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The insect olfactory system is challenged to decipher valid signals from among an assortment of chemical cues present in the airborne environment. In the moth, Heliothis virescens, males rely upon detection and discrimination of a unique blend of components in the female sex pheromone to locate mates. The effect of variable odor mixtures was used to examine physiological responses from neurons within sensilla on the moth antenna sensitive to female sex pheromone components. Increasing concentrations of heliothine sex pheromone components applied in concert with the cognate stimulus for each neuronal type resulted in mixture suppression of activity, except for one odorant combination where mixture enhancement was apparent. Olfactory receptor neuron (ORN) responses were compared between moths with intact and transected antennal nerves to determine whether specific instances of suppression might be influenced by central mechanisms. Type A sensilla showed little variation in response between transected and intact preparations; however, recordings from type B sensilla with transected antennal nerves exhibited reduced mixture suppression. Testing by parallel stimulation of distal antennal segments while recording and stimulating proximal segments dismissed the possibility of interneuronal or ephaptic effects upon sensillar responses. The results indicate that increasing concentrations of "noncognate" odorants in an odor mixture or antennal nerve transection can produce variation in the intensity and temporal dynamics of physiological recordings from H. virescens ORNs.

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The neurophysiology and antennal lobe projections of olfactory receptor neurons (ORNs) within sexually isomorphic short trichoid sensilla of male Heliothis virescens (Noctuidae: Lepidoptera) were investigated using cut-sensillum recording and cobalt-lysine staining. A total of 202 sensilla were sorted into 14 possible sensillar categories based on odor responses and physiology of ORNs within. Seventy-two percent of the sensilla identified contained ORNs stimulated by conspecific odors. In addition, a large number of ORNs were specifically sensitive to ss-caryophyllene, a plant-derived volatile (N = 41). Axons originating from ORNs associated with individual sensilla were stained with cobalt lysine (N = 67) and traced to individual glomeruli in the antennal lobe. ORNs with responses to female sex pheromone components exhibited similar axonal projections as those previously described from ORNs in long sensilla trichodea in male H. virescens. Antennal lobe axonal arborizations of ORNs sensitive to hairpencil components were also located in glomeruli near the base of the antennal nerve, whilst those sensitive to plant odorants projected to more medial glomeruli. Comparisons with ORNs described from female H. virescens supports the notion that glomeruli at the base of the antennal nerve are associated with conspecific and interspecific odorants, whereas those located medially are associated with plant volatiles.

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Within insect species, olfactory signals play a vital role in communication, particularly in the context of mating. During courtship, males of many moth species release pheromones that function as aphrodisiacs for conspecific females, or repellants to competing conspecific males. The physiology and antennal lobe projections are described of olfactory receptor neurons within an antennal sensillum present on male Heliothis virescens F. (Lepidoptera: Noctuidae) moths sensitive to conspecific male H. virescens-produced pheromone components. Olfactory receptor neurons responded to hexadecanyl acetate and octadecanyl acetate hairpencil components, and Z11-hexadecenyl acetate, an odorant used by closely related heliothine species in their female produced pheromone, which is antagonistic to male H. virescens responses. This acetate-sensitive sensillum appears homologous to a sensillum type previously described in females of this species, sharing similar physiology and glomerular projection targets within the antennal lobe. Wind tunnel observations indicate that H. virescens hairpencil odors (hexadecanyl acetate, octadecanyl acetate) function to antagonize responses of conspecific males following a female sex pheromone plume. Thus, male-male flight antagonism in H. virescens appears to be mediated by this particular sensillum type.

