Chang-Whan Kim

Differences in degeneration of the apical scolopidial organ in the labial palp of Lepidoptera during pupal development

SummaryThe apical scolopidial organ (ASO) in the labial palp of six species from four families of Lepidoptera was studied in pupal and imaginal stages using electron microscopy. The organ houses three sensory units, each of which consists of one sensory cell and two enveloping cells at early pupal stage in all the species studied. The distal part of the ASO is connected with the epidermis of the tip of the labial palp. Proximally it is attached to the primordium of the palpal nerve. The axons of the sensory cells run within this nerve to the central nervous system. There are two main differences in the differentiation of the ASO in the species examined during postembryonic development: (1) the sensory cells of the ASO degenerate at different rates; and (2) the ASO may or may not change its position within the palp. In Pieris brassicae and Pieris napi (Pieridae), all three sensory cells undergo stepwise degeneration. Consequently, no sensory cells are left in the imago in these species. However, in the Rhodogastria sp. (Arctiidae), only one sensory cell of the ASO degenerates during pupal life. Two remain, therefore, in the imaginal stage. Their dendritic outer segments and axons are normal, and their appearance does not differ from that in early pupal life. The same process was also observed in Rhodogastria bubo (Arctiidae), Autographa gamma (Noctuidae) and Aglais urticae (Nymphalidae). In addition to the degeneration of the sensory cells the ASO turns through about 180° in P. brassicae and P. napi so that its tip points to the base of the palp in the imagines of these species.

Axonal pathfinding in developing labial palps of the butterflies, Pieris rapae and Pieris brassicae

SummaryElectron microscopy was used to study the developmental changes in the pupal labial palp of Pieris rapae L. and P. brassicae L. prior to axogenesis of the peripheral receptor organs. Two cells emigrate from the anlage of the labial palp-pit organ (LPPO). They develop growth cones and filopodia, which are directed distally. The filopodia insert into stem cells of the LPPO. The perikarya of the cells proceed into the hemolymph space of the palp such that, eventually, these cells bridge the gap between the LPPO and the apical scolopidial organ (ASO) of the labial palp. This bridge is established at about 11% of total pupal development. Only 6 h later, at 15% of pupal development, the cells are no longer visible. We suggest that these cells are luminal neurons and name them pit-organ luminal (POL) cells. These POL cells may act as primary guiding structures for the axons of the sensory cells of the LPPO, which are assumed to orientate along this structure once they reach the ASO.