Spatial arrangement of different types of pheromone-sensitive sensilla in a male moth

Male moths perceive the female-emitted sex pheromone components by thousands of antennal sensilla of the same general morphology [1]. Different physiological types of pheromonereceptive sensilla, each with receptor cells responding to a specific component, have been distinguished [2, 3]. We have found that in the Heart and Dart moth Agrotis exclamationis (L.) (Lepidoptera:Noctuidae), the pheromone components are detected by two different receptor cells in different sensilla, which differ in their morphology and in their spatial localization on the antenna. The spatial arrangement of receptor cells on the male antenna might be an adaptation to optimize the detection of the minor component in the female pheromone. The antenna of male A. exclamationis consists of 80-90 segments with a total length of about 10 mm. It has no lateral branches, and the basal and medial 2/3 of the length (about 40 segments) has a relatively large diameter (250-300 gm) compared with the distal filiform part (150-250 ~tm). Two morphOlogical types of single-walled sensilla [1] dominate on the basal and medial part of the antenna. The most conspicuous type forms dense bundles of each about 20 sensilla at the lateral margins of all the basal and medial antennal segments (Fig. 1 A). These sensilla are slightly curved and have a length of about 200 gm, and a diameter of 4-5 gm at the base. More central to these there is a second type of sensilla which has a gradually decreasing length, the shortest towards the midline with a length of about 100 gm and a basal diameter of 3-4 gin. In the transmission electron microscope, another feature emerges which further emphasizes the difference in morphology between the lateral and medial sensilla besides their length and diameter; the sensilla in the marginal bundles are innervated by a single sensory cell, whereas those closer to the midline are innervated by two sensory cells (Fig. 1 B). The transition zone between the two morphological types may be sharply defined, or in some cases with a certain intermingling of the two types. Unbranched sensory processes emerge from the neurons and extend to the distal part of the hair in both kinds of sensilla. The number of pores in the hair wall is moderate and without any striking difference between the two kinds of sensilla. The two main pheromone components in A. exclamationis are (Z)-5-tetradecenyl acetate (Z5-14:OAc) and (Z)-9tetradecenyl acetate (Z9-14:OAc). Bestmann et al. [4] found these compounds in a 93:7 ratio in females matching a maximal trap catch of males at a ratio of 95:5. Vrkoc et al. [5] reported that addition of the dodecenyl and hexadecenyl acetate fractions from the female increased the behavioral activity of the tetradecenyl acetates, but no additional compounds could be identified. Using single-cell techniques, Priesner [6] found specific receptors on the male antenna for the two main components mentioned above, but also for three other compounds: (Z)-7-dodecenyl acetate (Z712:OAc), (Z)-7-tetradecenyl acetate (Z7-14: OAc), and (Z)-I 1-tetradecenyl acetate (Z11-14: OAc). In a first experiment we obtained identical electroantennogram dose-response curves for Z5and Z9-14: OAc. The threshold value was 1 gg loaded on the filter paper in the stimulus syringe. We then measured the electrophysiological response of 50 lateral and 50 medial sensilla, with the tip-recording technique [7]. Their responses to the following compounds (10 gg applied on filter paper source), were recorded: Z5-10:OAc, Z3-12:OAc, Z5-12:OAc, Z7-12:OAc, Z5 14:OAc, Z7-14:OAc, Z9-14:OAc, Z l l 14 :OAc , Z7-16:OAc, Z I I 16:OAc, Z5 14:OH, and Z9-14:OH. Among the lateral sensilla three physiological types were found: the most