Recruitment of ectodermal attachment cells via an EGFR-dependent mechanism during the organogenesis of Drosophila proprioceptors

Drosophila proprioceptors (chordotonal organs) are structured as a linear array of four lineage-related cells: a neuron, a glial cell, and two accessory cells, called cap and ligament, between which the neuron is stretched. To function properly as stretch receptors, chordotonal organs must be stably anchored at both edges. The cap cells are anchored to the cuticle through specialized lineage-related attachment cells. However, the mechanism by which the ligament cells at the other edge of the organ attach is not known. Here, we report the identification of specialized attachment cells that anchor the ligament cells of pentascolopidial chordotonal organs (lch5) to the cuticle. The ligament attachment cells are recruited by the approaching ligament cells upon reaching their attachment site, through an EGFR-dependent mechanism. Molecular characterization of lch5 attachment cells demonstrated that they share significant properties with Drosophila tendon cells and with mammalian proprioceptive organs.     

Structure and physiology of the locust femoral chordotonal organ

The connective chordotonal organs (COs) in the femora of the prothoracic and mesothoracic legs of the locust Schistocerca gregaria are divided into two parts, the proximal and the distal scoloparia. The proximal scoloparium contains about 150 small neurons and is anchored to the femoral cuticle. The distal scoloparium contains about 50 larger neurons and is connected at its proximal end to both the cuticle and the flexor tibiae muscle.Records were made from the distal scoloparium, classifying units by spike size. The tibial position/total activity response curve is ∪-shaped but when a small number of units is selected the responses occur only when the tibia is on one side of its centre position. The tonic responses display considerable hysteresis and a degree of adaptation which varies with the tibial angle. Units with phasic and phasic-tonic responses are common and their responsiveness depends on the range of angles the tibia is moved through. The same units respond strongly to flexor tibiae contraction with the tibia either fixed or free, and so may serve as receptors for tension in that muscle.The CO mediates phasic resistance reflexes in all three extensor tibiae motoneurons and tonic reflexes in the extensor ‘slow’ neuron. It is suggested that the very detailed information furnished by the CO is used in a complex way in the control of the femoral muscles.     

Morphology and neurophysiology of tarsal vibration receptors in the water strider Aquarius paludum (Heteroptera: Gerridae)

Substrate vibratory information receptors are extensively studied in insects and spiders, however for water surface dwelling species little data is available. We studied the vibration receptive organs in tarsi of the water strider Aquarius paludum, using light, transmission and scanning electron microscopes, and recorded the neural activity of the organs in response to vibrational stimuli, which were afterwards analysed with a custom made spike sorting program.We found that the tarsal chordotonal organ has one set of three scoloparia: one in the tarsomere I and two in the tarsomere II, all of which consisted of a few scolopidia. The chordotonal organ clearly responded to vibratory stimulation. Furthermore, we found that a pair of large subapical emergent dorsal setae, which had been deemed mechanosensory by previous authors, are not so. In turn, four ventral subapical trichobothria that are in direct contact with the water surface during locomotion, proved to be mechanosensory. The anatomical and ultrastructural observations support these electro-physiological results.     

Central projections of sensory cells of the midleg of the locust,Schistocerca gregaria

The central projections of trichoid hairs and of some scolopidial organs of the mesothoracic leg of the locust Schistocerca gregaria were studied by using nickel chloride backfilling and single cell recording. Trichoid hair sensilla on different parts of the legs project somatotopically in the ventral part of the ipsilateral neuropile of the mesothoracic ganglion. Generally, distally located receptors have their terminal arborizations in ventro-lateral areas of the neuropile, and proximally located receptors in ventro-medial areas. The axons of the subgenual organ and tarsal chordotonal organs project into the intermediate neuropile.     

