Directional hearing by mechanical coupling in the parasitoid fly Ormia ochracea

Structure, development, and evolution of insect auditory systems

This paper provides an overview of insect peripheral auditory systems focusing on tympanate ears (pressure detectors) and emphasizing research during the last 15 years. The theme throughout is the evolution of hearing in insects. Ears have appeared independently no fewer than 19 times in the class Insecta and are located on various thoracic and abdominal body segments, on legs, on wings, and on mouth parts. All have fundamentally similar structures-a tympanum backed by a tracheal sac and a tympanal chordotonal organ-though they vary widely in size, ancillary structures, and number of chordotonal sensilla. Novel ears have recently been discovered in praying mantids, two families of beetles, and two families of flies. The tachinid flies are especially notable because they use a previously unknown mechanism for sound localization. Developmental and comparative studies have identified the evolutionary precursors of the tympanal chordotonal organs in several insects; they are uniformly chordotonal proprioceptors. Tympanate species fall into clusters determined by which of the embryologically defined chordotonal organ groups in each body segment served as precursor for the tympanal organ. This suggests that the many appearances of hearing could arise from changes in a small number of developmental modules. The nature of those developmental changes that lead to a functional insect ear is not yet known.

Size and scale effects as constraints in insect sound communication

For optimal transfer of power to the surrounding medium, a sound source should have a radius of 1/6 to 1/4 of the sound wavelength. Sound-waves propagate from the source as compressions and rarefactions of the fluid medium, which decay by spreading and viscous losses. Higher frequencies are more easily refracted and reflected by objects in the environment, causing degradation of signal structure. In open air or water, the sound spreads spherically and decays by the inverse square law. If the sound is restricted to two dimensions rather than three, it decays as the inverse of range, whereas waves within a rod decay largely due to viscous losses; such calls are usually rather simple pulses and rely on the initial time of arrival because of multiple pathlengths or different propagation velocities in the environment. Because of the relationship between calling success and reproductive success, singing insects are under selective pressure to optimize the range, and to maintain the specificity, of their calls. Smaller insects have less muscle power; because of their small sound sources, higher frequencies will be radiated more efficiently than lower frequencies, but in order to produce brief loud pulses from a long-duration muscle contraction they may use both a frequency multiplier mechanism and a mechanical power amplifier. Airborne insect sounds in the range from 1 to 5 kHz tend to have sustained puretone components and a specific pattern of pulses which propagate accurately. Where the song frequency is higher, the pulses tend to become briefer, with a rapid initial build-up that gives a reliable time of onset through obstructed transmission pathways. These scale effects may be related both to the sound-producing mechanism and the auditory system of the receiver. Tiny insects have the special acoustic problem of communicating with only a small amount of available power. Some, such as fruit flies, communicate at low frequencies, at close range, by generating air currents; these currents may also be used to waft specific pheromones. Other small insects, such as Hemiptera, beetles, etc., communicate using substrate vibration. This enables long-range communication, but signal structure degrades with distance from the source; vibration signals tend to be confined to certain types of linear substrate, such as vegetation.

The brain has a body: adaptive behavior emerges from interactions of nervous system, body and environment

Studies of mechanisms of adaptive behavior generally focus on neurons and circuits. But adaptive behavior also depends on interactions among the nervous system, body and environment: sensory preprocessing and motor post-processing filter inputs to and outputs from the nervous system; co-evolution and co-development of nervous system and periphery create matching and complementarity between them; body structure creates constraints and opportunities for neural control; and continuous feedback between nervous system, body and environment are essential for normal behavior. This broader view of adaptive behavior has been a major underpinning of ecological psychology and has influenced behavior-based robotics. Computational neuroethology, which jointly models neural control and periphery of animals, is a promising methodology for understanding adaptive behavior.