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Single-cell electrophysiological recordings were obtained from olfactory receptor neurons (ORNs) in sensilla trichodea on male antennae of hybrids formed mainly by crossing female Heliothis subflexa with male Heliothis virescens ('SV hybrids'). We recorded from the A-, B-, and C-type sensilla trichodea, with the latter two types housing ORNs exhibiting response profiles to different pheromone components that we had previously found to be characteristic for each species. For both the B- and the C-type SV hybrid sensilla, most of the ORNs exhibited a spike amplitude and ORN co-compartmentalization within sensilla that more strongly resembled the ORNs of parental H. subflexa rather than those of H. virescens. The overall mean dose-response profiles of the ORNs in hybrid C- and B-type sensilla were intermediate between those of the H. virescens and H. subflexa parental type ORNs. However, not all hybrid ORNs were intermediate in their tuning spectra, but rather ranged from those that closely resembled H. subflexa or H. virescens parental types to those that were intermediate, even on the same antenna. The most noteworthy shift in ORN responsiveness in hybrid males was an overall increase in sensitivity to Z9-14:Ald exhibited by Z9-16:Ald-responsive ORNs. Heightened cross-responsiveness to Z9-14:Ald by hybrid ORNs correlates well with observed behavioral cross-responsiveness of hybrids in which Z9-14:Ald could substitute for Z9-16:Ald in the pheromone blend, a behavior not observed in parental types. The hybrid ORN shifts involving greater sensitivity to Z9- 14:Ald also correlate well with studies of hybrid male antennal lobe interneurons that exhibited a shift toward greater cross-responsiveness to Z9-14:Ald and Z9- 16:Ald. We propose that the differences between parental H. virescens, H. subflexa, and SV hybrid male pheromone ORN responsiveness to Z9-16:Ald and Z9-14:Ald are most logically explained by an increased or decreased co-expression of two different odorant receptors for each of these compounds on the same ORN.

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The neurophysiology and antennal lobe projections of olfactory receptor neurons housed within short trichoid sensilla of female Heliothis virescens F. (Noctuidae: Lepidoptera) were investigated using a combination of cut-sensillum recording and cobalt-lysine staining techniques. Behaviorally relevant odorants, including intra- and inter-sexual pheromonal compounds, plant and floral volatiles were selected for testing sensillar responses. A total of 184 sensilla were categorized into 25 possible sensillar types based on odor responses and sensitivity. Sensilla exhibited both narrow (responding to few odors) and broad (responding to many odors) response spectra. Sixty-six percent of the sensilla identified were stimulated by conspecific odors; in particular, major components of the male H. virescens hairpencil pheromone (hexadecanyl acetate and octadecanyl acetate) and a minor component of the female sex pheromone, (Z)-9-tetradecenal. Following characterization of the responses, olfactory receptor neurons within individual sensilla were stained with cobalt lysine (N=39) and traced to individual glomeruli in the antennal lobe. Olfactory receptor neurons with specific responses to (Z)-9-tetradecenal, a female H. virescens sex pheromone component, projected to the female-specific central large female glomerulus (cLFG) and other glomeruli. Terminal arborizations from sensillar types containing olfactory receptor neurons sensitive to male hairpencil components and plant volatiles were also localized to distinct glomerular locations. This information provides insight into the representation of behaviorally relevant odorants in the female moth olfactory system.

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Studies on numerous insect species suggest that male-produced sex pheromones play a role in attracting females; as aphrodisiacs, making females more quiescent; or as a means of inhibiting competing males. Male heliothine moths display abdominal hairpencils during courtship, but the specific effects of the odors released on female behavior have not yet been elucidated. This study investigates the role of male hairpencil compounds in female Heliothis virescens mating behavior. Female H. virescens were exposed to filter paper loaded with hairpencil extracts of male H. virescens, Heliothis subflexa and Helicoverpa zea, and observed for behavioral responses to odors. Single synthetic compounds found in the H. virescens hairpencil blend were also tested. In mating assays between single male and female H. virescens it was found that: (i) antennectomized females mated less frequently than sham-operated females; (ii) females mated less frequently with males whose hairpencils had been surgically removed; (iii) females mated with males with ablated hairpencils if a filter paper loaded with one male equivalent of H. virescens hairpencil extract was presented simultaneously; and (iv) this effect was species-specific, as presentation of H. subflexa or H. zea hairpencil extracts did not restore mate acceptance. This study suggests that odors released by male hairpencils are important in mate acceptance by female H. virescens, and may play a role in mate choice and species isolation.

