Location

Ears can be located just about anywhere on insects (Fig. 1). There is one striking difference between insect ears and vertebrate ears: those of the latter, because of developmental constraints, are always on opposite sides of the head and always on the head, whereas insect ears have been found on virtually every part of the body. Of course, in any given species of insect, ears are always found on the same part of the body, but from group to group, ears may be found on the head, legs, wings, thorax, or abdomen.

Although the ears of most insects are clearly recognized by the presence of a conspicuous tympanal membrane either on the body's external surface or within an ear cavity, the ears of other insects are morphologically cryptic. For example, some hawk moths (superfamily Sphingoidea) possess hearing organs in their mouthparts, and by inflating their palps while they are feeding on flowers at dusk, they create functional ears that alert them to the echolocation calls of bats. The "cyclopean" ear of the praying mantis is located within a deep groove between the hind legs, and the tympanal membranes are not morphologically distinct from surrounding body parts, even to the trained eye. Even more cryptic are the ears of the Madagascar hissing cockroach, Gromphadorhina portentosa, in which there is no obvious eardrum or tympanal membrane overlying the internal chor-dotonal organs contained within the tibiae of its forelegs.

Although most insects possess a single pair of hearing organs, there are at least two reports of insects with multiple ears. One lineage of praying mantids has two sets of ears: one located between the mesothoracic legs and tuned to ultrasonic frequencies (25 to 40 kHz) and another between the metatho-racic legs and tuned to lower frequencies (2—4 kHz). The multieared bladder grasshopper (B. membracioides) of South Africa has six pairs of serially repeated abdominal ears that function in detecting mating calls at distances of ~2 km.

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