Ectopic neurons and the organization of insect sensory systems

Summary The transplantation of appendages from one place to another on the body of crickets (Acheta domesticus) has been used to study the similarities and differences between the sensory systems of various ganglia. Mesothoracic legs have been transplanted to the abdomen in place of a cercus and cerci have been transplanted to thoracic leg stumps. After the ectopic sensory neurons had time to regenerate into the CNS, they were stained and their axonal arborizations examined. The results, which were concerned primarily with bristle receptors, revealed that bristle afferents on ectopic cerci arborized in ventral neuropil (the ventralmost association center) and leg afferents arborized in a ventral anterior region of the terminal abdominal ganglion. The results support the idea that each ganglion contains only a few distinct regions of neuropil (probably three), each receiving separate subsets of the afferent projection.The ectopic cerci were also shown to excite interneurons in the thoracic ganglia whose dendrites were located in the most ventral neuropil. These neurons normally respond to thoracic bristle afferents. Thus, the segregation of afferent axons has a correlate in the interneurons they excite.     

Transplantation of neurons reveals processing areas and rules for synaptic connectivity in the cricket nervous system

In order to assess the nature of spatial cues in determining the characteristic projection sites of sensory neurons in the CNS, we have transplanted sensory neurons of the cricket Acheta domesticus to ectopic locations. Thoracic campaniform sensilla (CS) function as proprioceptors and project to an intermediate layer of neuropil in thoracic ganglia while cercal CS transduce tactile information and project into a ventral layer in the terminal abdominal ganglion (TAG). When transplanted to ectopic locations, these afferents retain their modality-specific projection in the host ganglion and terminate in the layer of neuropil homologous to that of their ganglion of origin. Thus, thoracic CS neurons project to intermediate neuropil when transplanted to the abdomen and cercal CS neurons project to a ventral layer of neuropil when transplanted to the thorax. We conclude that CS can be separated into two classes based on their characteristic axonal projections within each segmental ganglion. We also found that the sensory neurons innervating tactile hairs project to ventral neuropil in any ganglion they encounter after transplantation. Ectopic sensory neurons can form functional synaptic connections with identified interneurons located within the host ganglia. The new contacts formed by these ectopic sensory neurons can be with normal targets, which arborize within the same layer of neuropil in each segmental ganglion, or with novel targets, which lack dendrites in the normal ganglion and are thus normally unavailable for synaptogenesis. These observations suggest that a limited set of molecular markers are utilized for cell–cell recognition in each segmentally homologous ganglion. Regenerating sensory neurons can recognize novel postsynaptic neurons if they have dendrites in the appropriate layer of neuropil. We suggest that spatial constraints produced by the segmentation and the modality-specific layering of the nervous system have a pivotal role in determining synaptic specificity. © 1993 John Wiley & Sons, Inc.     

Distribution and specific central projections of mechanoreceptors in the thorax and proximal leg joints of locusts

Summary Tactile hairs on the locust thorax can be divided into two classes by their external morphology and their central projection pattern: Short hairs, 10–100 m in length, which are assembled in distinct plates and rows, and long hairs, 100–800 m in length, which are distributed all over the body and are organized in large fields or aligned along the ridges of the appendages.The sensory fibers of the first class arborize in the lateral dorsal neuropile of thoracic ganglia and then extend further into the ipsilateral half of the corresponding ganglion in three main bundles from which fine rami of fibers end in the intermediate neuropile. In all three thoracic ganglia the projection pattern of homologous hair plates is similar.The sensory fibers of the second class exclusively terminate in special median ventral neuropiles, the ventral association center (VAC) and ventralmost ventral association center (VVAC). In addition fibers from meso- and metathoracic hairs, located close to the longitudinal midline of the animal, may terminate in the contralateral VAC and with one branch project to the next anterior ganglion through the ipsilateral connective. In contrast, fibers from prothoracic hairs were not found to leave their ganglion.With support by the DFG Neurale Mechanismen des VerhaltensSome of the studies were started at Universität Bielefeld, Fakultät für Biologie II (Abtlg. Prof. Dr. P. Görner)     

Anatomical and physiological observations on the organization of mechanoreceptors and local interneurons in the central nervous system of the wandering spider Cupiennius salei

