Loss of Midgut Alkalinity in Chilled or Narcotized Mosquito Larvae1

Larvae of Culex pipiens, Aedes aegypti, and Anopheles stephensi normally maintain the contents of the anterior midgut at about pH 10. If chilled to inactivity, pH declines to near neutrality in 3-5 hours. On return to room temperatures, activity recommences within 5 min and the anterior midgut pH starts to rise a few minutes later, generally attaining normal values within 2 hours. Larvae of C. pipiens narcotized by CO2 or chloroform became inactive in 5-10 min, with concurrent decline in midgut pH to neutrality; if the narcotizing agent was removed at the 1st signs of inactivity, activity recommenced about 15 (CO2) and 45 (chloroform) min later, accompanied, in those individuals that ultimately recovered normal activity, by the gradual development of high alkalinity in the midgut. For midgut alkalinity to redevelop after inactivation, it is crucial that the spiracles be open at the water surface and tracheal ventilation unhindered. Since pH begins to decline within a minute of removing the intact gut from a live larva, it is concluded that gut contents are virtually unbuffered and that maintenance of high pH in the normal larva must be a continuously active metabolic process, apparently dependent on an unrestricted air supply.