Developmental Origins

In vertebrates, ears are always located on the head, behind the eyes, and above the jaw, but in insects they are virtually anywhere on the body. Why are they not confined to one place like those of vertebrates? Evolutionary and developmental origins of the vertebrate ear, drawing from embryological anlagen (organs or structures in their earliest state) such as the gill arches, neural crest, and the otic capsule, have constrained the ears to their canonical position in the head. Insect ears require but two structural modifications, cuticular and spiracular, combined with innervation by a chordotonal organ. In the insect body, chordotonal organs do not function only as hearing organs, but are actually widely distributed throughout the body, where they act as detectors of self-induced body movements (proprio-ceptors) or substrate vibrations. Insect bodies are made up of a series of hard cuticular plates joined by flexible membranes, and chordotonal organs are frequently suspended between moving joints. We now know through developmental studies and comparative anatomy that it is not so difficult to "construct" an ear by making a few peripheral modifications to an existing proprioceptor and its surrounding cuticular and tracheal structures. By simply thinning the cuticle, enlarging the surrounding tracheal air sacs to allow membrane vibration, and mechanically isolating the sensory organ from body movements, a proprioceptor can be converted into a sound-pressure receiver (Fig. 2B). For example, the bat-detecting ears on the thorax of noctuoid moths are thought to have evolved from proprioceptors monitoring wing movements. Given the jointed, segmental body plan of any insect, the ubiquitous branching of the respiratory tracheae lining the inner face of the cuticle, and the widespread occurrence of chordotonal organs that span different segments in the body and appendages, the precursors of an insect ear can be found virtually at every joint in the body and appendages. There seem to be few developmental constraints in positioning an ear, should an adaptive need arise.

Given so many possibilities for developing an ear, how does it come about that a particular insect possesses an ear in one place, say its forelegs, and another species in another, say its abdomen? One can imagine a host of anatomical, biophysical, ecological, and evolutionary constraints and advantages that might play important roles in the selection process. Such factors as the distances between the ears, the degree of protection offered by surrounding structures, the availability of tracheal air sacs, or the preexisting connections to the central nervous system may all be important. For example, if the function of hearing in a flying nocturnal insect like a moth is to detect and avoid echolocating bats, then a proprioceptor with preestab-lished neural connections to wing flight musculature would be a better ear candidate than, say, a leg proprioceptor. Or, possibly, the reason the thorax and abdomen are so "busy" with ears (Fig. 1) could be because these locations offer maximum interaural distance and a high degree of protection.

0 0

Post a comment