Central Projections of Gustatory Receptor Neurons in the Medial and the Lateral Sensilla Styloconica of <i>Helicoverpa armigera</i> Larvae

<div><p>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, <i>Helicoverpa armigera</i>. 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.</p></div

The brain and the suboesophageal ganglion of the fifth instar larvae of <i>H. armigera</i>.

<p>(A) Diagram of the larval head including the brain (Br) and the suboesophageal ganglion (SOG). (B) Confocal image of the brain showing the cell body layer and the central neuropils. (C) Confocal image of the SOG showing the cell body layer (CBL) and the three fused neuromeres, from anterior to posterior: the mandibular neuromere (MnNe), the maxillary neuromere (MxNe), and the labial neuromere (LbNe). (D) 3D reconstruction of the whole brain. (E) 3D reconstruction of the SOG. A anterior, D dorsal, L lateral, M medial, P posterior, V ventral. Oe oesophagus, CC circumoesophageal connectives, MxN maxillary nerve, LbN labial nerve, MnN mandibular nerve, TC tritocerebrum, PC protocerebrum, Ca calyces of the mushroom body, AL antennal lobe, CB central body, Lo lobles, Ne neuromere. Scale bars: 250 µm in A, 100 µm in B, C, D, and E.</p

Morphologies of the two sensilla styloconica in the fifth instar larvae of <i>H. armigera</i>.

<p>(A) Ventral view of the head showing a pair of antennae (An), the labrum (Lr), the mandible (Mn), the labium (La), the maxillary palps (MP) on the maxilla, and the spinneret (Sp). (B) Close-up view showing the two sensilla styloconica, i.e. the lateral sensillum styloconicum (Ls) and the medial sensillum styloconicum (Ms) located on the maxilla (Ma). (C) Close-up view of the medial and the lateral sensillum styloconicum located on one galea (G). (D) Near view showing the socketed conic tip (Co) inserted into the large peg of a sensillum styloconicum. Scale bars: 500 µm in A, 200 µm in B, 50 µm in C, and 5 µm in D.</p

Images of the larval suboesophageal ganglion and brain with stained afferents originating from the medial sensillum.

<p>(A, B) Confocal images from two preparations showing axon projections in the suboesophageal ganglion (SOG). In A, a preparation containing two labeled axon is presented, and in B, another preparation containing three stained fibers. As indicated by arrows in both images, the axons entered the SOG via the maxillary nerve (MxN). In B, the arrow to the left points to the circumoesophageal connective (CC). (B’-B’’) High resolution images of axon terminals in the SOG can be seen (same preparation as in B). In B’, the target region of the terminals, i.e. the maxillary neuromerebeing situated between the dotted lines is shown. In B’’, the detailed pattern of the axon terminals giving off processes in the ventro-lateral (left arrow) and the dorso-medial (right arrow) neuromere are shown. The arrowhead points to a few arborizations crossing the midline and the asterisk indicates a tract in the SOG, which was not stained by the synapsin antibody. (B’’’) 3D AMIRA-reconstruction of the SOG including target regions (indicated by red) of axons originating from the medial sensillum (arrows; made from the preparation shown in B-B’’). The stained axons in the MxN and the CC are indicated by single lines in the AMIRA model. (C-C’) Confocal images showing the axon terminals in the brain (same preparation as presented in B-B’’). In C, the arrow points to the innervated brain region, i.e. the tritocerebrum (TC). In C’, the stained axon terminals in the TC are shown in a closer view. (C’’) 3D AMIRA reconstruction of the TC including the target region (arrow) of the stained axons (made from the preparation presented in C-C’). The stained axons entering the TC are indicated by one single line in the AMIRA model. AL antennal lobe, Ca calyces, PC protocerebrum. A anterior, D dorsal, L lateral, M medial, P posterior, V ventral. Scale bars: 100 µm in A, B, B’, B’’’, C, and C’’; 50 µm in B’’ and C’.</p