tailieunhanh - Entomology 3rd edition - C.Gillott - Chapter 12

12 Sensory Systems 1. Introduction Organisms constantly monitor and respond to changes in their environment (both external and internal) so as to maintain themselves under the most favorable conditions for growth and reproduction. | 12 Sensory Systems 1. Introduction Organisms constantly monitor and respond to changes in their environment both external and internal so as to maintain themselves under the most favorable conditions for growth and reproduction. The structures that receive these environmental cues are sense cells and the cues are always forms of energy for example light heat kinetic as in mechanoreception and sound reception and potential as in chemoreception the sense of smell and taste Dethier 1963 . The sensory structures use the energy to do work namely to generate a message that can be conducted to a decoding area the central nervous system so that an appropriate response can be initiated. The message is of course in the form of a nerve impulse. Sensory structures are generally specialized so as to respond to only one energy form and are usually surrounded by accessory structures that modify the incident energy. As Dethier 1963 noted the small size and exoskeleton of insects have had marked influence on their sensory and nervous systems. Smallness and therefore short neural pathways provide for a very rapid response to stimuli. However it also means that there are relatively few axons and therefore a limited number of responses to a given stimulus. This has led to a situation in insects where stimulation of a single sense cell may trigger a series of responses. Further almost all insect sense cells are primary sense cells that is they not only receive the stimulus but initiate and transmit information to the central nervous system in other words they are true neurons. In contrast in vertebrates almost all sensory systems include both a specialized secondary sense cell and a sensory neuron that transmits information to the central nervous system. The cuticle provides protection and support by virtue ofits rigid inert nature yet sense cells must beable to respond to very subtle minute energy changes in the environment. Thus only where the cuticle is sufficiently weakened thinner