Abstract: 【Aim】 This study aims to clarify the ultramorphology and developmental characteristics of different instar nymphs of Ornithodoros lahorensis, the dominant species in Xinjiang, northwestern China, so as to provide a morphological basis for the study of morphological characteristics and age division of soft ticks. 【Methods】 The 1st-5th instar nymphs of O. lahorensis were cultured under laboratory conditions, and their morphological characteristics were observed in detail under stereo microscope and scanning electronic microscope (SEM). 【Results】 Under stereo microscope, the body of the 1st-5th instar nymphs of O. lahorensis is oval in shape, with the narrow front end and the wide back end, pale yellow or yellowish brown, and with four pairs of legs. Under scanning electronic microscope, the 1st-5th instar nymphs have the following common characteristics: the cuticle is wrinkled with humps, their Haller’s organs show transverse fissure, their hypostomes are spear-shaped, their spiracles are semicircled, and their anus is visible as an oval structure composed of leaf-like petals and without transverse postanal groove. The differential characteristics between the 1st-5th instar nymphs are as follows: the 1st instar nymphs have long and thick setae, but have no humps, their hypostomes have obvious notched and pointed apexes and the dental formula is 2|2, without spiracular plate. The 2nd instar nymphs have no obvious humps, their hypostomes have slightly notched pointed apexes and the dental formula is 3|3, with a spiracular plate. The 3rd instar nymphs have obvious humps. The 4th instar nymphs have six pairs of anal setae. The 5th instar nymphs have seven pairs of anal setae. 【Conclusion】 The morphological structures of the 1st-5th instar nymphs of O. lahoernsis observed under stereo microscope are basically similar. The morphological characteristics of setae, humps, hypostome, dental formula, spiracles, spiracular plate, anal setae, medium postanal grooves, and preanal grooves observed under SEM can be used as the identification features of the 1st-5th instar nymphs of O. lahoernsis. This study presents the first detail descriptions of the morphological structure of different instar nymphs of O. lahoernsis, providing the basis for the instar division and life history study of soft ticks.

Key words: Ornithodoros lahorensis, nymph, morphology, ultrastructure, instar division