A pair of lateral chordotonal organs is found in abdominal segments 2 to 8 in larvae of Anax imperator Leach, Aeshna cyanea r(Müll.)and Libellula depressa L. The ultrastructure of the chordotonal organs has been determined. Each contains three scolopidia embedded in a connective tissue strand. A single type 1 cilium from a bipolar neuron innervates each scolopidium and is surrounded by a scolopale cell which forms the intracellular scolopale, extracellular space and distal cap. Surrounding the distal region of the scolopidium is the attachment cell. Activity from the longitudinal and vertical stretch receptors has been recorded from an intact larva. These receptors behave antagonistically to each other during rhythmic ventilation. Electrophysiological recordings also indicate that the chordotonal organ is active during expiration. This suggests a relaxation sensitive organ, a finding which is discussed in relation to its ultrastructure.