Summary In the wandering spider Cupiennius salei, the functional neuroanatomy of leg mechanosensory receptor neurons and interneurons associated with a single leg neumere was investigated by combined intracellular recording and Lucifer yellow ionophoresis. Trichobothria axons that selectively respond to air currents and to low-frequency airborne vibrations have arborizations restricted to ventral regions of the appropriate leg neuromere. Receptor afferents that respond selectively to substrateborne vibrations are distributed ventrally in the corresponding leg neuromere and extend into certain interganglionic tract neuropiles. Golgi impregnation and intracellular dye filling show that local interneurons originate in ventral sensory neuropiles of leg neuromeres and ascend dorsally to terminate amongst dendrites of motor neurons. Local interneurons generally show higher thresholds for vibration stimuli than do receptors. Local interneurons typically receive inputs from one or several types of receptors. Some respond to stimulation of a single leg, others respond to stimulation of several legs on the same side of the body. The functional morphology of the receptor afferents is correlated with known physiological characteristics of slit sensilla and trichobothria. Structure and activity of the local interneurons are compared with analogous interneurons in other arthropods.     

Central projections of olfactory receptor neurons from single antennal and palpal sensilla in mosquitoes

In insects, olfactory receptor neurons (ORNs) are located in cuticular sensilla, that are present on the antennae and on the maxillary palps. Their axons project into spherical neuropil, the glomeruli, which are characteristic structures in the primary olfactory center throughout the animal kingdom. ORNs in insects often respond specifically to single odor compounds. The projection patterns of these neurons within the primary olfactory center, the antennal lobe, are, however, largely unknown.We developed a method to stain central projections of intact receptor neurons known to respond to host odor compounds in the malaria mosquito, Anopheles gambiae. Terminal arborizations from ORNs from antennal sensilla had only a few branches apparently restricted to a single glomerulus. Axonal arborizations of the different neurons originating from the same sensillum did not overlap.ORNs originating from maxillary palp sensilla all projected into a dorso-medial area in both the ipsi- and contralateral antennal lobe, which received in no case axon terminals from antennal receptor neurons. Staining of maxillary palp receptor neurons in a second mosquito species (Aedes aegypti) revealed unilateral arborizations in an area at a similar position as in An. gambiae.     

Morphological and physiological development of anterior thoracic stretch receptors in two isopods, <Emphasis Type="Italic">Armadillidium vulgare</Emphasis> and <Emphasis Type="Italic">Ligia exotica</Emphasis>

Abdominal muscle receptor organs (MROs) monitor the position and movement of abdomen in crustaceans. Thoracic segments of decapods are fused and immovable. It is speculated that MROs had retrograded simple shape, N-cells that lost receptor muscles, a receptor cell and accessory nerves. We focused on the effect of segmental movement in respect to thoracic N-cells and MROs in isopods that have movable thoracic segments. Armadillidium vulgare rolled up its body segments. Ligia exotica swam by quick movement of the posterior thoracic segments. Both isopods possessed N-cells and MROs in the thorax. N-cells were a simple structure, but N-cells from the second and third thoracic segments of A. vulgare had a muscle strand. MROs^T3–T4 (from the third and fourth thoracic segments) of A. vulgare had two receptor muscles. MROs^T3–T4 of L. exotica had one long receptor muscle. N-cells of both species and MROs of A. vulgare showed slowly adapting stretch-activated discharges. MROs of L. exotica showed both slowly and rapidly adapting discharges. The stretch-activated responses of N-cells and MROs inhibited each other. N-cells or MROs in the thorax of isopods are not related to the segmental structure. The morphology and physiology of N-cells and MROs are specialized to species–specific behaviors.     

