Developmental constraint of insect audition

Functional Diversity of Vibrational Signaling Systems in Insects

Communication by substrate-borne mechanical waves is widespread in insects. The specifics of vibrational communication are related to heterogeneous natural substrates that strongly influence signal transmission. Insects generate vibrational signals primarily by tremulation, drumming, stridulation, and tymbalation, most commonly during sexual behavior but also in agonistic, social, and mutualistic as well as defense interactions and as part of foraging strategies. Vibrational signals are often part of multimodal communication. Sensilla and organs detecting substrate vibration show great diversity and primarily occur in insect legs to optimize sensitivity and directionality. In the natural environment, signals from heterospecifics, as well as social and enemy interactions within vibrational communication networks, influence signaling and behavioral strategies. The exploitation of substrate-borne vibrational signaling offers a promising application for behavioral manipulation in pest control.

Early postembryogenic development of the subgenual organ complex in the stick insect Sipyloidea sipylus

Stick insects have elaborate mechanosensory organs in their subgenual organ complex in the proximal tibia, particularly the distal organ with scolopidial sensilla in linear arrangement. For early postembryonic developmental stages of Sipyloidea sipylus (Phasmatodea: Necrosciinae), the neuroanatomy of the scolopidial organs in the subgenual organ complex and the campaniform sensilla is documented by retrograde axonal tracing, and compared to the adult neuroanatomy. Already after hatching of the first larval instars are the sensory structures of subgenual organ and distal organ as well as tibial campaniform sensilla differentiated. In the distal organ, the full set of sensilla is shown in all larval stages examined. This finding indicates that the sensory organs differentiate during embryogenesis, and are already functional by the time of hatching. The constancy of distal organ sensilla over postembryonic stages allows investigation of the representative number of sensilla in adult animals as well as in larval instars. Some anatomical changes occur by postembryogenic length increase of the distal organ, and grouping of the anterior subgenual sensilla. The embryonic development of scolopidial sensilla is similar for auditory sensilla in hemimetabolous Orthoptera (locusts, bushcrickets, crickets) where tympanal membranes develop during postembryogenic stages, conferring a successive gain of sensitivity with larval moults.

Sensing of Substrate Vibrations in the Adult Cicada Okanagana rimosa (Hemiptera: Cicadidae)

Detection of substrate vibrations is an evolutionarily old sensory modality and is important for predator detection as well as for intraspecific communication. In insects, substrate vibrations are detected mainly by scolopidial (chordotonal) sense organs found at different sites in the legs. Among these sense organs, the tibial subgenual organ (SGO) is one of the most sensitive sensors. The neuroanatomy and physiology of vibratory sense organs of cicadas is not well known. Here, we investigated the leg nerve by neuronal tracing and summed nerve recordings. Tracing with Neurobiotin revealed that the cicada Okanagana rimosa (Say) (Hemiptera: Cicadidae) has a femoral chordotonal organ with about 20 sensory cells and a tibial SGO with two sensory cells. Recordings from the leg nerve show that the vibrational response is broadly tuned with a threshold of about 1 m/s2 and a minimum latency of about 6 ms. The vibratory sense of cicadas might be used in predator avoidance and intraspecific communication, although no tuning to the peak frequency of the calling song (9 kHz) could be found.

Postembryonic development of the auditory system of the cicada Okanagana rimosa (Say) (Homoptera: Auchenorrhyncha: Cicadidae)

Cicadas (Homoptera: Auchenorrhyncha: Cicadidae) use acoustic signalling for mate attraction and perceive auditory signals by a tympanal organ in the second abdominal segment. The main structural features of the ear are the tympanum, the sensory organ consisting of numerous scolopidial cells, and the cuticular link between sensory neurones and tympanum (tympanal ridge and apodeme). Here, a first investigation of the postembryonic development of the auditory system is presented. In insects, sensory neurones usually differentiate during embryogenesis, and sound-perceiving structures form during postembryogenesis. Cicadas have an elongated and subterranian postembryogenesis which can take several years until the final moult. The neuroanatomy and functional morphology of the auditory system of the cicada Okanagana rimosa (Say) are documented for the adult and the three last larval stages. The sensory organ and the projection of sensory afferents to the CNS are present in the earliest stages investigated. The cuticular structures of the tympanum, the tympanal frame holding the tympanum, and the tympanal ridge differentiate in the later stages of postembryogenesis. Thus, despite the different life styles of larvae and adults, the neuronal components of the cicada auditory system develop already during embryogenesis or early postembryogenesis, and sound-perceiving structures like tympana are elaborated later in postembryogenesis. The life cycle allows comparison of cicada development to other hemimetabolous insects with respect to the influence of specially adapted life cycle stages on auditory maturation. The neuronal development of the auditory system conforms to the timing in other hemimetabolous insects.