Structure of the subgenual organ in the green lacewing, Chrysoperla carnea

Functional Morphology of Leg Mechanosensory Organs in Early Postembryonic Development in the Stick Insect (Sipyloidea chlorotica)

The subgenual organ complex of stick insects has a unique neuroanatomical organisation with two elaborate chordotonal organs, the subgenual organ and the distal organ. These organs are present in all leg pairs and are already developed in newly hatched stick insects. The present study analyses for the first time the morphology of sensory organs in the subgenual organ complex for a membrane connecting the two sensory organs in newly hatched insects (Sipyloidea chlorotica (Audinet-Serville 1838)). The stick insect legs were analysed following hatching by axonal tracing and light microscopy. The subgenual organ complex in first juvenile instars shows the sensory organs and a thin membrane connecting the sensory organs resembling the morphology of adult animals. Rarely was this membrane not detected, where it is assumed as not developed during embryogenesis. The connection appears to influence the shape of the subgenual organ, with one end extending towards the distal organ as under tension. These findings are discussed for the following functional implications: (1) the physiological responses of the subgenual organ complex to mechanical stimuli after hatching, (2) the influence of the membrane on the displacement of the sensory organs, and (3) the connection between the subgenual organ and distal organ as a possible functional coupling.

The 3D ultrastructure of the chordotonal organs in the antenna of a microwasp remains complex although simplified

Insect antennae are astonishingly versatile and have multiple sensory modalities. Audition, detection of airflow, and graviception are combined in the antennal chordotonal organs. The miniaturization of these complex multisensory organs has never been investigated. Here we present a comprehensive study of the structure and scaling of the antennal chordotonal organs of the extremely miniaturized parasitoid wasp Megaphragma viggianii based on 3D electron microscopy. Johnston's organ of M. viggianii consists of 19 amphinematic scolopidia (95 cells); the central organ consists of five scolopidia (20 cells). Plesiomorphic composition includes one accessory cell per scolopidium, but in M. viggianii this ratio is only 0.3. Scolopale rods in Johnston's organ have a unique structure. Allometric analyses demonstrate the effects of scaling on the antennal chordotonal organs in insects. Our results not only shed light on the universal principles of miniaturization of sense organs, but also provide context for future interpretation of the M. viggianii connectome.

Vibration detection in arthropods: Signal transfer, biomechanics and sensory adaptations

In arthropods, the detection of vibrational signals and stimuli is essential in several behaviours, including mate recognition and pair formation, prey detection, and predator evasion. These behaviours have been studied in several species of insects, arachnids, and crustaceans for vibration production and propagation in the environment. Vibration stimuli are transferred over the animals' appendages and the body to vibrosensory organs. Ultimately, the stimuli are transferred to act on the dendrites of the mechanosensitive sensilla. We refer to these two different levels of transfer as macromechanics and micromechanics, respectively. These biomechanical processes have important roles in filtering and pre-processing of stimuli, which are not carried out by neuronal components of sensory organs. Also, the macromechanical transfer is posture-dependent and enables behavioural control of vibration detection. Diverse sensory organs respond to vibrations, including cuticular sensilla (slit sensilla, campaniform sensilla) and internal chordotonal organs. These organs provide various adaptations, as they occur at diverse body positions with different mechanical couplings as input pathways. Macromechanics likely facilitated evolution of vibrosensory organs at specific body locations. Thus, vibration detection is a highly complex sensory capacity, which employs body and sensory mechanics for signal filtering, amplification, and analysis of frequency, intensity and directionality.

CONVERGENT EVOLUTION OF COURTSHIP SONGS AMONG CRYPTIC SPECIES OF THE CARNEA GROUP OF GREEN LACEWINGS (NEUROPTERA: CHRYSOPIDAE: CHRYSOPERLA)

Although traits of related species are likely to be similar due to common ancestry, mating signals are an exception. In singing insects, for example, song similarity has been documented only for allopatric or allochronic species pairs, and even then, not often. Where song similarity does occur, it has been logically attributed to the inheritance of ancestral traits rather than convergence. It is quite common for related, sympatric insect species to differ dramatically in calling song, which is predicted by evolutionary theory to maximize intraspecific mating success. Given that there are a limited number of ways to make sounds on anatomically similar organs and given that there would be no selective pressure for songs to differ in widely separated geographic areas, convergence in songs among related species living on different continents might be expected. Here we present the first well-documented case of such convergence, in a group of sibling, cryptic species characterized by substrate-borne vibrational mating songs. In this example from green lacewings of the carnea group of the genus Chrysoperla, a variety of statistical tests shows that one species in North America and another in Asia possess songs that are strikingly similar to each other. DNA data demonstrate that the species involved belong to divergent speciose lineages, and behavioral data demonstrate that the convergent songs are readily accepted by members of both species.