Embryonic development and evolutionary origin of the Orthopteran auditory organs

Two different types of ears characterize the order of Orthopteran insects. The auditory organs of grasshoppers and locusts (Caelifera) are located in the first abdominal segment, those of bushcrickets and crickets (Ensifera) are found in the tibiae of the prothoracic legs. Using neuron-specific antibody labelling, we describe the ontogenetic origin of these two types of auditory organs, use comparative developmental studies to identify their segmental homologs, and on the basis of homology postulate their evolutionary origin. In grasshoppers the auditory receptors develop by epithelial invagination of the body wall ectoderm in the first abdominal segment. Subsequently, at least a part of the receptor cells undergo active migration and project their out-growing axons onto the next anterior intersegmental nerve. During this time the receptor cells and their axons express the cell-cell adhesion molecule, Fasciclin I. Similar cellular and molecular differentiation processes in neighboring segments give rise to serially homologous sensory organs, the pleural chordotonal organs in the pregenital abdominal segments, and the wing-hinge chordotonal organs in the thoracic segments. In more primitive earless grasshoppers pleural chordotonal organs are found in place of auditory organs in the first abdominal segment. In bushcrickets the auditory receptors develop in association with the prothoracic subgenual organ from a common developmental precursor. The auditory receptor neurons in these insects are homologous to identified mechanoreceptors in the meso- and metathoracic legs. The established intra- and interspecies homologies provide insight into the evolution of the auditory organs of Orthopterans.     

Evolution of the metathoracic tympanal ear and its mesothoracic homologue in the Macrolepidoptera (Insecta)

Two independent methods of comparison, serial homology and phylogenetic character mapping, are employed to investigate the evolutionary origin of the noctuoid moth (Noctuoidea) ear sensory organ. First, neurobiotin and Janus green B staining techniques are used to describe a novel mesothoracic chordotonal organ in the hawkmoth, Manduca sexta, which is shown to be serially homologous to the noctuoid metathoracic tympanal organ. This chordotonal organ comprises a proximal scolopidial region with three bipolar sensory cells, and a long flexible strand (composed of attachment cells) that connects peripherally to an unspecialized membrane ventral to the axillary cord of the fore-wing. Homology to the tympanal chordotonal organ in the Noctuoidea is proposed from anatomical comparisons of the meso- and metathoracic nerve branches and their corresponding peripheral attachment sites. Second, the general structure (noting sensory cell numbers, gross anatomy, and location of peripheral attachment sites) of both meso- and metathoracic organs is surveyed in 23 species representing seven superfamilies of the Lepidoptera. The structure of the wing-hinge chordotonal organ in both thoracic segments was found to be remarkably conserved in all superfamilies of the Macrolepidoptera examined except the Noctuoidea, where fewer than three cells occur in the metathoracic ear (one cell in representatives of the Notodontidae and two cells in those of other families examined), and at the mesothoracic wing-hinge (two cells) in the Notodontidae only. By mapping cell numbers onto current phylogenies of the Macrolepidoptera, we demonstrate that the three-celled wing-hinge chordotonal organ, believed to be a wing proprioceptor, represents the plesiomorphic state from which the tympanal organ in the Noctuoidea evolved. This ’trend toward simplicity’ in the noctuoid ear contrasts an apparent ’trend toward complexity’ in several other insect hearing organs where atympanate homologues have been studied. The advantages to having fewer rather than more cells in the moth ear, which functions primarily to detect the echolocation calls of bats, is discussed. Accepted: 18 June 1999     

Ultrastructure of the attachment of Pyrsonympha to the hind-gut wall of Reticulitermes tibialis

Protozoa of the genus Pyrsonympha are found attached to the cuticular lining of the hind-gut wall of the termite Reticulitermes tibialis by a specialized anterior structure referred to here as the ‘attachment organelle’. The structure of the hind-gut, the attachment organelle, and the nature of the attachment are described using light and electron microscopy. The attachment organelle is a highly folded membranous structure containing large 20 to 26 nm filaments. Although no unambiguous functional significance can presently be assigned to this attachment, it may serve to prevent loss of the protozoa from the intestinal tract or it may be involved in the nutritional symbiosis between termite and protozoa.     

