References

Howse, P. E. 1970. Brain structure and behavior in insects. Ann. Rev. Entomol. 20: 359-379.

Miller, T. A. 1979. Insect neurophysiological techniques. Springer, New York. Treherne, J. E. 1974. Insect neurobiology. North Holland, Amsterdam.

Integumentary Sense Organs

The arthropod would be isolated from its environment by the nonliving, encapsulating cuticle, but thousands of structures sensitive to external stimuli (Dethier 1963), collectively called sensilla, cover the surface. They are especially numerous on the antennae, tarsal pads, and palpi and communicate with the nervous system via nerves of the peripheral system. The anatomy of sensilla is extremely varied, each type specifically adapted to the perception of a certain subset of stimuli important to the animal's safety and other life processes.

The most common and often most abundant sensilla are hairlike extensions (setae, chaetae) responsible for mechanorecep-tion (Mclver 1975). These may respond to touch, stretching, or bending, directly from an outward force or by pressure from another part of the body (often they are found in articulations). The hair base may be simple and level with the surface or recessed and quite complex, as is the trichobothria of arachnids.

Portions of the body wall can be innervated so that deformations transmit information about mechanical stresses. Even slight vibrations from air currents or compression waves are perceived. For example, masses of stretch receptors in the swollen, subbasal segment of the antennae of mosquitoes and other flies (Johnston's organ) respond to deflexions of the flagel-lum by air movements, giving these insects an acute sense of hearing. When the integument is especially sensitive in this way, and there are structural modifications for reception such as thin, vibrating membranes, these portions are considered auditory or gans (Michelsen 1979). Such are the thoracic and abdominal tympana of many moths and grasshoppers, fore tibial hearing pits of katydids, and acoustical windows in the cicada thorax.

Sensilla are often structured for the reception of chemicals in air or liquids. Such chemoreceptors (Slifer 1970) usually have thin or porous walls so the molecules may pass through the outer part of the organ and stimulate inner receptive surfaces. They may be extremely sensitive. Calculations for the sex attractant of the domestic silk moth indicate that a single molecule may elicit a response.

Certain sensilla also react to ambient temperature changes, radiant heat, pressure, humidity, and surface moisture (Alt-ner and Loftus 1985). Perception of related factors internally are by direct cellular sensitivity.

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