Morphological Requirements

There is tremendous morphological diversity of insect ears (Fig. 1). The multitude of different ear designs and locations reflects the unique physical and behavioral challenges faced by each insect. Yet despite their many differences, most ears follow a similar morphological plan. Each typically consists of three identifiable substructures: a tympanal membrane, a tracheal air chamber, and a chordotonal sensory organ.

Tympanal Membrane

The tympanal membrane (eardrum) is a thinned region of exoskeleton, typically supported by a chitinous ring and stretched across an enlarged air-filled cavity. Sound impinges upon the membrane, setting it and its associated nerve cells into motion. The thickness of the membrane can vary from 1 to over 100 |lm. The ultra-sound-sensitive ears of many nocturnal Lepidoptera (butterflies and moths), for example, are so thin that they are transparent. Such fragile membranes are typically protected within body cavities or by external flaps of cuticle. In contrast, the thicker, opaque tympanal membranes of some diurnal butterflies or grasshoppers are conspicuously positioned on the outer surface of the body.

Tracheal Air Chamber

The internal face of the eardrum backs onto an enlarged air-filled chamber, which forms part of the tracheal respiratory system. In some ears, the air chambers are connected directly to other sound input sources (spiracles or contralateral ears) or resonating chambers via the tracheal system. In a few rare cases [e.g., green lacewings (Chrysopidae) and some water bugs (Corixidae)] the tympanic chambers are largely fluid filled.

Attachment cell Scolopale cap

Lumen

Scolopale cell

Sensory cell

Attachment cell Scolopale cap

Lumen

Scolopale cell

Sensory cell

FIGURE 1 A schematic drawing of a generalized insect showing 15 body locations where tympanal ears have been identified. Each number represents a position on the body where an ear has evolved independently in one or more taxa, although all species within a taxonomic division do not necessarily possess ears. (1) Lepidoptera: Sphingidae (Choerocampini, Acherontini). Location: palp—pilifer region. (2) Diptera: Sarcophagidae, Tachinidae. Location: ventral inflation of prosternum, between coxa. (3) Coleoptera: Scarabidae, Dynastinae. Location: dorsolateral region of prosternum. (4) Orthoptera: Ensifera: Gryllidae, Tettigoniidae. Location: tibia of foreleg. (5) Heteroptera: Hydrocorisae (water boatmen). Location: lateral mesothorax, ventral to wing base. (6) Lepidoptera: Papilionoidea, Hedyloidea; Neuroptera: Chrysopidae. Location: base of ventral forewing. (7) Dictyoptera: Mantodea. Location: within a deep groove between the metathoracic legs. (8) Lepidoptera: Noctuoidea. Location: within a cavity on the posterior metathorax. (9) Lepidoptera: Pyraloidea. Location: within a cavity on ventral surface of first abdominal segment. (10) Lepidoptera: Geometridae. Location: within a cavity on anterior side of first abdominal segment. (11) Lepidoptera: Drepanidae. Location: internalized tympanal membrane located between two air-filled chambers on first abdominal segment.

(12) Orthoptera: Acrididae. Location: lateral surface of first abdominal segment.

(13) Coleoptera: Cicindelidae. Location: dorsal surface of first abdominal segment, beneath the elytra. (14) Homoptera: Cicadidae. Location: within cavity on lateral second abdominal segment. (15) Lepidoptera: Uraniidae. Location: within cavity at the anterior (females) or posterior (males) end of the second abdominal segment. (Illustration by M. Nelson.)

Chordotonal Organ

Associated with the inner surface of the tympanal membrane is one to several chordotonal organs. Chordotonal organs are specialized mechanoreceptors unique to insects and crustaceans, but not unique to ears. Each chordotonal organ comprises one or more individual sensory units called scolopidia, and each scolopidium consists of three cells arranged in a linear array: a sensory cell, a scolopale cell, and an attachment cell (Fig. 2A). The total number of scolopidia in an ear ranges from one in some moths (Notodontidae) to almost 2000 in the bladder grasshopper (Bullacris membracioides). A chordotonal organ may attach directly to the inner surface of the tympanic

FIGURE 2 Insect tympanal sensory receptors. (A) A typical tympanal scolopidial organ, consisting of three cell types. The dendrite of a bipolar sensory neuron projects into a fluid-filled space (lumen) formed by the walls of a enveloping scolopale cell. The distal tip of the dendrite inserts into the scolopale cap, an extracellular secretion of the scolopale cell. The attachment cell connects the sensory neuron and scolopale cell to the tympanal membrane, either directly or indirectly via a tracheal air sac. A chordotonal organ may have from one to several thousand scolopidia. (B) A schematic diagram depicting the hypothetical transition from a wing-hinge proprioceptive chordotonal organ to a tympanal hearing chordotonal organ. The top two images show a chordotonal organ functioning as a proprioceptor monitoring wing movements. At the bottom the chordotonal organ has been mechanically isolated within a rigid tympanal cavity and attaches to a thinned region of cuticle (the tympanic membrane) that detects sounds. (A was modified, with permission, from E. G. Gray, 1960, The fine structure of the insect ear, Philos. Trans. R. Soc. B 243, 75-94. Illustrations by M. Nelson.)

membrane (e.g., Fig. 4E) or to tracheal air sacs indirectly associated with the tympanum. The axons of the sensory neurons collectively make up the auditory nerve that forms the neural link between the mechanosensory stimulation of the eardrum and its "perception" by the central nervous system. Vibrations of the eardrum and/or air chamber cause bioelectric currents to flow in the sensory cell, initiating action potentials in the auditory nerves and signaling neurons in the auditory pathways of the nervous system.

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