Abstract: 【Aim】This study aims to compare the difference in the external morphology and internal microstructure of compound eyes and the changes of ommatidial structure and pigment granules between a diurnal moth Phauda flammans and a nocturnal moth Spodoptera litura under natural light and completely dark conditions, so as to lay a foundation for further exploring the survival and reproductive mechanisms of diurnal and nocturnal moths based on vision.【Methods】The external morphology of the compound eyes of P. flammans and S. litura adults was observed, and the parameters such as the number of ommatidia and length of compound eyes were measured by using scanning electron microscopy. The internal tissue structure of the compound eyes of adults were observed using paraffin sectioning technology. The light- and dark-adapted states of compound eyes of adults under natural light and completely dark conditions were observed using digital microscope. 【Results】The length ［(1.67±0.05) mm］ and width［(1.57±0.02) mm］ of compound eyes and number of ommatidia (8 816.38±25.56) of S. litura adults were significantly greater than those of P. flammans adults［(0.74±0.11) mm, (0.66±0.01) mm and 820.55, respectively］. Under natural light and completely dark conditions, the compound eyes of S. litura adults underwent a significant transition in light- and dark-adapted states, while no obvious changes of compound eyes of P. flammans adults were observed. Under natural light condition, the pigment granules of P. flammans were evenly distributed on both sides of the rhabdom and crystalline cone, while the pigment granules of S. litura were distributed between the crystalline cone and the clear zone. Under completely dark condition, the pigment granules in the retinula cells of both species moved to the distal region of the ommatidia, and the pigment granules on both sides of the crystalline cone increased. Under completely dark condition, the length of crystalline cone, rhabdom and retinula cell nucleus of P. flammans significantly reduced, while the distance from retinula cell nucleus to basement membrane of P. flammans significantly increased, the width of rhabdom of S. litura increased significantly, but the other ommatidial structure of S. litura showed no significant changes, as compared with those under natural light condition. Under natural light condition, the length of ommatidia, length of crystalline cone and distance from the retinula cell nucleus to basement membrane of P. flammans were significantly smaller than those of S. litura, while the corneal thickness, length of rhabdom, and length and width of retinula cell nucleus of P. flammans were significantly larger than those of S. litura. Under completely dark condition, the length of ommatidia, length of crystalline cone and distance from retinula cell nucleus to basement membrane of P. flammans were significantly smaller than those of S. litura, while the corneal thickness, length of rhabdom, width of crystalline cone and length and width of retinula cell nucleus of P. flammans were significantly larger than those of S. litura.【Conclusion】The compound eyes of P. flammans and S. litura adults are apposition eyes and superposition eyes, respectively. Although the moving direction of pigment granules in the ommatidia of P. flammans is the same as that in S. litura under different light and dark conditions, the changes of adaptational states of compound eye, crystalline cone and rhabdom structure in the ommatidium of the two moths are different, which may be related to their different photosensitive mechanisms.

Key words:  Diurnal insect, nocturnal insect, compound eye, light/dark adaptation; microstructure, photoreceptive mechanism