Evolution and ultrastructure of the bovine spermatogonia precursor cell line

We have used a cytochemical technique to investigate the distribution of acetylcholinesterase (AChE) activity in the deutocerebrum of the brain of the sphinx moth Manduca sexta. To distinguish between extra-and intracellular pools of the enzyme, some brains were treated prior to histochemical staining with echothiophate, an irreversible AChE inhibitor which penetrates cell membranes very slowly and, therefore, inhibits only extracellular AChE. In the antennal nerve, fascicles of presumably mechanosensory fibers show echothiophateinsensitive AChE activity. They bypass the antennal lobe and project to the antennal mechanosensory and motor center of the deutocerebrum. In the antennal lobe, fibers in the coarse neuropil, cell bodies in the lateral cell group, and all glomeruli exhibit AChE activity. In most ordinary glomeruli, echothiophate-sensitive AChE activity is concentrated in the outer cap regions, corresponding to the terminal arborizations of olfactory afferents. A previously unrecognized glomerulus in the ventro-median antennal lobe shows uniform and more intense AChE-specific staining that the other glomeruli. No AChE activity appeared to be associated with malespecific pheromone-sensitive afferents in the macro-glomerular complex. About 67 interneurons with somata in the lateral cell group of the antennal lobe show echo-thiophate-insensitive AChE activity. These neurous seem to be members of two types of antennal-lobe projection neurons with fibers passing through the outer-antenno-cerebral tract to the protocerebrum. AChE-stained arborizations of these neurons appear to invade all glomeruli, including three distinguishable subunits of the male-specific macroglomerular complex. In echothiophate-treated animals, the projections of one of these types of fiber form large terminals in the lateral horn of protocerebrum, which partly protrude into the adjacent glial cell layer. The results suggest that extracellularly accessible AChE is associated with ordinary olfactory receptor terminals but apparently not with pheromone-sensitive afferents. Intracellular AChE appears to be present in antennal mechanosensory fibers and in two types of olfactory projection neurons of the antennal lobe. The study provides further evidence for cholinergic neurotransmission of most antennal afferents. The AChE-containing interneurons might be cholinergic as well or use the enzyme for functions unrelated to hydrolysis of acetylcholine.Abbreviations ACh acetylcholine - AChE acetylcholinesterase - AL antennal lobe - AMMC antennal mechanosensory and motor center - ChAT choline acetyltransferase - IACT inner antenno-cerebral tract - MGC macroglomerular complex     

Lateral ocellar nerve projections in the dragonfly brain

Summary The central projections of the lateral ocellar neurons of the dragonfly were examined using whole nerve cobalt iontophoresis, supplemented by sectioning of the nerve and brain for inspection in the light and electron microscopes. At E.M. level the presence of cobalt in filled axon profiles and cell bodies was confirmed by analysis of X-ray energy spectra in the microscope.The pathways, cell body sites and terminal arborizations of four large (7–25 m diameter) lateral ocellar neurons are described. Two of these fibers arborize in the ipsilateral posterior neuropil of the protocerebrum and two cross the brain and arborize in the contralateral posterior neuropil. Within each half of the posterior neuropil, two spatially separated regions of ocellar input have been identified. These regions receive median ocellar input plus input from either the ipsi- or contralateral ocellus, but not both. The arborizations of the contralateral fibers are more extensive than those of the ipsilateral fibers.One of the contralateral neurons crosses the brain in the region of the protocerebral bridge giving off a collateral in that region before descending to the posterior neuropil. This collateral arborizes almost immediately in a region receiving input from arborizations of a number of small ocellar neurons (those less than 5 m in diameter) from the ipsilateral ocellar nerve, together with small neurons from the median ocellar nerve, forming a region in each half of the brain which receives input from all three ocelli. The small lateral ocellar neurons associated with these arborizations have cell bodies adjacent to the lateral ocellar tracts.This work was supported in part by National Institute of Health Grants 2 RO1 EY-00777 and 1 KO4 EY-00040     

Trithorax: A new homoeotic mutation of Drosophila melanogaster causing transformations of abdominal and thoracic imaginal segments

Summary A new homoeotic mutation, trithorax ¹ (trx ¹), of Drosophila melanogaster is described which causes intersegmental transformations in both the adult thorax and abdomen. Specifically, the metathorax and ventral prothorax are partially transformed to mesothroax, whilst abdominal segments are partially transformed towards the first abdominal segment. Three variables are shown to influence the expressivity of the mutant phenotype: (1) presence of wild type copies of the gene in the female parent; (2) culture temperature during the first four hours of embryogenesis; (3) exposure of embryos to ether vapour at the blastoderm stage. In addition it is shown that phenocopies of the mutant phenotype can be induced by ether treatment of heterozygous trx ¹ embryos with a frequency higher than that of bithorax phenocopies in wild type embryos.These findings are together taken to imply that the mutated gene functions early during embryogenests, and that a common mechanism underlies the determination of all the thoracic and abdominal segments.     

