Fine Structure of the Galeal Styloconic Sensilla of Larval Lymantria dispar (Lepidoptera: Lymantriidae)

Lepidopteran larvae possess two pairs of styloconic sensilla located on the maxillary galea. These sensilla, namely, the lateral and medial styloconic sensilla, are each composed of a smaller cone, which is inserted into a style. They are thought to play an important role in host plant selection and are the main organs involved in feeding. Ultrastructural examination of these sensilla of fifth instar Lymantria dispar (L.) larvae reveal that they are each ≈70 µm in length and 30 µm in width. Each sensillum consists of a single sensory peg inserted into the socket of a large style. Each peg bears a slightly subapical terminal pore averaging 317 nm in lateral sensilla and 179 nm in medial sensilla. Each sensillum houses five bipolar neurons. The proximal dendritic segment of each neuron gives rise to an unbranched distal dendritic segment. Four of these dendrites terminate near the tip of the sensillum below the pore and bear ultrastructural features consistent with contact chemosensilla. The fifth distal dendrite terminates near the base of the peg and bears ultrastructural features consistent with mechanosensilla. Thus, these sensilla each bear a bimodal chemo-mechanosensory function. The distal dendrites lie within the dendritic channel and are enclosed by a dendritic sheath. The intermediate and outer sheath cells enclose a large sensillar sinus, whereas the smaller ciliary sinus is enclosed by the inner sheath cell. The neurons are ensheathed successively by the inner, intermediate, and outer sheath cells.