A novel acoustic signalling system in mosquitoes: exploring the biophysical and neurophysiological basis for interactive behaviour in an insect

Mosquitoes hear with their antennae and Johnston, who discovered the mosquito auditory organ at the base of the antenna 150 years ago, speculated that audition was involved with mating behaviour. Indeed, the plumose antennae of males are sensitive to the flight tone of other mosquitoes and are used to detect and locate females. Mate detection through audition was, therefore, thought to be the prerogative of the male. The antennae of females are simpler in form, but they are more sensitive than any other insect, yet nothing was known about the auditory behaviour of females until Gibson & Russell discovered that they also respond to mosquito flight tones. Sexual selection by flying mosquitoes is an acoustic dialogue which they enter into when they are within hearing range of each other (~ 10 cm); each responds to the sound of the other by altering their own flight tone, until the tones converge in the case of male-female couples, or dramatically diverge in the case of same-sex pairs. These acoustic interactions can, therefore, reveal to an individual whether its interaction is with a partner of the same or opposite sex, and may hold the key to understanding how mosquitoes identify mates of the right species. In our recent studies we established that mosquitoes communicate with each other in a way that appears to be unique amongst the invertebrates. Rather than emitting and receiving discreet calls, mosquitoes continuously send out a signal, in the form of their own wing beats, while continuously monitoring sound inputs. The information to which they are sensitive is encoded somehow in the way each mosquito in a duet alters its wing-beat frequency over time. To discover what this sex-specific behaviour is, we will investigate the biophysics and neurophysiology of sound detection in mosquitoes, using advanced techniques we have developed for vertebrate hearing, to characterise the way mosquito antennae detect and respond to sound and to discover how mosquitoes then use auditory information to interact with each other. The beating wings of mosquitoes produce complex sounds, made up of multiple harmonics, with the fundamental frequency randomly shifting by a few Hz over the course of a few milliseconds. We will map the sound field around a tethered flying mosquito to see if there are features that might facilitate acoustic interactive behaviour with nearby mosquitoes. When a tethered flying mosquito detects sound other than its own wing-beats, it reduces its random shifts in wing-beat frequency and tries to match its wing-beat frequency to the other sound. We will use this response to test the sensory acuity of mosquitoes to sound. This information will be used to determine which components of the flight tone trigger a response to other mosquitoes. So far, we have found that changes in ambient light intensity and wavelength alter wing-beat frequency and increases in temperature and sound levels increase the antennal resonance in tethered mosquitoes. We will explore how environmental changes influence sound reception and flight sounds, and the possible role of efferent neurones in controlling JO frequency tuning and sensitivity. Tethered mosquitoes will be used for our basic studies, and results will be verified with free-flying mosquitoes. Our data will be assimilated into simulation models, which will then be used to test against live mosquitoes. Our aims will be achieved when the model mosquito elicits natural responses in live mosquitoes. The results of our study will provide a wealth of new information about this remarkable form of communication in mosquitoes and will extend our understanding of the biophysical and neurophysiological properties of hearing in insects. The findings will be of significance to scientists interested in auditory systems throughout the animal kingdom and to those involved with the control of mosquitoes that transmit life-threatening pathogens to humans.

The project is based on our discovery that mosquitoes enter into complex auditory interactions which enable species-specific sex recognition (Gibson & Russell, 2006). Each of a pair of tethered mosquitoes flying within auditory range alters its wing-beat frequency in response to the flight tone of the other. Within seconds the flight tone frequencies of opposite-sex pairs become closely matched while the flight tones of same-sex pairs eventually diverge. These acoustic interactions ultimately reveal to an individual whether the other mosquito is of the same or opposite sex. We now aim to identify the sex-specific characteristics of these interactions that make sex-recognition possible. Our research subject will be Culex quinquefasciatus Say (Cx quinx) because we can record from it in free-flight and its mating behaviour, sensory limitations and crepuscular activity indicate that males probably rely on acoustic cues to detect females and to avoid each other in swarms. We will repeat our previous analysis of Toxorhynchytes brevipalpis audition using Cx quinx, to assess the biophysics of sound production and the sensitivity and frequency tuning of the mechanosensitive Johnston's organ (JO) at the base of the antenna and we will compare these results with behavioural audiograms. We will see if the JO responds to self-generated flight tones and if it generates distortion products that might be exploited by mosquitoes for acoustic interaction. We will analyse acoustic interactions between same- and opposite-sex pairs of Cx quinx in both tethered- and free-flying pairs of mosquitoes, using simulation models to help identify the essential sex-specific elements of the interactions. We will explore the role of efferent neurones in controlling JO frequency tuning and sensitivity. Our aims will be achieved when the model mosquito elicits natural responses in live mosquitoes.