Development of the Insect Auditory System

Synaptic ultrastructure of Drosophila Johnston's organ axon terminals as revealed by an enhancer trap

The role of auditory circuitry is to decipher relevant information from acoustic signals. Acoustic parameters used by different insect species vary widely. All these auditory systems, however, share a common transducer: tympanal organs as well as the Drosophila flagellar ears use chordotonal organs as the auditory mechanoreceptors. We here describe the central neural projections of the Drosophila Johnston's organ (JO). These neurons, which represent the antennal auditory organ, terminate in the antennomechanosensory center. To ensure correct identification of these terminals we made use of a beta-galactosidase-expressing transgene that labels JO neurons specifically. Analysis of these projection pathways shows that parallel JO fibers display extensive contacts, including putative gap junctions. We find that the synaptic boutons show both chemical synaptic structures as well as putative gap junctions, indicating mixed synapses, and belong largely to the divergent type, with multiple small postsynaptic processes. The ultrastructure of JO fibers and synapses may indicate an ability to process temporally discretized acoustic information.

Variation in number and differentiation of the abdominal infrared receptors in the Australian 'fire-beetle' Merimna atrata (Coleoptera, Buprestidae)

Most individuals of the Australian 'fire-beetle' Merimna atrata have two pairs of IR receptors which are located ventrolaterally on the second and third abdominal sternite. An IR receptor consists of a specialized IR absorbing area, which is innervated by a neural complex. This complex contains one thermoreceptive multipolar neuron with a unique terminal dendritic mass (TDM) and two scolopidia and was termed 'sensory complex'. However, also individuals with one pair of IR receptors on the second sternite and beetles with three pairs on the second, third, and fourth sternites were found. Additionally, beetles having one or two pairs of IR receptors may have preliminary stages of IR receptors on the third and fourth sternite, respectively. We found two kinds of preliminary stages, both of which are characterized by a much less pronounced absorbing area. In all five abdominal sternites segmental nerves are attached to the cuticle with a neural complex. Investigation of complexes of non-IR sternites suggests that the sensory cells inside the sensory complex of an IR receptor have developed from common internal stretch receptors. From our results it can be hypothesized that the IR sensory system in Merimna atrata has not yet reached a stage, which can be regarded as evolutionary stable.

Embryonic development of the central projection of auditory afferents (Schistocerca gregaria, Orthoptera, Insecta)

The auditory system of Schistocerca gregaria is a well investigated sensory network in the adult grasshopper. Here we present a first study on the embryonic development of this neuronal network. Focussing on the auditory receptor cells we show that they differentiate axonal processes at around 45% of embryonic development. These axons fasciculate with the intersegmental nerve and enter the central nervous system by 45-50% of development. First collaterals sprout into the major arborization area, the frontal auditory projection area of the metathoracic ganglion by 60%. This projection increases in density until an adult-like morphology is established by 90% of development. Furthermore, by the end of embryogenesis all three types of receptor fiber projections can be distinguished. This development is independent of a hearing ability, which develops much later during postembryonic life. The auditory projection co-develops with the fusion of neuromeres to the metathoracic ganglion, the formation of the target neuropile areas and the expression of the synapse associated molecule synapsin. Fasciclin I and Lachesin, both potential axon-guidance molecules, are expressed strongly on both, peripheral and central auditory pathways and, although much weaker, within the synaptic target area.

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.