The organization of plurisegmental mechanosensitive interneurons in the central nervous system of the wandering spider Cupiennius salei

Summary In spiders the bulk of the central nervous system (CNS) consists of fused segmental ganglia traversed by longitudinal tracts, which have precise relationships with sensory neuropils and which contain the fibers of large plurisegmental interneurons. The responses of these interneurons to various mechanical stimuli were studied electrophysiologically, and their unilateral or bilateral structure was revealed by intracellular staining. Unilateral interneurons visit all the neuromeres on one side of the CNS. They receive mechanosensory input either from a single leg or from all ipsilateral legs via sensory neurons that invade leg neuromeres and project into specific longitudinal tracts. The anatomical organization of unilateral interneurons suggests that their axons impart their information to all ipsilateral leg neuromeres. Bilateral interneurons are of two kinds, symmetric and asymmetric neurons. The latter respond to stimulation of all legs on one side of the body, having their dendrites amongst sensory tracts of the same side of the CNS. Anatomical evidence suggests that their terminals invade all four contralateral leg neuromeres. Bilaterally symmetrical plurisegmental interneurons have dendritic arborizations in both halves of the fused ventral ganglia. They respond to the stimulation of any of the 8 legs. A third class of cells, the ascending neurons have unilateral or bilateral dendritic arborizations in the fused ventral ganglia and show blebbed axons in postero-ventral regions of the brain. Their response characteristics are similar to those of other plurisegmental interneurons. Descending neurons have opposite structural polarity, arising in the brain and terminating in segmental regions of the fused ventral ganglia. Descending neurons show strong responses to visual stimulation. Approximately 50% of all the recorded neurons respond exclusively to stimulation of a single type of mechanoreceptor (either tactile hairs, or trichobothria, or slit sensilla), while the rest respond to stimulation of a variety of sensilla. However, these functional differences are not obviously reflected by the anatomy. The functional significance of plurisegmental interneurons is discussed with respect to sensory convergence and the coordination of motor output to the legs. A comparison between the response properties of certain plurisegmental interneurons and their parent longitudinal tracts suggests that the tracts themselves do not reflect a modality-specific organization.Abbreviations BPI bilateral plurisegmental interneuron - CNS central nervous system - FVG fused ventral ganglia - LT longitudinal tract - PI plurisegmental interneuron - PSTH peristimulus timehistogram - UPI unilateral plurisegmental interneuron     

Distribution and morphology of descending brain neurons in the cricket Gryllus bimaculatus

The number and distribution of descending brain neurons have been investigated in the cricket. The results are based on retrograde labeling of these cells with either Lucifer yellow or Neurobiotin via whole or small split portions of the cervical connectives. Various groups of cells and single neurons have been identified, and the morphology of more than 40 cells is described. Nearly 200 descending brain neurons can be stained via one cervical connective. Their perikarya are concentrated in clusters that occur ipsi- and contralateral to the filled connective and that lie dorsal and ventral in the brain. Descending cells only arborize in the nonglomerular neuropils of the brain and never branch in the optic lobe. Cells descending ipsilaterally never arborize in the contralateral hemisphere, whereas contralateral descending neurons often branch in both hemispheres. Irrespective of soma position, cells can arborize in the ventral and/or dorsal neuropils of the brain. Neurons with somata in the protocerebrum often have branches in the deutocerebrum and vice versa. The main arborizations of the cells from the prominent ventral i5 group are found in the same part of the protocerebrum. In contrast, various cells arborize in the ventral posterior deutocerebrum, but their somata are not located in different clusters. Thus, neurons from the same cluster may, but need not necessarily, arborize in the same brain area.     