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Single-cell electrophysiological recordings were obtained from olfactory receptor neurons in sensilla trichodea on male antennae of the heliothine species Heliothis subflexa and the closely related congener H. virescens. A large percentage of sensilla (72% and 81%, respectively, of all sensilla sampled) contained a single odor-responsive receptor neuron tuned to the major pheromone component of both species, Z-11-hexadecenal. A second population of sensilla on H. subflexa antennae (18%) housed receptor neurons that were tuned to Z-9-hexadecenal but also responded with less sensitivity to Z-9-tetradecenal. A similar population of sensilla (4%) on H. virescens male antennae housed receptor neurons that were shown to be tuned specifically only to Z-9-tetradecenal, with no response to even high dosages of Z-9-hexadecenal. A third population of sensilla (comprising 8% and 16% of the sensilla sampled in H. subflexa and H. virescens, respectively) housed two olfactory receptor neurons, one of which was tuned to Z-11-hexadecenyl acetate and the other tuned to Z-11-hexadecenol. In H. subflexa the Z-11-hexadecenyl acetate-tuned neuron also responded to Z-9-tetradecenal with nearly equivalent sensitivity. The behavioral requirements of males of these two species for distinct pheromonal blends was, therefore, reflected by the subtle differences in the tuning properties of antennal olfactory receptor neurons.

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Single-cell electrophysiological recordings were obtained from olfactory receptor neurons housed in sensilla trichodea along the adult antennae arising from transplantation of the antennal imaginal discs between larval male Helicoverpa zea and Heliothis virescens. The olfactory receptor neurons from the majority of type C sensilla sampled on transplanted antennae displayed response characteristics consistent with those of the species that donated the antennae. However, some of the sensilla type C sampled in either transplant type contained olfactory receptor neurons that responded in a manner typical of the recipient species or other neurons that have not previously been found in the type C sensilla of either species. The single-cell data help to explain behavioral results showing that some transplant males do fly upwind to both species' pheromone blends, an outcome not expected based on known antennal sensory phenotypes. Our results suggest that host tissue can influence antennal olfactory receptor neuron development, and further that because of a common phylogenetic ancestry the donor tissue has the genetic capability to produce a variety of sensillar and receptor types.

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The neural computations used to represent olfactory information in the brain have long been investigated. Recent studies in the insect antennal lobe suggest that precise temporal and/or spatial patterns of activity underlie the recognition and discrimination of different odours, and that these patterns may be strengthened by associative learning. It remains unknown, however, whether these activity patterns persist when odour intensity varies rapidly and unpredictably, as often occurs in nature. Here we show that with naturally intermittent odour stimulation, spike patterns recorded from moth antennal-lobe output neurons varied predictably with the fine-scale temporal dynamics and intensity of the odour. These data support the hypothesis that olfactory circuits compensate for contextual variations in the stimulus pattern with high temporal precision. The timing of output neuron activity is constantly modulated to reflect ongoing changes in stimulus intensity and dynamics that occur on a millisecond timescale.

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Chemical signals mediate many of life's processes. For organisms that use these signals to orient and navigate in their environment, where and when these cues are encountered is crucial in determining behavioral responses. In air and water, fluid mechanics impinge directly upon the distribution of odorous molecules in time and space. Animals frequently employ behavioral mechanisms that allow them to take advantage of both chemical and fluid dynamic information in order to move toward the source. In turbulent plumes, where odor is patchily distributed, animals are exposed to a highly intermittent signal. The most detailed studies that have attempted to measure fluid dynamic conditions, odor plume structure, and resultant orientation behavior have involved moths, crabs, and lobsters. The behavioral mechanisms employed by these organisms are different but generally integrate some form of chemically modulated orientation (chemotaxis) with a visual or mechanical assessment of flow conditions in order to steer up-current or upwind (rheo- or anemo-taxis, respectively). Across-stream turns are another conspicuous feature of odor-modulated tracks of a variety of organisms in different fluid conditions. In some cases, turning is initiated by detection of the lateral edges of a well-defined plume (crabs), whereas in other animals turning appears to be steered according to an internally generated program modulated by odor contacts (moth counterturning). Other organisms such as birds and fish may use similar mechanisms, but the experimental data for these organisms is not yet as convincing. The behavioral strategies employed by a variety of animals result in orientation responses that are appropriate for the dispersed, intermittent plumes dictated by the fluid-mechanical conditions in the environments that these different macroscopic organisms inhabit.

