Temperature will influence the components of the katydid sound production system in a manner similar to that for cicadas. Although the speed of contraction will be determined by the ultrastructure of the muscle (particularly the amount of sarcoplasmic reticulum) (Josephson, 1975), individual muscles will still exhibit temperature-dependent contraction kinetics (Josephson, 1973, 1981, 1984). Temporal parameters of katydid song, such as wing stroke and tooth strike rates, have been shown to be temperature dependent (Frings and Frings, 1957; Shaw, 1968; Walker et al., 1973;
Walker, 1975a, 1975b; Samways, 1976a; Gwynne and Bailey, 1988; Stiedl etal., 1994) as expected in ectotherms. For example, the chirp rate of Neoconocephalus ensiger (Harris) correlates to ra with a Q10 of about two (Frings and Frings, 1957). Even though temperature varies the acoustic parameters of some katydid calls, there are species who can alter the rate of their calls to remain in synchrony with the temporal patterns of calls produced by males at different temperatures (Samways, 1976a), as seen in some crickets.
Although some katydids appear to be ectotherms, others are endothermic like cicadas (Heath and Josephson, 1970; Josephson, 1973, 1984). There is a low correlation between song components and ra in these species (Walker et al., 1973). The high rate of wing movement necessary to generate their acoustic signals suggests that katydids would elevate Th (or Tmuscle) in order to be able to contract their wing muscles fast enough to produce their calling song.
Recordings of Th in the field and in the laboratory illustrate that Neoconocephalus robustus (Scudder) actively regulates its Th during acoustic activity with Th measured as much as 15°C above ambient during activity (Heath and Josephson, 1970). Recordings of muscle electrical potentials show that the animals elevate Th through a period of shivering thermogenesis where antagonistic wing muscles are activated in a synchronous fashion causing Th to elevate without wing movement. When the animals reach a Th sufficient to support calling (33.5°C), the electrical activity of the antagonistic wing musculature begins to occur out of phase. This stimulates the opening and closing of the wings which can result in sound production (Heath and Josephson, 1970) (Figure 7.6).
The wing musculature of N. robustus is synchronous (Josephson and Halverson, 1971). The high frequency of wing movements needed to generate the song can only be accomplished with elevated Tmuscie. The increase in percentage of sarcoplasmic reticulum within the muscles (Josephson and Halverson, 1971) can account for the high contraction frequency, but sufficient contractile speed and tension must also be produced if sound is to be generated. The Q10 effect would prohibit rapid wing movements without the elevation of Th (or Tmuscle). In addition, Josephson (1973) showed that endothermy and the elevated Tmuscie were necessary to produce the song in Euconocephalus nasutus (Thunberg). Measurements of the contraction kinetics demonstrate that E. nasutus muscles would enter tetany if muscle and ambient temperatures were equal. Only by elevating muscle temperature can the contractions occur rapidly enough to produce the song (Josephson, 1973). Similar changes to morphology are seen in rapidly contracting cicada muscles which also operate at temperatures well above ambient (Josephson and Young, 1985).
The endothermic species expend significantly greater energy than ectothermic species. It was estimated that N. robustus would expend 0.8% of its body mass in one hour of singing just to regulate its Th (additional energy would be required to move the wings and generate sound energy). This raises the question why the species would expend that much energy to thermoregulate when ectothermy is a viable alternative. Heath and Josephson (1970) suggested that the endothermy used by N. robustus is a means to isolate the species from the cogener, N. ensiger, with which it shares a habitat. The elevated Th of N. robustus permits the production of a loud, highly distinctive song [150 to 200 chirps sec"1 (Josephson and Halverson, 1971)] in comparison to the soft intermittent song (10 to 15 chirps sec"of the presumed ectothermic N. ensiger (Heath and Josephson, 1970).
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