Organization of the antennal motor system in the sphinx moth Manduca sexta

The antennae of the sphinx moth Manduca sexta are multimodal sense organs, each comprising three segments: scape, pedicel, and flagellum. Each antenna is moved by two systems of muscles, one controlling the movement of the scape and consisting of five muscles situated in the head capsule (extrinsic muscles), and the other system located within the scape (intrinsic muscles) and consisting of four muscles that move the pedicel. At least seven motoneurons innervate the extrinsic muscles, and at least five motoneurons innervate the intrinsic muscles. The dendritic fields of the antennal motoneurons overlap one another extensively and are located in the neuropil of the antennal mechanosensory and motor center. The density of motoneuronal arborizations is greatest in the lateral part of this neuropil region and decreases more medially. None of the motoneurons exhibits a contralateral projection. The cell bodies of motoneurons innervating the extrinsic muscles are distributed throughout an arching band of neuronal somata dorsal and dorsolateral to the neuropil of the antennal mechanosensory and motor center, whereas the cell bodies of motoneurons innervating the intrinsic muscles reside mainly among the neuronal somata situated dorsolateral to that neuropil. Received: 30 March 1996 / Accepted: 23 June 1996     

Parallel motor pathways from thoracic interneurons of the ventral giant interneurons system of the cockroach,Periplaneta americana

The data described here complete the principal components of the cockroach wind-mediated escape circuit form cercal afferents to leg motor neurons. It was previously known that the cercal afferents excite ventral giant interneurons which then conduct information on wind stimuli to thoracic ganglia. The ventral giant interneurons connect to a large population of interneurons in the thoracic ganglia which, in turn, are capable of exciting motor neurons that control leg movements. Thoracic interneurons that receive constant short latency inputs from ventral giant interneurons have been referred to as type A thoracic interneurons (TI_As). In this paper, we demonstrate that the motor response of TI_As occurs in adjacent ganglia as well as in the ganglion of origin for the TI_A. We then describe the pathway from TI_As to motor neurons in both ganglia. Our observations reveal complex interactions between thoracic interneurons and leg motor neurons. Two parallel pathways exist. TI_As excite leg motor neurons directly and via local interneurons. Latency and amplitude of post-synaptic potentials (PSPs) in motor neurons and local interneurons either in the ganglion of origin or in adjacent ganglia are all similar. However, the sign of the responses recorded in local interneurons (LI) and motor neurons varies according to the TI_A subpopulation based on the location of their cell bodies. One group, the dorsal posterior group, (DPGs) has dorsal cell bodies, whereas the other group, the ventral median cells, (VMC) has ventral cell bodies. All DPG interneurons either excited postsynaptic cells or failed to show any connection at all. In contrast, all VMC interneurons either inhibited postsynaptic cells or failed to show any connection. It appears that the TI_As utilize directional wind information from the ventral giant interneurons to make a decision on the optimal direction of escape. The output connections, which project not only to cells within the ganglion of origin but also to adjacent ganglia and perhaps beyond, could allow this decision to be made throughout the thoracic ganglia as a single unit. However, nothing in these connections indicates a mechanism for making appropriate coordinated leg movements. Because each pair of legs plays a unique role in the turn, this coordination should be controlled by circuits didicated to each leg. We suggest that this is accomplished by local interneurons between TI_As and leg motor neurons.     

The distribution of neurones immunoreactive for β-tyrosine hydroxylase,dopamine and serotonin in the ventral nerve cord of the cricket,Gryllus bimaculatus

The cellular localization of the biogenic amines dopamine and serotonin was investigated in the ventral nerve cord of the cricket, Gryllus bimaculatus, using antisera raised against dopamine, -tyrosine hydroxylase and serotonin. Dopamine-(n<-70) and serotonin-immunoreactive (n<-120) neurones showed a segmental arrangement in the ventral nerve cord. Some neuromeres, however, did not contain dopamine-immunoreactive cell bodies. The small number of stained cells allowed complete identification of brain and thoracic cells, including intersegmentally projecting axons and terminal arborizations. Dopamine-like immunostaining was found primarily in plurisegmental interneurones with axons descending to the soma-ipsilateral hemispheres of the thoracic and abdominal ganglia. In contrast, serotonin-immunostaining occurred predominantly in interneurones projecting via soma-contralaterally ascending axons to the thorax and brain. In addition, serotonin-immunoreactivity was also present in efferent cells and afferent elements. Serotonin-immunoreactive, but no dopamine-immunoreactive, varicose fibres were observed on the surface of some peripheral nerves. Varicose endings of both dopamine-and serotonin-immunoreactive neurones occurred in each neuromere and showed overlapping neuropilar projections in dorsal and medial regions of the thoracic ganglia. Ventral associative neuropiles lacked dopamine-like immunostaining but were innervated by serotonin-immunoreactive elements. A colocalization of the two amines was not observed. The topographic representation of neurone types immunoreactive for serotonin and dopamine is discussed with respect to possible modulatory functions of these biogenic amines in the central nervous system of the cricket.     