Motor innervation of the supercontracting longitudinal ventro-lateral muscles of the blowfly larva

The gross motor innervation of the abdominal longitudinal ventro-lateral muscles of the larva of Calliphora erythrocephala is described. Two of these muscles, 6A and 7A, are innervated by the same two multiterminally-ending axons, and thus comprise a single motor unit. No difference is found between the axon diameters in the main nerve trunks, but there is a difference where the axons run over the muscle surface. Only the dorsal, inner, surface of the muscle is innervated. Electro-physiological results show two sizes of EPSP: the large fast EPSP presumably corresponds to the thicker axon and the small slow one to the thinner axon. Preliminary work indicates that it is not possible to distinguish between the two axons with electron microscopy; the presynaptic regions possess both ‘classical’ synaptic and ‘neurosecretory’ type vesicles, and have no glial cell covering.     

Features of the receptors of the alar system of locusts which have lost their ability to fly

Using two species of locusts, Romalia microptera Beavy and Podisma pedestris L., receptors of the wing apparatus are described: campaniform sensillas of the wing, hair receptors of the tegula, chordotonal organ and thorax stretch receptor. A comparative analysis of the receptors mentioned with the homologous sensitive organs, participating in the control of wing movements, is performed in well flying species (Locusta migratoria migratorioides and Schistocerca gregaria). Loss of ability to fly is accompanied with a sharp decrease in the wing campaniform sensillas and in the tegula proprioceptive hairs. Simultaneously, there is loss of connection between the thorax receptors and the wing elements that are present in good flyers. The thorax stretch receptor begins to innervate the longitudinal dorsal muscle, as it is observed in the abdominal segments. The data obtained make it possible to speak about homology of the tergal chordotonal organs and the thorax and abdomen stretch receptors and about the pathways of their evolution, when the insects obtain and loose their ability to fly.     

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.     

A strand receptor with a central cell body synapses upon spiking local interneurones in the locust

Summary Movements of the femoro-tibial joint of a locust hind leg are monitored by three classes of proprioceptors; a chordotonal organ (Usherwood et al. 1968), multipolar joint receptors (Coillot and Boistel 1968) and a strand receptor innervated by a single afferent with a central cell body (Bräunig 1985). All three classes are excited by imposed or voluntary extension of the tibia. The strand receptor (fe-tiSR) spikes tonically and at a frequency dependent upon the position of the joint whilst the multipolar joint receptors give overlapping information but for a more restricted range. The afferent from the strand receptor makes an excitatory connection with a spiking local interneurone in the midline group of the metathoracic ganglion. The central latency and consistency with which the EPSP follows each sensory spike suggests that the connection is direct. This interneurone also receives convergent inputs from neurones in the chordotonal organ, but not from multipolar joint receptors. Neither the strand receptor nor the multipolar joint receptors apparently synapse upon leg motor neurones that we have tested, in contrast to receptors in the chordotonal organ.     

Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Fast and slow receptor muscles

Receptor muscles of the abdominal muscle receptor organs of the crayfish, Procambarus clarkii, were examined by electron microscopy. Both the fast and the slow receptor strand comprises a single muscle fibre which is divided by invagination of the cell membrane into numerous cytoplasmic processes in its intermediate region (the so-called intercalated tendon). Most of these myofibrillar processes insert in this region, but some of them pass through the intermediate region without interruption and join the other portion of the fibre. Thus the receptor muscles, whilst maintaining cytoplasmic continuity throughout their whole length, are modified in their intermediate regions, becoming fasciculated and providing spaces which are occupied by the connective tissue and the dendrites of the sensory neurone. Clear-cut differences in fine structure are shown between the muscle of the two types of receptor unit. The fast receptor muscle shows the typical features of arthropod fast muscles, including short sarcomere length (on average 3.3 μm), cylindrical myofibrils, well-developed sarcoplasmic reticulum, and regular hexagonal array of the myofilaments. By contrast, the slow receptor muscle fibre is characterized by long sarcomeres (average 6.5 μm) and unique organization of the myofilaments, with very thick ‘thick’ filaments having diameters in the range of 25–36 nm surrounded by about 12 thin filaments.     

