Nymphal development of the auditory system in the praying mantis Hierodula membranacea Burmeister (Dictyoptera, Mantidae).

Like other praying mantises, Hierodula membranacea has a single midline ear on the ventral surface of the metathorax. The ear comprises a deep groove with two tympana forming the walls. A tympanal organ on each side contains 30-40 scolopophorous sensillae with axons that terminate in the metathoracic ganglion in neuropil that does not match the auditory neuropil of other insects. Nymphal development of the mantis ear proceeds in three major stages: 1) The tympanal organ is completely formed with a full complement of sensillae before hatching; 2) the infolding and rotations that form the deep groove are completed primarily over the first half of nymphal development; and 3) over the last five instars (of ten), the tympana thicken and broaden to their adult size and shape, and the impedance-matching tracheal sacs also enlarge and move to become tightly apposed to the inner surfaces of the tympana. Auditory sensitivity gradually increases beginning with the fifth instar and closely parallels tympanum and tracheal sac growth. Late instar nymphs have auditory thresholds of 70-80 dB sound pressure level (SPL). Appropriate connections of afferents to a functional interneuronal system are clearly present by the eighth instar and possibly much earlier. The pattern of auditory system ontogeny in the mantis is similar to that in locusts and in noctuid moths, but it differs from crickets. In evolutionary terms, it is significant that the metathoracic anatomy of newly hatched mantis nymphs matches very closely the anatomy of the homologous regions in adult cockroaches, which are closely related to mantises but are without tympanal hearing, and in mantises that are thought to be primitively deaf.