Ultrastructure of the dung-beetle Johnston's organ

The enclosed TEM images were taken of the pedicel of the antenna of Kheper lamarcki – a South African, ball-rolling dung-beetle. These straight-line orientating beetles have been shown to use wind as a compass cue. Therefore, we hypothesised that the pedicel of these beetles contains the chordotonal Johnston’s organ, a mechanosensor known to support wind orientation. The pedicel of two dung-beetles were studied (LJ1.1. and LJ3.1., distinguished in the file names), at three levels of depth (denoted by I, II, or III in the file names). The samples were fixated in a modified Karnovsky fixative, post-fixated in osmium tetroxide, and embedded in AGAR. The embedded samples were then sectioned into 55-70 nm sections and stained with uranyl acetate and Reynold’s lead citrate. The images were viewed in ImageJ2 (v. 2.16.0). Indeed, the micrographs confirm the presence of the stretch receptive Johnston’s organ. The data reveals that the beetles’ Johnston’s organ is heterodynal and amphinematic, and its individual sensory units are arranged in approximately 110 clusters, with 1-5 units per cluster. Each sensory unit contains three sensory neurons, two of which display Type 1 cilia, and one with a Type 2 cilium.