Simple SummaryTo further explore the host recognition mechanism of the monophagous insectAgasicles hygrophilaSelman and Vogt (Coleoptera: Chrysomelidae), scanning electron microscopy images were used to examine and compare the external morphology and distribution of the sensilla of this species in both male and female adults. The results showed that there were 12 types and 46 subtypes of sensilla, some of which displayed sexual dimorphism. We found a new type of sensor, reported here for the first time, and named it sensilla petal-shaped (Sps) based on its morphological characteristics. This sensor may be related to host recognition. In addition, the potential functions of the structure are discussed. This information extends the study of the sensorium of monophagous insects and lays a morphological foundation for the exploration of the host recognition mechanism ofA. hygrophila.AbstractAgasicles hygrophilaSelman and Vogt (Coleoptera: Chrysomelidae) is the key natural enemy ofAlternanthera philoxeroides(Mart.) Griseb, an invasive weed worldwide. To understand the morphology ofA. hygrophilaand further explore the specific host localization mechanism, scanning electron microscopy was used to observe and study the morphological characteristics of sensilla on the head appendages, tarsi, and external genital segments ofA. hygrophila. Twelve types and forty-six subtypes of sensilla were observed. These contain various types of head appendices, including sensilla chaetica, sensilla trichodea, sensilla basiconca, sensilla coeloconica, sensilla styloconica, Böhm bristles, sensilla campaniform, sensilla terminal, sensilla dome, sensilla digit-like, sensilla aperture, and many subtypes. A new type of sensor was reported for the first time, which may be related to host plant recognition. This sensor was located on the distal segment of the maxillary palps ofA. hygrophilaand was named as sensilla petal-shaped based on its morphological characteristics. Sensilla chaetica, sensilla trichodea, and sensilla basiconca are also found on the tarsi and external genital segments. In addition, sensilla basiconica 4, sensilla coeloconica 1 and 2, sensilla styloconica 2, Böhm bristles 2, and sensilla campaniform 1 were only found in females. On the contrary, sensilla styloconica 3, sensilla coeloconica 3, and sensilla dome were only found in males. Numbers and sizes of the sensilla were also different between males and females. The potential functions related to structure were discussed in comparison with previous investigations on beetles and other monophagous insects. Our results provide a microscopic morphological basis for further research on the localization and recognition mechanism ofA. hygrophilaand its obligate host.