Ailanthus altissima, causing serious economic and ecological losses. The external morphology and internal ultrastructure of the compound eyes of two related weevils were investigated with light microscopy, scanning electron microscopy, and transmission electron microscopy. E. scrobiculatus and E. brandti possess a pair of reniform apposition compound eyes and contain about 550 ommatidia per eye. The interommatidial angle of E. scrobiculatus and E. brandti are 7.08 ± 0.31° and 4.84 ± 0.49°, respectively. The corneal thickness, rhabdom length, and ommatidium length of E. scrobiculatus are significantly greater than those of E. brandti. Under light-adapted conditions, the pigment granules are mainly distributed at the junction of the cone and the rhabdom, and the diameter and the cross-sectional area of the middle end of the rhabdom is increased in the two weevil species. Under dark-adapted conditions, the pigment granules shift longitudinally and are evenly distributed on both sides of the cone and the rhabdom, and the diameter and cross-sectional area of the middle end of the rhabdom are decreased. The discrepancy in visual structure is beneficial for adaptation to niche differentiation of the two related species. The present results suggest that the two weevils possess different visual organ structures to perceive visual information in the external environment.