Development of fast singing muscles in a katydid.

In males of the katydid Neoconocephalus robustus, mesothoracic wings are used in flight (wing stroke frequence = 20 Hz) and stridulation (200 Hz), while the metathoracic wings are used in flight alone. Most mesothoracic wing muscles produce much briefer isometric twitches than metathoracic counterparts. The mesothoracic first tergocoxal muscle (TCX1) has a twitch duration (onset to 50% relaxation, 35 degrees C) of 6-8 ms and the metathoracic TXC1 a twitch duration of 12-15 ms. The TCX1 muscles from animals one and two instars from adulthood produce twitches similar in duration to those of the adult metathoracic TCX1. The twitch duration of the mesothoracic TCX1 acquires its adult brevity gradually over the first 5 days of adult life. Both TCX1 muscles increase greatly in size and mitochondrial content around the time of the terminal molt. During this period the mesothoracic TCX1 develops narrower myofibrils and a smaller ratio of fibril volume to sarcoplasmic reticulum volume than is characteristic of the metathoracic TCX1. Changes in the ultrastructure of the mesothoracic TCX1 precede changes in contraction kinetics around the time of the terminal molt so that there is not a strict correlation between muscle structure and performance during the period of rapid growth.