Central projections of gustatory receptor neurons in the medial and the lateral sensilla styloconica of Helicoverpa armigera larvae

Food selection behavior of lepidopteran larvae is predominantly governed by the activation of taste neurons present in two sensilla styloconica located on the galea of the maxilla. In this study, we present the ultrastructure of the sensilla styloconica and the central projection pattern of their associated receptor neurons in larvae of the heliothine moth, Helicoverpa armigera. By means of light microscopy and scanning electron microscopy, the previous findings of two morphologically fairly similar sensilla comprising a socketed conic tip inserted into a large peg were confirmed. However, the peg size of the medial sensillum was found to be significantly bigger than that of the lateral sensillum. The sensory neurons derived from each sensillum styloconicum were mapped separately using anterograde staining experiments combined with confocal laser-scanning microscopy. For determining the afferents' target regions relative to each other, we reconstructed the labeled axons and placed them into a common reference framework. The sensory axons from both sensilla projected via the ipsilateral maxillary nerve to the suboesophageal ganglion and further through the ipsilateral circumoesophageal connective to the brain. In the suboesophageal ganglion, the sensory projections targeted two areas of the ipsilateral maxillary neuropil, one located in the ventrolateral neuromere and the other adjacent to the neuromere midline. In the brain, the axon terminals targeted the dorso-anterior area of the ipsilateral tritocerebrum. As confirmed by the three-dimensional reconstructions, the target regions of the neural projections originating from each of the two sensilla styloconica were identical.

Comparative external ultrastructure and diffusion pathways in styloconic sensilla on the maxillary galea of larval Mamestra configurata (Walker) (Lepidoptera: Noctuidae) and five other species

The external ultrastructure of sensilla on the maxillary galea are investigated in Mamestra configurata and five other lepidopterous larvae using scanning electron microscopy. The galea and lacinia, comprising the mesal lobe of the maxilla, are either completely separate, fused, or incompletely fused in these species. The distal surface of the mesal lobe of all species examined bears two styloconic sensilla, three basiconic sensilla, and three trichoid sensilla, whereas the midventral wall of this lobe bears a campaniform sensillum. The latter sensillum is visible in only three of the six species examined. The styloconic and basiconic sensilla occupy a ventro anterior location, whereas the trichoid sensilla are positioned dorsoposteriorly. Interspecific comparisons of galeal size, as well as sensillar size, shape, and position are made for all species. The styloconic sensilla are the only sensillar type permeable to an aqueous solution of cobalt chloride when viewed by brightfield light microscopy in all species examined. Cobalt ions permeate through the terminal pore of each styloconic peg and percolate through the fenestrated fibrillar pore matrix, located directly below the pore. These ions permeate along the dendritic channel and accumulate in the adjacent sensillar sinus surrounding the peg and/or style by way of a presumably permeable dendritic sheath in all species, but to varying extents. The cuticular sidewall pores surrounding the terminal pore also appear to be permeable to cobalt ions in all the species examined. In most species examined, the styloconic sensilla are only minimally permeable to mercury ions. In these species, mercury ions permeate through the terminal pore, but become trapped within the plug of fenestrated fibrils within it. The sidewall pores are not permeable to mercury ions in any of the species examined. The styloconic sensilla are not permeable to lead ions in M. configurata or Malacosoma lutescens, the only species tested. © 1996 Wiley-Liss, Inc.