Schematic drawing of the wind tunnel (length, 250 cm; width, 90 cm; height, 90 cm).

<p>Females were released from a platform 50(distance between sources, 20 cm) were placed at the upwind entrance to the wind tunnel. These consisted of filter papers loaded with synthetic flower odors. Headspace volatiles from non-flowering plants placed in a glass cylinder outside the tunnel were released close to the source of flower volatiles.</p

Attraction of <i>M. sexta</i> females to plant and flower odors.

<p>(a) No-choice experiment: Percentage of moths that flew upwind towards the presented odor source (duf) and reached the source with extended proboscis (sc). (b) Two-choice experiment with two single odor sources presented in the wind tunnel (20 cm apart). (b-i) Number of first source contacts. (b-ii) Total number of approaches per moth within 5 min. (c) Two-choice experiment, presenting a single flower blend stimulus and a combined flower and plant odor. (c-i) Number of first source contacts. (c-ii) Total number of approaches per moth within 5 min. Error bars depict the standard deviation.</p

Host Plant Odors Represent Immiscible Information Entities - Blend Composition and Concentration Matter in Hawkmoths

<div><p>Host plant choice is of vital importance for egg laying herbivorous insects that do not exhibit brood care. Several aspects, including palatability, nutritional quality and predation risk, have been found to modulate host preference. Olfactory cues are thought to enable host location. However, experimental data on odor features that allow choosing among alternative hosts while still in flight are not available. It has previously been shown that <i>M. sexta</i> females prefer <i>Datura wrightii</i> compared to <i>Nicotiana attenuata</i>. The bouquet of the latter is more intense and contains compounds typically emitted by plants after feeding-damage to attract the herbivore’s enemies. In this wind tunnel study, we offered female gravid hawkmoths (<i>Manduca sexta</i>) odors from these two ecologically relevant, attractive, non-flowering host species. <i>M. sexta</i> females preferred surrogate leaves scented with vegetative odors form both host species to unscented control leaves. Given a choice between species, females preferred the odor bouquet emitted by <i>D. wrightii</i> to that of <i>N. attenuata</i>. Harmonizing, i.e. adjusting, volatile intensity to similar levels did not abolish but significantly weakened this preference. Superimposing, i.e. mixing, the highly attractive headspaces of both species, however, abolished discrimination between scented and non-scented surrogate leaves. Beyond ascertaining the role of blend composition in host plant choice, our results raise the following hypotheses. (i) The odor of a host species is perceived as a discrete odor ‘Gestalt’, and its core properties are lost upon mixing two attractive scents (ii). Stimulus intensity is a secondary feature affecting olfactory-based host choice (iii). Constitutively smelling like a plant that is attracting herbivore enemies may be part of a plant’s strategy to avoid herbivory where alternative hosts are available to the herbivore.</p> </div

Effects of host blend composition and intensity on host choice in <i>M. sexta</i>.

<p>(A) Choice experiments with gravid <i>M. sexta</i> females were performed in a wind tunnel. Plants were placed in glass boxes outside the wind tunnel where they could not be seen by the moths. Pumps delivered plant headspace to two surrogate leaves serving as visual stimuli inside the wind tunnel. Two host plants, <i>D. wrightii</i> and <i>N. attenuata</i>, were tested (I) against a clean air control, (II) with their plant headspaces mixed together 1:1 against a clean air control, (III) against a conspecific plant whose headspace was diluted with clean air, and (IV) against each other, with <i>N. attenuata</i> headspace either not manipulated or diluted with clean air. Plant headspace and clean air were mixed in a 1:4 (vol/vol) ratio resulting in a 5-fold dilution. (B) The percentage of first choices made in the corresponding experiments. Sample size is given next to each experiment. Asterisks denote significant differences between sources (Binomial Test, *** p<0.001; ** p<0.01; *p<0.05). (C) Boxplots depict preference indices calculated from the number of contacts to each source. Values close to 1/-1 represent a high preference for one source; 0 means no preference. The black line delineates the median; color distribution within the box represents the percentage of contacts to each source. Asterisks above the boxes denote indices significantly different from 0 (Wilcoxon Signed Ranks Test, *** p<0.001; ** p<0.01; * p<0.05). Preference indices resulting from experiments in which the plant headspace of both species is offered superimposed or separately against clean air differed significantly (Kruskal-Wallis Test, p<0.0001, and Dunn’s post hoc test, ** p<0.01, * p<0.05). Furthermore, preference indices derived from interspecific choice experiments were significantly different from each other (Mann-Whitney U Test, p<0.05).</p

Hypothesized function, abundance external morphology and dendritic structure (ODS: outer dendritic segment, TB: tubular body) of sensilla on antennae (A) and maxillary (M) and labial palps (L) in <i>Melolontha melolontha</i> larvae.

<p>Hypothesized function, abundance external morphology and dendritic structure (ODS: outer dendritic segment, TB: tubular body) of sensilla on antennae (A) and maxillary (M) and labial palps (L) in <i>Melolontha melolontha</i> larvae.</p

Structure of the pore-like openings and support cells of the antennal pore plates of <i>M. melolontha</i> larvae.

