Abstract: 【Aim】 To characterize the morphology of the labial palp and the labial-palp pit organ (LPO), including the morphology, type and distribution of LPO sensilla in the oriental armyworm, Mythimna separata Walker. 【Methods】 Light and scanning electron microscopy was used to observe the labial palp, the LPO and the LPO sensilla of M. separata adults. 【Results】 The results showed that the labial palp of M. separata adult is a cylindrical appendage with three segments. Each segment varies in length and the 2nd one is the longest. The labial palp shows sexual dimorphism, and in females its length is 2 463.50±143.65 μm, while in males its length is 2 566.11±70.28 μm (t=2.722, df=34, P=0.012). The pit organ located on the 3rd segment of the labial palp is about 280 μm deep, 50 μm wide at the opening and 32 μm wide inside the pit, with no significant differences between females and males. Two sensillum types were found in both sexes, i.e., hair-shaped and club-shaped sensilla, which are located on the upper and lower of the pit, respectively. The hair-shaped sensilla are significantly shorter in females (18.20±0.84 μm) than in males (21.24 ± 0.34 μm) (t=3.335, df=30, P=0.003). The other main type, the club-shaped sensilla, on the other hand, are significantly longer in females (14.69±0.48 μm) than in males (12.31±0.49 μm) (t=3.452, df=21, P=0.002). 【Conclusion】 The LPO sensilla of M. separata belong to two morphological types, both showing sexual dimorphism in length. The results of this study about the morphological properties of the LPO sensilla provide basic knowledge for further investigation on their physiology and function. 
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Key words: Mythimnaseparata, labial palp, labial-palp pit organ, sensilla, carbon dioxide sensilla, sexual dimorphism