Experientia 42 (1986), Birkh~user Verlag, CH-4010 BaseI/Switzerland 99 Sulzman, F.M., Fuller, C.A., and Moore-Ede, M.C., Effects of phasic and tonic light inputs on the circadian organization of the squirrel monkey. Photochem. Photobiol. 34 (1981) 249-256. 100 Takahashi, J.S., and Menaker, M., Role of the suprachiasmatic nuclei in the circadian system of the house sparrow, Passerdomes- ticus. J. Neurosci. 2 (1982) 815-828. 101 Takahashi, J.S., and Menaker, M., Entrainment of the circadian system of the house sparrow: a population of osciUators in pinealec- tomized birds. J. comp. Physiol. (A) 146 (1982) 245-253. 102 Takahashi, J. S., and Menaker, M., Multiple redundant circadian oscillators within the isolated avian pineal gland. J. comp. Physiol. (A) 154 (1984) 435M40. 103 Takahashi, J. S., and Zatz, M., Regulation of circadian rhythmicity. Science 217 (1982) t 104-1111. 104 Tnrek, F.W., Are the suprachiasmatic nuclei the location of the biological clock in mammals? Nature 292 (1981) 289-290. 105 Vernikos-Danellis, J., and Winget, C. M., The importance of light, postural and social cues in the regulation of the plasma cortisol rhythm in man, in: Advances Bioscience, 19, Chronopharmacol- ogy, pp. t01-106. Eds A. Reinberg and F. Halberg. Pergamon, Oxford 1979. 106 Weitzman, E.D., Czeisler, C.A., and Moore-Ede, M.C., Sleep- wake, neuroendocrine and body temperature circadian rhythms under entrained and non-entrained (free-running) conditions in man, in: Biological Rhythms and their Central Mechanism, pp. 199-227. Eds M. Suda, O. Hayaishi and H. Nakagawa. Elsevier, North-Holland, New York 1979. 107 Weitzman, E. D., Zimmerman, J. C., Czeisler, C. A., and Ronda, J., Cortisol secretion is inhibited during sleep in normal man. J. clin. Endocr. Metab. 56 (1983) 352-358. 108 Wever, R.A., The Circadian System of Man: Results of Experi- ments under Temporal Isolation. Springer-Verlag, New York 1979. 109 Wever, R.A., On varying work-sleep schedules, in: Biological Rhythms, Sleep and Shift Work, pp. 35-60. Eds L. C. Johnson, D.I. Tepas, W. P. Colquhoun and M. H. Colligan. Spectrum, New York 1981. 13 110 Wever, R.A., Behavioural aspects of circadian rhythmicity, in: Rhythmic Aspects of Behaviour, pp. 105-171. Eds F.M. Brown, R.C. Graeber, Lawrence Erlbaum, London 1982. 111 Wever, R.A., Influence of physical workload on free-running circa- dian rhythms of man. Pflfigers Arch. 381 (1979) 119 126. 112 Wever, R.A., Fractional desynchronization of human circadian rhythms. A method for evaluating entrainment limits and func- tional interdependencies. Pfliigers Arch. 396 (1983) 128-137. 113 Wever, R. A., Toward a mathematical model of circadian rhythmic- ity, in: Mathematical Models of the Circadian Sleep-Wake Cycle, pp. 17 77. Eds M.C. Moore-Ede and C.A. Czeisler. Raven Press, New York 1984. 114 Wever, R. A., Sex differences in human circadian rhythms: intrinsic periods and sleep fractions. Experientia 40 (1984) 1226-1234. 115 Wever, R.A., Polasek, J., and Wildgruber, C.M., Bright light af- fects htmmn circadian rhythms. Pfliigers Arch. 396 (1983) 85-87. 116 Wiedenmann, G., Splitting in a circadian activity rhythm: the ex- pression of bilaterally paired oscillators. J. comp. Physiol. (A) 150 (1983) 51-60. 117 Winfree, A.T., Circadian timing of sleepiness in man and woman. Am. J. Physiol. 243 (1982) R193 R204. 118 Winfree, A.T., Impact of a circadian clock on the timing of human sleep. Am. J. Physiol. 245 (1983) R497 R504. 119 Winfree, A.T., Exploratory data analysis: published records of uncued human sleep-wake cycles, in: Mathematical Models of the Circadian Sleep-Wake Cycle, pp. 187-199. Eds M.C. Moore-Ede and C.A. Czeister. Raven Press, New York 1984. 120 Zulley, J., Wever, R.A., and Aschoff, J., The dependence of onset and duration of sleep on the circadian rhythm of rectal temperature. Pfl/igers Arch. 391 (1981) 314-318. 0014-4754/86/010001-1351.50 + 0.20/0 9 Birkh/iuser Verlag Basel, 1986 Sensitivity variations in insect chemoreceptors; a review I W.M. Blaney 2, L. M. Schoonhoven 3 and M. S.J. Simmonds 4 Behavioral Entomology Group, Department of Zoology, Birkbeck College, Malet Street, London WC1E 7HX (England) Key words. Insects; chemoreceptor; receptor sensitivity; feeding behavior; central regulating mechanisms; peripheral regulating mechanisms; sensory behavior. Introduction The feeding behavior of insects, like that of other animals, depends heavily on neural input from their chemical senses. Therefore the chemoreceptors of insects have attracted much interest and the gustatory sense of blowflies, and to a lesser extent of lepidopterous larvae, are among the best studied chemoreceptor systems in the invertebrates 23"6~ These studies have concentrated on de- termining receptor specificities and sensitivities, with the aim of elucidating the neural code which governs food selection behavior. Concomitant observations on the structure of chemoreceptors have revealed their micro- 76 architecture . Since there is no unequivocal evidence for the existence of efferent neural control of insect chemore- ceptors, most studies on the relationships between sen- sory input and insect behavior assume receptor activity to be solely dependent on stimulus characteristics. A grow- ing number of reports, however, indicate that receptor sensitivity may vary depending on developmental stage, feeding history and/or physiological state of the insect. Such peripheral neural changes and the processes which regulate them are the subject of this review. Changes in receptor characteristics Inconstancies in sensory input to the central nervous system (cns) under standardized stimulus conditions may be due to either changes in the accessibility of the recep- tors to the stimulus or to sensitivity changes in the recep- tors per se. The gustatory pegs on the palps of locusts exemplify sensilla which become unresponsive to chemi- cals after the insect has finished a meal, due to the closure of their distal orifices 9. Some reports in the literature suggest that in flies and caterpillars also, a partial con-