Synaptic input from serial chordotonal organs onto segmentally homologous interneurons in the grasshopper Schistocerca gregaria

We investigated the synaptic inputs from the serially homologous pleural, tympanal and wing-hinge chordotonal organs onto a set of identified homologous interneurons (714, 539, 529) in the ventral nerve cord of the grasshopper Schistocerca gregaria. Cobalt backfills show that afferents from all chordotonal organs project into stereotypic tracts in the central nervous system in which intracellular staining reveals the interneurons to have dendritic arborizations. Neuron 714 was found to receive excitatory bilateral synaptic input from all the serial chordotonal organs tested, from the second thoracic segment down to the seventh abdominal segment. Neuron 531, by contrast, only receives input from the chordotonal afferents on the first abdominal segment; those on the axon side are excitatory, while those on the soma side are inhibitory. The pattern of chordotonal input onto neuron 529 is similar to that seen for neuron 714, with the exception that neuron 529 receives no input from the forewing chordotonal organs. The pattern of afferent connectivities onto neurons 714, 531 and 529 differs with respect to those afferents which synapse directly or indirectly with the respective neuron. The synaptic inputs demonstrate a segmental specialization in the chordotonal system and thereby offer an insight into information processing in a modular sensory system.     

Range fractionation in the locust metathoracic femoral chordotonal organ

Summary Insect femoral chordotonal organs are internal proprioceptors which monitor the position and movements of the femur-tibia joint of the leg. The locust (Locusta migratoria) metathoracic femoral chordotonal organ is composed of approximately 100 neurones with a variety of response properties. In this study intracellular recordings were used to examine the range fractionation of phasic and tonic responses to tibial movements. Some neurones responded across the full range of leg angles, while others had restricted response ranges, and could therefore act as labeled lines. Neurones with maximal firing at mid-angles are described for the first time in a locust femoral chordotonal organ. Responses are discussed in terms of underlying structural constraints on signal transduction.Abbreviation (mt) FCO (metathoracic) femoral chordotonal organ     

Bimodal innervation of the infrared organ of Merimna atrata (Coleoptera,Buprestidae) by thermo- and mechanosensory units

The pyrophilous Australian “fire-beetle” Merimna atrata approaches forest fires and possesses abdominal infrared (IR) organs. Each round IR organ is centrally innervated by a sensory complex showing two different units: one thermoreceptive multipolar neuron and one mechanosensitive chordotonal organ (CO) consisting of two scolopidia. We investigated the CO and found that the scolopidia are mononematic (the scolopale cap remains below the cuticle) and monodynal (one sensory cell per scolopidium). The dendrites of the scolopidia extend anteriorly and are attached by their caps to the cuticle about in the middle of the absorbing area. Structural features at the site of innervation suggest that the CO measures minute thermal deformations caused by IR absorption. Therefore, an additional photomechanic component which has been described for the IR receptors of pyrophilous jewel beetles of the genus Melanophila can be proposed for the IR organ of Merimna. Because scolopidia can measure displacements in the subnanometer range, the CO may enhance the sensitivity of the IR organ. The sensory complex of the Merimna IR organ shows the same units and similar cuticular modifications as the tympanal organs of some noctuid moths. Therefore, a parallel evolution of insect ears and the Merimna IR organ is discussed.     

An integumentary gland secreting a territorial marking pheromone in Atta sp.: Detailed structure and histochemistry

An abdominal pheromone-producing gland in Atta sp. was examined using light and electron microscopy techniques. The gland is composed of a bunch of juxtaposed secretory units in which the secretory ductules open on to a cribellum close to the sting base.The structure and cycles of the secreting units are described. Each includes a secretory cell with an ‘end apparatus’, ductule cells and epidermal cells. The secretory cycle of glycoproteins accumulated in the ‘end apparatus’ is discussed and a functional interpretation of the morphological components of the application system is proposed.     

