Hamann’s organ in Leptodirus hohenwarti a highly specialized cave Bathysciinae, has been studied under the TEM, SEM and light microscope. This receptor organ located in the 7th, 9th and 10th antennal articles and previously referred to as the “vesicule olfactive” and as the “antennal organ” or “antennal vesicle”, reaches its highest degree of structural complexity in leptodirus. This paper attempts to establish some degree of synonymy among the terms used by earlier authors in describing the various antennal parts and sensilla. Five types of sensilla to be found in the organ are described, namely cribrose-stick sensilla, cribrose-utricular sensilla, star-shaped sensilla, claviform sensilla and branching setae. Comparisons within Bathysciinae species and among the latter and other subfamilies of Catopidae reveal differences in the number of vesicles and in the number and structures of sensilla, these differences appear to depend on i) the degree of phylogenetic relationships among taxa and 2) the degree of specialization to cave environment. The considerable complexity of Hamann’s organ, unrivalled by other insects organs, apart from light receptors, suggests that it has a plurality of functions. Its hygroreceptor role, supported by recent experimental work, is discussed here.