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The rules governing the central discrimination of odors are complex and poorly understood, but a growing body of evidence supports the hypothesis that olfactory glomeruli may represent functionally distinct coding modules in the brain. Testing this hypothesis requires that both the functional characteristics and the spatial position of the glomerulus under study be uniquely identifiable. To address these questions, we examined a specialized array of glomeruli (the macroglomerular complex; MGC) in the antennal lobe of male moths that receives input from olfactory receptor cells tuned specifically to female-released odorants that either promote upwind flight (conspecific sex pheromones) or inhibit it (interspecific antagonists). By using a three-dimensional reconstruction method based on high-resolution laser-scanning confocal microscopy, we generated precise spatial maps of the MGC glomeruli in two related noctuid species with similar pheromone chemistry, Heliothis virescens and Helicoverpa zea. To determine the breadth of tuning of individual MGC glomeruli in processing information about these social signals, we used intracellular recording and staining methods to examine the responses of projection (output) neurons that innervate MGC glomeruli and that each project an axon to higher integrative centers. In both species, a close correspondence was found between the odor specificity of the projection neurons and the glomerulus (or glomeruli) supplied by them. The binary blend of pheromone components for each species was represented by neural activity in only two distinct glomeruli in both H. virescens and H. zea. Odorants that antagonize upwind flight when they are added to the respective pheromonal blends evoked excitatory activity in output neurons restricted to a third glomerulus in the MGCs of both species. In summary, these results suggest that the selective activation of different combinations of functionally distinct MGC glomeruli is a general means for discriminating these specific attractant and antagonist chemical signals in the brain.

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The macroglomerular complex (MGC) is a sexually dimorphic array of contiguous glomeruli in male moths, located at the base of the antennal nerve where it enters the primary olfactory processing area of the brain, the antennal lobe. A new three-dimensional reconstruction method was used to map this glomerular array, and images obtained with the laser scanning confocal microscope revealed the precise location and spatial arrangement of the MGC glomeruli in two related species of Heliothine moth. Intracellular recording and staining of projection neurons (PNs) that arborize within the MGC has revealed that information about the two attractive components of the pheromone blend is segregated to different glomeruli in both species. In one species, Helicoverpa zea, a third glomerulus serves as the locus for the processing of odors that antagonize upwind flight. Thus, activation of different but overlapping sets of glomeruli within the MGC may provide a neural substrate for discriminating attractant vs antagonist chemical signals.

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We characterized single upwind surges of flying male Heliothis virescens moths in response to individual strands of pheromone generated experimentally in a wind tunnel. We then showed how this surge functions in this species as a basic 13.4-cm, 0.38-sec-long building block that is strung together repeatedly during typical male upwind flight in a normal pheromone plume. The template for a single iteration, complete with crosswind casting both before and after the straighter upwind surging portion, was exhibited by males flying upwind to pheromone and experiencing filament contacts just frequently enough to produce successful upwind flight to the source, as hypothesized by an earlier model. Also as predicted, with more frequent filament contact by males, only the straightest upwind portions of the surges were reiterated, producing direct upwind flight with little crosswind casting. Electroantennogram recordings made from males in free flight upwind in a normal point source pheromone plume further support the idea that a high frequency of filaments encountered under the usual pheromone plume conditions promotes only these repeated straight surges. In-flight electroantennogram recordings also showed that when filament contacts cease, the casting, counterturning program begins to be expressed after a latency period of 0.30 sec. Together these results provide a plausible explanation for how male and female moths, and maybe other insects, fly successfully upwind in an odor plume and locate the source of odor, using a surging-casting, phasic-tonic response to the onset and disappearance of each odor strand.