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

Abstract

Lepidopteran larvae possess two pairs of styloconic sensilla located on the maxillary galea. These sensilla, namely the lateral and medial styloconic sensilla, are each comprised 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 approximately 70 um in length and 30 um 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 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 cell. The neurons are ensheathed successively by the inner, intermediate, and outer sheath cells. Keywords sensillum; gustation; gypsy moth; ultrastructure; styloconic Gypsy moth larvae, Lymantria dispar (L.) are major pest defoliators in most of the United States and destroy millions of acres of trees annually. This larva is highly polyphagous and displays a wide host plant preference, feeding on the foliage of hundreds of plants, but favoring leaves of deciduous hardwood trees, such as oak, maple, and sweet gum (Mosher 1915; Liebhold et al. 1995;Shields et al. 2003).Lepidopteran larvae, such as gypsy moth larvae, possess gustatory and olfactory chemosensilla located on the antennae, maxillary palps, and epipharynx, as well as a pair of styloconic sensilla (lateral and medial) on each maxillary galea. Food plant recognition is predominantly governed by the activity of the styloconic sensilla (Schoonhoven 1972). In the literature, the lateral and medial styloconic sensilla have been referred to as "large cones" (Forbes 1910), "sensilla styloconica" and "Z2" and "Z1" (Schoonhoven and Dethier (1966), "sensilla styloconica" and Ss-I" and "Ss-II (Ishikawa 1967), "sensilla styloconica" and " SsII" and SsI (Ma, 1972); "uniporous pegs" and "LST" and "MST" (Albert 1980); "uniporous pegs" and "l" and Vonnie D.C. Shields, Biological Sciences Dept., Towson University, 8000 York Road, Towson, MD 21252, Phone: 410-704-3130, Fax: 410-704-2405, vshields@towson "m" (Devitt and Smith 1982), and "sensilla styloconica" and "LSS" and "MSS" (Grimes and Neunzig 1986).The styloconic sensilla are considered to be bimoda...

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

“…Generally, their morphology including a socketed conic tip inserted into the large peg was similar to that of corresponding sensilla formerly described in other lepidopteran species [5], [7], [9]–[12], [14], [15], [17], [18]. The significant difference in size between the pegs of the two sensilla styloconica, as found here, has not been reported previously, however.…”

Characteristics of morphology, electrophysiology, and central projections of two sensilla styloconica in Helicoverpa assulta larvae

“…Also, the characteristic arrangement of the socketed conic tip inserted into the large peg is in correspondence to previous findings in other lepidopterous caterpillars [5,[22][23][24][25][26][27][28][29], indicating a conservative morphology of sensilla styloconica over lepidopterous species. Interestingly, the different size of the two sensilla in H. assulta, the medial being significantly longer than the lateral, is in accordance with our previous findings on H. armigera [14].…”

“…Both light and electron microscopy have shown that each sensillum styloconica is innervated by five bipolar neurons, three or four of which function as gustatory receptor neurons (GRNs) and one as a mechanosensitive neuron [5][6][7]. The GRNs in lepidopterous insects have been reported to detect distinct stimuli such as nutrients, stimulants, and deterrents, and to show different response patterns [5,[8][9][10][11].…”

Comparisons of Contact Chemoreception and Food Acceptance by Larvae of Polyphagous Helicoverpa armigera and Oligophagous Bombyx mori

“…The two sensilla styloconica seem to play a particularly important role in acceptance of host plants (Schoonhoven 1987). Each styloconic sensillum houses four gustatory receptor neurons (GRNs) with specific response spectra for plant compounds (summarized by Schoonhoven and Van Loon 2002; Shields 2009). Typically, some of the neurons respond to phagostimulants, primary plant metabolites like sugars and amino acids that evoke feeding.…”