The D1 family dopamine receptor,DopR, potentiates hind leg grooming behavior in Drosophila

Drosophila groom away debris and pathogens from the body using their legs in a stereotyped sequence of innate motor behaviors. Here, we investigated one aspect of the grooming repertoire by characterizing the D1 family dopamine receptor, DopR. Removal of DopR results in decreased hind leg grooming, as substantiated by quantitation of dye remaining on mutant and RNAi animals vs. controls and direct scoring of behavioral events. These data are also supported by pharmacological results that D1 receptor agonists fail to potentiate grooming behaviors in headless DopR flies. DopR protein is broadly expressed in the neuropil of the thoracic ganglion and overlaps with TH‐positive dopaminergic neurons. Broad neuronal expression of dopamine receptor in mutant animals restored normal grooming behaviors. These data provide evidence for the role of DopR in potentiating hind leg grooming behaviors in the thoracic ganglion of adult Drosophila. This is a remarkable juxtaposition to the considerable role of D1 family dopamine receptors in rodent grooming, and future investigations of evolutionary relationships of circuitry may be warranted.     

Organization of motor neurons innervating the proboscis musculature in Drosophila melanogaster meigen (diptera : Drosophilidae)

The present study addresses the question as to how the motor neurons involved in feeding in Drosophila melanogaster Meigen (Diptera : Drosophilidae) are organized. The motor neurons have been visualized both by Golgi-silver impregnation and by intramuscular injection of horseradish peroxidase, and analyzed in light of the existing information on taste sensory system and the feeding behaviour. The motor neurons have been broadly classified into the following types: labial nerve motor neurons, pharyngeal nerve motor neurons, and accessory pharyngeal nerve motor neurons, depending on the nerve through which their axons exit. The arborization of all the motor neurons is confined to the suboesophageal ganglion (SOG). All of them have predominantly ipsilateral and some contralateral arborizations. Their dendrites predominantly occupy the ventral region of the neuropil of the SOG and partially overlap the taste sensory projections, thereby providing an opportunity for interaction with the taste sensory input. The pharyngeal motor neurons arborize more extensively in the ventral tritocerebram, anteroventral. and mid-ventral neuropil, whereas the dendritic fields of labial motor neurons are confined to the mid-ventral neuropil. There is a functional segregation in motor neuron organization: cibarial muscles involved in sucking are innervated by pharyngeal motor neurons, while the proboscis muscles involved in positioning, of the proboscis are innervated by labial motor neurons. We have also observed projections of the stomodaeal nerve in the tritocerebrum.     

Sensory Projections from Ectopic Appendages in an insect: Inherent Specificity and Influence of Location

Peripheral and central pathfinding by sensory axons from appendages was investigated in the fly Sarcophaga bullata . (a) Supernumerary appendages (haltere, wing, antenna and leg) were produced by imaginal disc transplantation at various ectopic sites, (b) Leg neuropil was deafferented by leg disc extirpation and in its place another leg disc was implanted. (c) The basal stalk of a leg disc connecting it with the thoracic ganglion was transected. Using cobalt chloride and HRP backfilling methods the pathways taken by the afferents from these experimentally altered appendages was examined. The results indicate that the larval nerves and the imaginal disc stalks act as guides for growing axons to locate their correct entry sites within the ventral ganglion. In the absence of these guides the axons follow any peripheral nerve, such as abdominal nerve, and enter the ganglion at inappropriate sites. However, within the ganglion they take particular routes, almost identical to those taken by axons from in situ appendages suggesting the existance of some kind of a labelled pathway. Deafferentation does not make the leg neuropil more attractive to ingrowing ectopic sensory axons.     