Ultrastructure and blood–nerve barrier of chordotonal organs in the Drosophila embryo

Chordotonal organs of Drosophila embryos have become models for studies of developmental biology and molecular genetics due to their consistent segmental placement and mutability. Our first goal was to find the origin and anatomical correlate of the blood–nerve barrier of this PNS proprioreceptor in wild type embryos. The concept of a blood–nerve barrier for the PNS of the Drosophila embryo is new, and the present data are the first in this regard. A second goal was to reveal the ultrastructure of these four-celled stretch receptors, focusing particularly on the ‘core’ of this organ: the bipolar neuron enclosed by a scolopale cell. These latter data have resulted in a graphic reconstruction of the chordotonal organ which reveals how the four consistent cells fit together. At Stage 13 we first observed a clearly recognizable scolopale cell with an enclosed neuron. Surprisingly, an operative blood–nerve barrier, comprised of occlusive pleated-sheet septate junctions, exists at this relatively early stage. A blood–brain barrier is not yet functioning in the CNS during this same stage, as the perineurium is not present until Stage 17. Cross-sectional views of a more mature chordotonal organ show that the neuron’s inner segment has a ‘tongue-in-groove’ formation which fits the dendrite into the scolopale cell. Other newly discovered fine structural features are: hemidesmosomes linking individual scolopale rod bundles to the inner dendrite, and a cap cell matrix bonding with the tip of the ciliary dendrite. Functional aspects of these findings are discussed.     

Colour and colour change in the grasshopper, Kosciuscola tristis

The males of the small grasshopper (Kosciuscola tristis), with a restricted range above 1830 m in the Australian Alps, exhibit a remarkable colour change. They are dark, almost black, when cold and change to a bright sky blue colour within minutes of exposure to warmth.Sections of cuticle fixed in the two conditions confirm that the cells underlying the cuticle contain two kinds of granules: large (diameter 1·0 μm) spherical, brown granules, and smaller (0·17 μm) less dense granules. In the blue (warm) condition the small granules are closely packed in the distal part of the cells, whereas the ‘black’ granules are found predominantly in the deeper proximal zones. Evidence is presented to suggest that the blue colour arises from Tyndall scattering of light by the suspension of small granules and is intensified by being seen against a dark background.In the black condition the black granules are found to have moved towards the surface, mingling with the smaller granules and ‘quenching’ the light scattering.The smaller granules are white in the isolated state. They consist of a mixture of uric acid and a pteridine, probably leucopterin.The epidermal cells contain numerous microtubules, which are directed towards the cell surface, that is, parallel to the direction of movement of the granules. It is possible that the microtubules are associated with the movement.     

An electrophysiological study of sound sensitive neurons in the ‘Primitive ear’ of Acheta domesticus

Crickets have two types of mechanisms for the reception of environmental sounds: (1) the tympanal organs in the two forelegs and (2) the freely articulated setal receptors on the abdominal ceri. The cercal setal receptors have hitherto received much less experimental attention as decoders of biologically significant sounds than have the tympano-receptors. In the present study the cercal auditory system of Acheta domesticus was examined electrophysiologically to determine its auditory frequency sensitivity, the tuning characteristics of individual units, and the synchronization between nerve impulses and stimulus frequency. Both pre- and postsynaptic units were examined in the fifth abdominal ganglion; several of the observed response patterns were compared with those of homologous cercal sensory neurons in Periplaneta americana. The results show that (1) A. domesticus possesses an elaborate array of cercal receptors which are highly sensitive to sounds, (2) the cercal setal receptors are more sensitive and numerous in the cricket than in the cockroach, and (3) the cercal auditory system can decode stimulus information by narrow tuning in individual cells and by synchronous discharge patterns; firing frequencies range up to 300 Hz in presynaptic sensory units and 60 Hz in the postsynaptic giants. The response patterns were related to the structure of the receptor and the behavioural adaptations of the insect.