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FIGURE 2 Left: head of an adult Indian stick insect (Carausius morosus) with a normal antenna on the left and a regenerated antenna with leglike morphology on the right. Right: head of adult C. morosus with two regenerated antennae with leglike morphology. [After Fig. 78 in Wigglesworth, V. B. (1971). 'The Principles of Insect Physiology." Chapman & Hall, London, with the kind permission of Kluwer Academic Publishers.]

FIGURE 2 Left: head of an adult Indian stick insect (Carausius morosus) with a normal antenna on the left and a regenerated antenna with leglike morphology on the right. Right: head of adult C. morosus with two regenerated antennae with leglike morphology. [After Fig. 78 in Wigglesworth, V. B. (1971). 'The Principles of Insect Physiology." Chapman & Hall, London, with the kind permission of Kluwer Academic Publishers.]

antennae are usually biological in origin and airborne (volatiles), although (depending on the insect species) the sampled chemical compounds are sometimes in a liquid or associated with a solid surface. The chemicals intercepted by antennae may alert the insect to the presence of prospective mates, food, suitable places to lay eggs, or predators.

The physical stimuli detected by mechanoreceptors on the antennae may be used by the insect to indicate air speed during flight, to detect vibrations of the air, or to detect solid boundaries in its environment by touch. While a single mechanosensory hair will send information to the brain about the local physical conditions existing at its microscopic location, an antenna also has mechanosensory organs that evaluate the physical forces acting on the antenna as a whole. These mechanosensory organs, located near the base of the antenna, include Johnston's organ, Böhm bristles, hair plates (groups of mechanosensory hairs), and campaniform sensilla (thin flexible patches of cuticle that are innervated). Johnston's organ is located in the pedicel and responds to changes of location or vibrations of the whole antenna. In contrast, the Böhm bristles, located near the scape—pedicel boundary, send information to the brain about the antennal position, rather than its movements. The variety of mechanosensory organs associated with the first two segments of the antennae are believed to act together to inform a flying insect about its air speed, because greater

FIGURE 3 A worker ant (Formica polyctena) cleans one of its antennae by dragging it across the specialized comb of right foreleg. (Reprinted by permission of the publisher from THE INSECT SOCIETIES by Edward O. Wilson, Cambridge, MA.: The Belknap Press of Harvard University Press, Copyright 1971 by the President and Fellows of Harvard College. Original drawing by Turid Holldobler.)

FIGURE 3 A worker ant (Formica polyctena) cleans one of its antennae by dragging it across the specialized comb of right foreleg. (Reprinted by permission of the publisher from THE INSECT SOCIETIES by Edward O. Wilson, Cambridge, MA.: The Belknap Press of Harvard University Press, Copyright 1971 by the President and Fellows of Harvard College. Original drawing by Turid Holldobler.)

flying speed will cause greater deflection of the antennae by the air rushing past. Contact chemosensory hairs, so called because the chemical compounds are usually detected when the insect is touching a liquid or solid surface with the antennae, often have mechanosensory capabilities as well and are usually located near the distal ends of antennae.

The function of the antennal sensory organs will be affected by their arrangement on the antennae. For example, sensory organs on the distal tip of a very long antenna will permit chemical or physical sampling of the environment far from the body of the insect. Close packing of sensory hairs will decrease the airflow in their vicinity, and hence will modify both the chemical and physical sampling of the environment by those hairs. The function of the antennae will also be dependent on the behaviors of the insect that will affect the airflow around the antennae, such as flying, wing fanning, postural changes, or oscillating the antennae. A structure projecting into the environment is liable to collect debris that might interfere with its sensory function; both antennal grooming behaviors and modifications of leg parts against which an antenna is scraped are common in insects (Fig. 3). In some insects, antennae are modified for nonsensory functions such as clasping mates during copulation (fleas and collembolans), holding prey items (beetle larvae), or forming a temporary physical connection between an underwater air reservoir and the atmosphere (aquatic beetles).

See Also the Following Articles

Chemoreception • Imaginal Discs • Mechanoreception • Pheromones

Further Reading

Hansson, B. S., and Anton, S. (2000). Function and morphology of the antennal lobe: New developments. Annu. Rev. Entomol. 45, 203—231. Heinzel, H., and Gewecke, M. (1987). Aerodynamic and mechanical properties of the antennae as air-current sense organs in Locusta migratoria. II. Dynamic characteristics. J. Comp. Physiol. A 161, 671-680.

Kaissling, K. E. (1971). Insect olfaction. In Handbook of Sensory Physiology (L. M. Beidler, ed.), Vol. IV of "Chemical Senses," Part 1, "Olfaction," pp. 351-431. Springer, Verlag, Berlin. Keil, T. A. (1999). Morphology and development of the peripheral olfactory organs. In "Insect Olfaction" (B. S. Hansson, ed.), pp. 5-48. SpringerVerlag, Berlin.

Loudon, C. and Koehl, M. A. R. (2000). Sniffing by a silkworm moth: Wing fanning enhances air penetration through and pheromone interception by antennae. J. Exp. Biol. 203, 2977-2990. Pass, G. (2000). Accessory pulsatile organs: Evolutionary innovations in insects. Annu. Rev. Entomol. 45, 495-518. Schneider, D. (1964). Insect antennae. Annu. Rev. Entomol. 9, 103-122. Schneiderman, A. M., Hildebrand, J. G., Brennan, M. M., and Tumlinson, J. H. (1986). Transsexually grafted antennae alter pheromone-directed behaviour in a moth. Nature 323, 801-803. Steinbrecht, R. A. (1987). Functional morphology of pheromone-sensitive sensilla. In "Pheromone Biochemistry" (G. D. Prestwich and G. J. Blomquist, eds.), pp. 353-384. Academic Press, London. Zacharuk, R. Y. (1985). Antennae and sensilla. In "Comprehensive Insect Physiology Biochemistry and Pharmacology" (G. A. Kerkut and L. I. Gilbert eds.), Vol. 6, pp. 1-69. Pergamon Press, New York.

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