Patterns of ventilation in dragonfly larvae.

Respiration in the aquatic larvae of anisopteran dragonilies is of interest from a comparative point of view in that it utilizes a tidal ventilation mechanism in an aquatic medium. In most animals where water is the respiratory medium the flow over the respiratory surfaces is generally undirectional and this probably requires less energy for the extraction of a given amount of oxygen. Tidal or reciprocating pumping mechanisms in water are only truly found here and in some sea-cucumbers. The respiratory surface in dragonfly larvae is also of interest since it is made up of tracheal gills so that the oxygen is transferred from an aquatic medium to a gaseous phase. The general nature of ventilation is well known but accounts of the detailed patterns are largely dependent upon direct observation. There are some excellent accounts, but inevitably they are based upon observation over a relatively short period and small number of individuals. Tonner (1936) divided the respiratory patterns into three main types. He and many other authors also observed the way in which the respiratory mechanisms for ventilating the rectal chamber had been made use of in the locomotion of these insects. Methods of more continuous recording previously used, mainly for the study of swimming were applied to the study of ventilation in this animal by recording pressures in the respiratory chambers using a condenser manometer (Hughes, 1958). The purpose of the present investigation was to give a more complete account of the patterns of respiration using electrophysiological recording methods which would also give information concerning the detailed mechanisms which produce the required flow of water over the respiratory surface. The respiratory patterns of different individuals of the same species were compared and also those found in the three genera Anax, Aeshna and Libellula. In general the results have served to confirm the major types of ventilation but have shown that the patterns of a given individual can vary from time to time and that there are variations between individuals. While there are certainly differences between the three genera, the overall patterns are similar and presumably the mechanisms producing them are essentially the same. Because of differences of opinion regarding the morphological relationships and functioning of the muscles and cavities concerted in ventilation it has also been necessary to investigate these aspects.