The horsehair worm, Chordodes formosanus, is known to manipulate external morphology and internal sex organ of its mantid host, Hierodula formosana. Such alteration on antennae was gender-derived on infected males and caused feminization. The male H. formosana emerge with larger numbers of antennal sensilla by spreading them toward anterior antennal segments, while the female keep that in the nymphal form. Thus, the growth in infected males was blocked and caused juvenilization, which is thought to result in intersex and castration. Juvenilization is composed of two ideas, general juvenilization and inhibition of sex differentiation, which are related to different physiological processes but confused by the similar outcome in H. formosana. Recently, we found both sexes of H. petallifera emerged with spreading antennal sensilla toward the anterior antennal segments but the sexual dimorphism was still created by wider spread in adult males. The dimorphism was eliminated in an artificially infected male while the infected female proceeded normal growth. This result suggests parasites inhibit hosts' sex differentiation and allochthonous stimulus intervention in insect sexual differentiation.