1. A study has been made of the patterns of ventilation in three genera of anisopteran dragonfly larvae by recording simultaneously the pressure changes in the branchial chamber and the dorso-ventral movements of the abdomen.
2. The three main types of ventilation described by Tonner have been confirmed and the characteristic pressure changes during normal, gulping, and chewing ventilation are described.
3. During normal ventilation there is a positive pressure phase of about 4 cm. water followed by a brief negative phase of ½ cm. water. The main expiratory phase is produced by the respiratory dorso-ventral muscles which are found in abdominal segments 4-9. Inspiration, which closely follows expiration, results from the elastic properties of the exoskeleton together with the activity of the two transverse muscles. One of these, lying at the anterior end of the fifth abdominal segment, forms the so-called diaphragm and divides the abdomen into two functional cavities.
4. Gulping ventilation is characterized by a maintained pressure of 20-40 cm. water during which there may be fluctuations associated with the chewing movements, which are mainly due to the intrinsic musculature of the colon and rectum.
5. Despite some differences in detail the main patterns of ventilation are very similar in Aeshna, Anax and Libellula.