<p>A, B: SEM. A: Here the pore-like openings are plugged. Note small dark spots spread over the surface. B: Higher magnification of a plug within a pore-like opening. C–J: TEM. C: Longitudinal section of a pore-like opening. Although the pore-plate cuticle is fully ruptured by the hour-glass-like duct, its outer half seems to be sealed. D: In this oblique section the duct appears somewhat oval. E: Dendritic branches project into the inner half of the duct. F: This section shows a cuticular protrusion in the duct. G: This protrusion extends as a cuticular thread between the dendritic branches. H: The epidermal support cells have punctual contacts with the pore-plate cuticle. This separates adjacent areas with dendritic branches. I: Mitochondria and electron-dense material are concentrated in the contact areas of the support cells. J: Desmosome-like densities can be observed in the apical membranes of the support cells. Abbr.: cT, cuticular thread; Cu, cuticle; dB, dendritic branch; De, desmosome; Mi, mitochondrion; SC, support cell.</p

Mean EAG and EPG amplitudes for recordings on antennae

<p>(<b>blue bars</b>)<b>, maxillary</b> (<b>pink bars</b>) <b>and labial palps</b> (<b>green bars</b>) <b>from third instar </b><b><i>M. melolontha</i></b><b> larvae whole-body mounts</b> (<b>n = 15 replicates on 6 animals</b> (<b>1–3 per animal</b>))<b>.</b> Response to respective controls (empty pipette, DCM, dist. water and DCM supplemented by 20% water) has been subtracted. The grey bars behind colored bars display gross responses without solvent correction. Asterisks indicate significantly higher responses to the tested compound than to respective solvents (Welch two sample t-tests with sqrt transformed data). Significance levels: *** at p<0.001; ** at p<0.01 and * at p<0.05. A: alcohols, ketones and aldehydes at a concentration of 1µg/µl in DCM. B: Undiluted compounds, stimulation with empty pipette and CO₂, C: Amines, esters at a concentration of 1µg/µl in DCM and acids at the same concentration in DCM supplemented by 20% dist. water. D: Monoterpenes, sesquiterpenes and solvents at a concentration of 1µg/µl in DCM.</p

S11 and S12 sensilla of the palps of <i>M. melolontha</i> larvae.

<p>A–B: SEM. A: S11 sensillum with a pointed tip on a maxillary palp. Dotted lines indicate approximate cutting planes of transverse sections shown in Figures C–F. B: S11 sensillum with a blunt tip from a different maxillary palp. C–G: TEM. C: Oblique section of the sensillum tip. Note the massive cuticle and sparse lumen. D: This section represents the middle portion of the shaft. A lumen is visible, but it is empty. E: Oblique section of the area where the shaft merges in the flexible cuticle of the socket. Note the minute lumen of the shaft. F: A little deeper inside the socket, a thick dendritic sheath with a single tubular body, attached to the flexible cuticle, becomes visible. G: Below the socket only one large ensheathed outer dendritic segment can be found. Note that the number of radial folds of the dendritic sheath changes in different section levels (see inset). H, I: SEM. H: Slightly bent S12 sensillum from the labial palp, bearing a subterminal pore opening (arrowhead). Dotted lines indicate approximate cutting planes of transverse sections shown in Figures J–L. I: The subterminal pore (arrowhead) of this S12 sensillum from a different labial palp opens much closer to the apex (cp. Figure H). J–M: TEM. J: Lamellate dendritic profiles are present in the apical part of the sensillum. K: In this section only two dendritic profiles are visible. L: Shortly above the socket only one dendrite remains inside the dendritic sheath. M: This single dendrite can also be found deeply below the sensillum socket. Abbr.: Cu, cuticle; dB, dendritic branches; dS, dendritic sheath; fCu, flexible cuticle; Mi, mitochondrion; Mv, microvilli; oD, outer dendritic segment; tB, tubular body; tP, terminal pore.</p

S5, S6 and S7 sensilla of the antennae of <i>M. melolontha</i> larvae.

<p>A–D: SEM. A: Lateral view on the egg-shaped S5 sensillum. Note the large, circular socket. B: Higher magnification of the tip of a S5 sensillum. Finger-like cuticular projections surround a terminal pore. C: Lateral view on a short, blunt S6 sensillum. D: S7 sensillum with a short, conical, bent shaft. Its tip seems to be damaged. The dotted line indicates the approximate cutting plane of the transverse sections shown in Figure E. E: Oblique transverse section of the S7 sensillum. The cuticle of the sensillum is penetrated by numerous pores which connect the outside with the lumen, where outer dendritic segments are present. Note the minute pore openings (arrowhead). Abbr.: Cu, cuticle; oD, outer dendritic segment; Po, pore; tP, terminal pore.</p

S10 sensillum of the palps of <i>M. melolontha</i> larvae.

<p>A: SEM. S10 sensillum from the maxillary palp. The surface is slightly sculptured. Dotted lines indicate approximate cutting planes of transverse sections shown in Figures B, D and G. B–K: TEM. B: Oblique transverse section of the apical part of the shaft. The cuticle is porous and the wide lumen is sparsely filled with thin dendritic branches. C: Bundles of short pore tubules are directed towards the lumen of the sensillum. The pore openings (arrowheads) on the surface of the sensillum are very small. D: Oblique transverse section of the basal part of the shaft, where the porous part of the cuticle merges in an non-porous part. Note an inflated outer dendritic segment. E: Small dendritic branches and the large inflated dendritic segment come in close contact with the pore tubles. F: Several dendritic branching points (arrowheads) are visible in this section. G: Oblique section of the socket. H: Magnification of the 18 dendritic segments shown in Figure G. Only few, loosely arranged electron-dense remnants of a dendritic sheath are present. I: This further posterior section shows 10 outer dendritic segments embedded in a matrix of dendritic sheath material. J: Four large outer dendritic segments are present below the socket. K: Finally, only two outer dendritic segments represent the entire innervation of the S10 sensillum. Abbr.: Cu, cuticle; dB, dendritic branches; dS, dendritic sheath; oD, outer dendritic segment; Po, pore; pT, pore tubules; RLy, receptor lymph; toC, tormogen cell.</p