Mapping the Auditory Space of Culex pipiens Female Mosquitoes in 3D.

The task of directional hearing faces most animals that possess ears. They approach this task in different ways, but a common trait is the use of binaural cues to find the direction to the source of sound. In insects, the task is further complicated by their small size and, hence, minute temporal and level differences between two ears. A single symmetric flagellar particle velocity receiver, such as the antenna of a mosquito, should not be able to discriminate between the two opposite directions along the vector of the sound wave. Paired antennae of mosquitoes presume the usage of binaural hearing, but its mechanisms are expected to be significantly different from the ones typical for the pressure receivers. However, the directionality of flagellar auditory organs has received little attention. Here, we measured the in-flight orientation of antennae in female Culex pipiens pipiens mosquitoes and obtained a detailed physiological mapping of the Johnston's organ directionality at the level of individual sensory units. By combining these data, we created a three-dimensional model of the mosquito's auditory space. The orientation of the antennae was found to be coordinated with the neuronal asymmetry of the Johnston's organs to maintain a uniformly shaped auditory space, symmetric relative to a flying mosquito. The overlap of the directional characteristics of the left and right sensory units was found to be optimal for binaural hearing focused primarily in front of, above and below a flying mosquito.

Frequency tuning of swarming male mosquitoes (Aedes communis, Culicidae) and its neural mechanisms.

The primary function of hearing in mosquitoes is believed to be intraspecific communication. This view dictated the principle of many behavioral studies, namely, the attraction of male mosquitoes to the sounds that mimicked a female tone. However, after the avoidance response to certain frequencies of sound was demonstrated, it became clear that attraction tests cannot fully account for all the capabilities of the mosquito auditory system. In addition, the tuning curves obtained by electrophysiological measurements differ from the behavioral ones. We designed a simple but robust field test based on responses of swarming mosquitoes to sound stimulation, but not limited to the attraction response. Here we report the auditory thresholds over a wide range of sound frequencies measured in the field from swarms of Aedes communis mosquitoes. In parallel, the auditory sensitivity of male mosquitoes taken from the same swarms was measured electrophysiologically. Surprisingly, we found high acoustic sensitivity; 26 dBSPL on average, in the frequency range 180-220 Hz (ambient temperature 12 °C). In addition, responses were found in the high-frequency range, 500-700 Hz (the so-called 'mirror channel'). Two types of auditory units were recorded: more sensitive broadband neurons and less sensitive units with distinct narrow (quality factor Q6 = 7.4) frequency tunings in the range 180-350 Hz. We propose that the former provides the detection of signal while the latter are used for frequency identification in order to make a behavioral choice.

Directional and frequency characteristics of auditory neurons in Culex male mosquitoes.

The paired auditory organ of the mosquito, the Johnston's organ (JO), being the receiver of the particle velocity component of sound, is directional by its structure. However, to date almost no physiological measurements of its directionality have been made. In addition, the recent finding on the grouping of the JO auditory neurons into antiphase pairs demands confirmation by different methods. Using the vector superposition of the signals produced by two orthogonally oriented speakers, we measured the directional characteristics of individual units as well as their relationships in physiologically distinguishable groups - pairs or triplets. The feedback stimulation method allowed us to discriminate responses of the two simultaneously recorded units, and to show that they indeed responded in antiphase. Units of different frequency tuning as well as highly sensitive units (thresholds of 27 dB SPVL and below) were found in every angular sector of the JO, providing the mosquito with the ability to produce complex auditory behaviors.

Frequency organization of the Johnston's organ in male mosquitoes (Diptera, Culicidae).

The Johnston's organs (JO) of mosquitoes are the most complex mechanosensitive organs yet found in insects. Previous findings on the behavior of mosquitoes suggest that, together with exceptional sensitivity, their auditory system can discriminate frequencies. Analysis of compound responses of the JO did not provide unambiguous evidence of such discrimination, nor did it help to find its mechanism. Using the feedback stimulation method, we measured the tuning frequencies of the JO sensory neurons. Here we present electrophysiological evidence that male mosquitoes of Culex pipiens possess at least eight groups of auditory neurons that are distinct in their frequency tuning, with individual frequencies ranging from 85 to 470 Hz. Most of the neurons are tuned to 190-270 Hz, which corresponds to the difference between male and female flight tones. Axons of the JO sensory units propagate graded amplified receptor potentials rather than all-or-none action potentials, are grouped into pairs or triplets and often respond in anti-phase to each other. Some features of the mosquito auditory system suggest an analogy to the retinal mechanisms. Together with our previous findings on frequency tuning in female mosquitoes of different species, this study presents evidence in favor of sophisticated frequency analysis of sound in mosquitoes.