Anatomy of the ocellar interneurons of acridid grasshoppers

Summary The anatomy of the small ocellar interneurons in the brain of the acridid grasshopper Schistocerca vaga was revealed by cobalt-filling the three ocellar nerves and subsequent reconstructions from silver-intensified (Timm's method) serial sections.In total, 61 small ocellar interneurons were repeatedly identified with arborizations in many areas of the brain and optic lobe, including in particular the posterior neuropil, ocellar tracts, protocerebral bridge, lobula, ventral bridge and tritocerebral crotch, calyces, and antenno-glomerular tracts.Each ocellar nerve contains the axons of small cells that arborize in the other two ocellar tracts; these tracts are sites of ocellar integration. Direct interactions between the ocelli and compound eyes are suggested by the projections of small ocellar interneurons into the proximal lobula. Small cell arborizations from all three ocelli are distributed across much of the protocerebral bridge, implying a role for the bridge as an ocellar neuropil within the brain.Four of the small interneurons could be seen in whole-mount preparations and are demonstrated to be identical in five species of acridid grasshoppers of two different subfamilies: Schistocerca vaga, S. gregaria, Gastrimargus africanus, Trimerotropis pallidipennis, and Arphia conspersa.     

Neuronal architecture of the antennal lobe in Drosophila melanogaster

Summary Computer reconstruction of the antennal lobe of Drosophila melanogaster has revealed a total of 35 glomeruli, of which 30 are located in the periphery of the lobe and 5 in its center. Several prominent glomeruli are recognizable by their location, size, and shape; others are identifiable only by their positions relative to prominent glomeruli. No obvious sexual dimorphism of the glomerular architecture was observed. Golgi impregnations revealed: (1) Five of the glomeruli are exclusive targets for ipsilateral antennal input, whereas all others receive afferents from both antennae. Unilateral amputation of the third antennal segment led to a loss of about 1000 fibers in the antennal commissure. Hence, about 5/6 of the approximately 1200 antennal afferents per side have a process that extends into the contralateral lobe. (2) Afferents from maxillary palps (most likely from basiconic sensilla) project into both ipsi-and contralateral antennal lobes, yet their target glomeruli are apparently not the same as those of antennal basiconic sensilla. (3) Afferents in the antennal lobe may also stem from pharyngeal sensilla. (4) The most prominent types of interneurons with arborizations in the antennal lobe are: (i) local interneurons ramifying in the entire lobe, (ii) unilateral relay interneurons that extend from single glomeruli into the calyx and the lateral protocerebrum (LPR), (iii) unilateral interneurons that connect several glomeruli with the LPR only, (iv) bilateral interneurons that link a small number of glomeruli in both antennal lobes with the calyx and LPR, (v) giant bilateral interneurons characterized by extensive ramifications in both antennal lobes and the posterior brain and a cell body situated in the midline of the suboesophageal ganglion, and (vi) a unilateral interneuron with extensive arborization in one antennal lobe and the posterior brain and a process that extends into the thorax. These structural results are discussed in the context of the available functional and behavioral data.Abbreviations AC antennal commissure - AMMC antennal mechanosensory and motor center - iACT, mACT, oACT inner/middle/outer antenno-cerebral tract - bACTI, uACTI bilateral/unilateral ACT relay interneuron - AN antennal nerve - AST antenno-suboesophageal tract - FAI fine arborization relay interneuron - GSI giant symmetric relay interneuron - LI local interneuron - LPR lateral protocerebrum - SOG suboesophageal ganglion - TI thoracic relay interneuron - bVI bilateral V-relay interneuron     

Topographical organization of the auditory pathway within the prothoracic ganglion of the cricket Gryllus campestris L.

Summary The topographical organization of the prothoracic ganglion of the cricket, Gryllus campestris L., is described from horizontal, transverse, and sagittal sections of preparations specially treated to elucidate longitudinal tracts, commissures, and areas of neuropil. These structures were compared to those reported from other insect thoracic ganglia, resulting in still further evidence for a common basic morphological pattern among insect central nervous systems.Six types of auditory interneurons, all existing as mirrorimage pairs, were identified through intracellular application of the dye Lucifer yellow, and then related to several morphological patterns. Two intrasegmental neurons (ON1, ON2) are similar in location of cell bodies and course of neurites and axons; three intersegmental neurons (AN1, AN2, TN1) are likewise similar to one another. The axons of the two intrasegmental neurons cross the midline of the ganglion in the newly described omega commissure. Axons of the other four types all course within the median portion of the ventral intermediate tract and project intersegmentally.All six neuron types arborize within the ventral portion of the ring tract, the same neuropilar region in which auditory sensory neurons terminate. The ring tract is therefore considered the most important region for auditory information processing within the cricket prothoracic ganglion.