Functional organisation of the metathoracic femoral chordotonal organ in the cricket Acheta domesticus

The metathoracic chordotonal organ of the cricket Acheta domesticus (Gryllidae) consists of two closely associated scoloparia situated proximally within the femur with their distal ends connected by a pair of ligaments to an apodeme arising from the tibia. The smaller scoloparium is associated with the dorsal ligament, which arises from the dorsal surface of the apodeme 150 µm from its proximal end. The larger scoloparium is attached directly to the proximal end of the apodeme by a larger ventral ligament. Both ligaments are composed of bundles of attachment cells containing densely packed microtubules. Longitudinally orientated, Acid-Fuchsin-staining fibrils are found in an extracellular matrix surrounding the individual attachment cells. Similar fibrils occur in the sheath surrounding each ligament. The fibrils are thickest and most densely packed in the sheath surrounding the ventral ligament. They are thinner and more sparsely distributed in the sheath of the dorsal ligament. The finest fibrils are found in the extracellular matrix surrounding individual attachment cells. Staining with phosphotungstic acid provides the first evidence that they are elastic. The ventral ligament also contains a spring-like cuticular core arising as a proximal extension of the apodeme. As femoro-tibial angle changes, the cuticular core changes in length, shortening with tibial extension and lengthening with flexion. Ventral ligament attachment cells terminate at different levels along the cuticular core. This arrangement provides a new possible mechanism for differential sensitivity of the sensory neurones associated with the attachment cells.