Chordotonal organs in hemipteran insects: unique peripheral structures but conserved central organization revealed by comparative neuroanatomy

Hemipteran insects use sophisticated vibrational communications by striking body appendages on the substrate or by oscillating the abdominal tymbal. There has been, however, little investigation of sensory channels for processing vibrational signals. Using sensory nerve stainings and low invasive confocal analyses, we demonstrate the comprehensive neuronal mapping of putative vibration-responsive chordotonal organs (COs) in stink bugs (Pentatomidae and Cydinidae) and cicadas (Cicadidae). The femoral CO (FCO) in stink bugs consists of ventral and dorsal scoloparia, homologous to distal and proximal scoloparia in locusts, which are implicated in joint movement detection and vibration detection, respectively. The ligament of the dorsal scoloparium is distally attached to the accessory extensor muscle, whereas that of the ventral scoloparium is attached to a specialized tendon. Their afferents project to the dorso-lateral neuropil and the central region of the medial ventral association center (mVAC) in the ipsilateral neuromere, where presumed dorsal scoloparium afferents and subgenual organ afferents are largely intermingled. In contrast, FCOs in cicadas have decreased dorsal scoloparium neurons and lack projections to the mVAC. The tymbal CO of stink bugs contains four sensory neurons that are distally attached to fat body cells via a ligament. Their axons project intersegmentally to the dorsal region of mVACs in all neuromeres. Together with comparisons of COs in different insect groups, the results suggest that hemipteran COs have undergone structural modification for achieving faster signaling of resonating peripheral tissues. The conserved projection patterns of COs suggest functional importance of the FCO and subgenual organ for vibrational communications.

Sensory neuroanatomy of stick insects highlights the evolutionary diversity of the orthopteroid subgenual organ complex

The subgenual organ is a scolopidial sense organ located in the tibia of many insects. In this study the neuroanatomy of the subgenual organ complex of stick insects is clarified for two species, Carausius morosus and Siyploidea sipylus. Neuronal tracing shows a subgenual organ complex that consists of a subgenual organ and a distal organ. There are no differences in neuroanatomy between the three thoracic leg pairs, and the sensory structures are highly similar in both species. A comparison of the neuroanatomy with other orthopteroid insects highlights two features unique in Phasmatodea. The subgenual organ contains a set of densely arranged sensory neurons in the anterior-ventral part of the organ, and a distal organ with 16-17 scolopidial sensilla in C. morosus and 20-22 scolopidial sensilla in S. sipylus. The somata of sensory neurons in the distal organ are organized in a linear array extending distally into the tibia, with only a few exceptions of closely associated neurons. The stick insect sense organs show a case of an elaborate scolopidial sense organ that evolved in addition to the subgenual organ. The neuroanatomy of stick insects is compared to that studied in other orthopteroid taxa (cockroaches, locusts, crickets, tettigoniids). The comparison of sensory structures indicates that elaborate scolopidial organs have evolved repeatedly among orthopteroids. The distal organ in stick insects has the highest number of sensory neurons known for distal organs so far.

Structure and sensory physiology of the leg scolopidial organs in Mantophasmatodea and their role in vibrational communication

Individuals of the insect order Mantophasmatodea use species-specific substrate vibration signals for mate recognition and location. In insects, substrate vibration is detected by mechanoreceptors in the legs, the scolopidial organs. In this study we give a first detailed overview of the structure, sensory sensitivity, and function of the leg scolopidial organs in two species of Mantophasmatodea and discuss their significance for vibrational communication. The structure and number of the organs are documented using light microscopy, SEM, and x-ray microtomography. Five scolopidial organs were found in each leg of male and female Mantophasmatodea: a femoral chordotonal organ, subgenual organ, tibial distal organ, tibio-tarsal scolopidial organ, and tarso-pretarsal scolopidial organ. The femoral chordotonal organ, consisting of two separate scoloparia, corresponds anatomically to the organ of a stonefly (Nemoura variegata) while the subgenual organ complex resembles the very sensitive organs of the cockroach Periplatena americana (Blattodea). Extracellular recordings from the leg nerve revealed that the leg scolopidial organs of Mantophasmatodea are very sensitive vibration receptors, especially for low-frequency vibrations. The dominant frequencies of the vibratory communication signals of Mantophasmatodea, acquired from an individual drumming on eight different substrates, fall in the frequency range where the scolopidial organs are most sensitive.

The structure and function of auditory chordotonal organs in insects

Insects are capable of detecting a broad range of acoustic signals transmitted through air, water, or solids. Auditory sensory organs are morphologically diverse with respect to their body location, accessory structures, and number of sensilla, but remarkably uniform in that most are innervated by chordotonal organs. Chordotonal organs are structurally complex Type I mechanoreceptors that are distributed throughout the insect body and function to detect a wide range of mechanical stimuli, from gross motor movements to air-borne sounds. At present, little is known about how chordotonal organs in general function to convert mechanical stimuli to nerve impulses, and our limited understanding of this process represents one of the major challenges to the study of insect auditory systems today. This report reviews the literature on chordotonal organs innervating insect ears, with the broad intention of uncovering some common structural specializations of peripheral auditory systems, and identifying new avenues for research. A general overview of chordotonal organ ultrastructure is presented, followed by a summary of the current theories on mechanical coupling and transduction in monodynal, mononematic, Type 1 scolopidia, which characteristically innervate insect ears. Auditory organs of different insect taxa are reviewed, focusing primarily on tympanal organs, and with some consideration to Johnston's and subgenual organs. It is widely accepted that insect hearing organs evolved from pre-existing proprioceptive chordotonal organs. In addition to certain non-neural adaptations for hearing, such as tracheal expansion and cuticular thinning, the chordotonal organs themselves may have intrinsic specializations for sound reception and transduction, and these are discussed. In the future, an integrated approach, using traditional anatomical and physiological techniques in combination with new methodologies in immunohistochemistry, genetics, and biophysics, will assist in refining hypotheses on how chordotonal organs function, and, ultimately, lead to new insights into the peripheral mechanisms underlying hearing in insects.

A contribution to the functional morphology of the femoral chordotonal organ in the green lacewing Chrysoperla carnea (Neuroptera)

The femoral chordotonal organ (FCO) and the subgenual organ (SGO) of the green lacewing Chrysoperla carnea were examined by conventional light and confocal laser scanning microscopy in order to search for neuroactive substances which are used for neurotransmission in sensory cells of these organs. Antibodies against serotonin, histamine and choline acetyltransferase were tested immunohistochemically. In the FCO, antiserum against serotonin strongly labelled cell bodies and axons of about 16 sensory cells. In the proximal scoloparium all 12 sensory cells showed immunoreaction with antiserotonin. In the distal scoloparium only four of 40 sensory cells were immunoreactive. These results suggest that different neuroactive substances are employed as neurotransmitters in the FCO of the green lacewing and that the proximal scoloparium and the distal scoloparium are functionally differentiated. Contrary to the FCO in the locust, acetylcholine was not found as a neurotransmitter in the FCO of the green lacewing. Additionally, histamine showed a negative result in the sensory cells of the FCO. Other neuroactive substances seem to be used as transmitters in the SGO because none of the tested antibodies showed positive reaction.

Communication with substrate-borne signals in small plant-dwelling insects

Vibratory signals of plant-dwelling insects, such as land bugs of the families Cydnidae and Pentatomidae, are produced mainly by stridulation and/or vibration of some body part. Signals emitted by the vibratory mechanisms have low-frequency characteristics with a relatively narrow frequency peak dominant around 100 Hz and differently expressed frequency modulation and higher harmonics. Such spectral characteristics are well tuned to the transmission properties of plants, and the low attenuation enables long-range communication on the same plant under standing wave conditions. Frequencies of stridulatory signals extend up to 10 kHz. In some groups, vibratory and stridulatory mechanisms may be used simultaneously to produce broadband signals. The subgenual organ, joint chordotonal organs, campaniform sensilla and mechanoreceptors, such as the Johnston's organ in antennae, are used to detect these vibratory signals. Species-specific songs facilitate mate location and recognition, and less species-specific signals provide information about enemies or rival mates.

Johnston's organ and central organ in Nezara viridula (L.) (Heteroptera, Pentatomidae)

The fine structure of Johnston's organ and central organ in Nezara viridula (Heteroptera, Pentatomidae) is described. Johnston's organ consists of 45 scolopidia distributed around the periphery of the distal part of the third antennal segment (distal pedicellite). The scolopidia are anchored separately in invaginations of joint cuticle between the pedicel and flagellum. The scolopidia are amphinematic and each scolopidium comprises three sensory cells and three enveloping cells. The latter are a proximal scolopale cell with a typical labyrinth, an attachment cell filled with many microtubules, and a distal accessory cell also filled with microtubules. Axons of 17 scolopidia gather and join one antennal nerve; 28 scolopidia of the opposite side, extend axons into the other antennal nerve. The central organ consists of seven mononematic scolopidia, which comprise of one or two sensory cells. They anchor in the same joint as the scolopidia of Johnston's organ. The sensory cell bodies of the central organ are located close to the antennal nerves, more proximally than those of Johnston's organ. The axons of four scolopidia join one antennal nerve and those of the remaining three scolopidia join the other antennal nerve. Enveloping cells similar to those in Johnston's organ are present in the central organ.