Neural networks in the mushroom bodies of the honeybee

The Kenyon cells (K cells) or intrinsic neurones of the honeybee's mushroom bodies are organised as a series of arrays. In the calyces the arrays form concentric rings that are represented by rectilinear layers in the α and β lobes. The inputs to the calyces have been revealed by intraneuropilar cobalt injection into the optic and antennal lobes. Neurones from the medulla project to the collar neuropil of the calyx while the relay neurones of the antennal lobe project to the lip neuropil of the calyx. Extrinsic neurones of unknown polarity penetrating the α and β lobes have branching patterns that reflect the layered pattern of the intrinsic neurones. The study illustrates the feasibility of producing a fine grain map of the optic lobe and antennal lobe inputs to the mushroom bodies. It is suggested that the map could be produced by making cobalt injections into individual identified antennal glomeruli and at known sites in the medulla retinotopic mosaic.     

Projection pattern of poreplate sensory neurones in honey bee worker,Apis mellifera L. (Hymenoptera : Apidae)

The projections of olfactory receptor cells of the poreplate sensilla were studied in the worker honey bee, Apis mellifera L. (Hymenoptera : Apidae) by filling single sensilla iontophoretically with cobalt chloride. Successful fillings of individual sensilla lead to staining of one to 22 sensory neurones. All stained receptor cell axons are uniglomerular. Seven fillings of poreplates from the 5th flagellar segment in different animals were compared to analyse the distribution patterns of the receptor cell axons in the antennal lobe. The sensory neurones of individual poreplates project to widely distributed glomeruli in the antennal lobe. The projection patterns of different poreplates are not the same, but may be overlapping.     

Glomerular interactions in olfactory processing channels of the antennal lobes

An open question in olfactory coding is the extent of interglomerular connectivity: do olfactory glomeruli and their neurons regulate the odorant responses of neurons innervating other glomeruli? In the olfactory system of the moth Manduca sexta, the response properties of different types of antennal olfactory receptor cells are known. Likewise, a subset of antennal lobe glomeruli has been functionally characterized and the olfactory tuning of their innervating neurons identified. This provides a unique opportunity to determine functional interactions between glomeruli of known input, specifically, (1) glomeruli processing plant odors and (2) glomeruli activated by antennal stimulation with pheromone components of conspecific females. Several studies describe reciprocal inhibitory effects between different types of pheromone-responsive projection neurons suggesting lateral inhibitory interactions between pheromone component-selective glomerular neural circuits. Furthermore, antennal lobe projection neurons that respond to host plant volatiles and innervate single, ordinary glomeruli are inhibited during antennal stimulation with the female’s sex pheromone. The studies demonstrate the existence of lateral inhibitory effects in response to behaviorally significant odorant stimuli and irrespective of glomerular location in the antennal lobe. Inhibitory interactions are present within and between olfactory subsystems (pheromonal and non-pheromonal subsystems), potentially to enhance contrast and strengthen odorant discrimination.     

Functional organization of the macroglomerular complex related to behaviourally expressed olfactory redundancy in male cabbage looper moths

Abstract. The neurophysiological bases for behaviourally expressed olfactory redundancy in the sex pheromone communication system of the cabbage looper moth, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae), were examined by coupling the cut-sensillum extracellular recording technique with a highly specific neuronal marking method for moth peripheral receptors. In seventy-two antennal sensilla, axonal pathways of cobalt-stained neurones could be traced into the male-specific macroglomerular complex in the antennal lobe. In T. ni males this comprises five glomeruli, two of which are subdivided into morphologically, and in some instances functionally identifiable, regions. Axonal arborizations of forty-eight neurones (single stainings) showed high fidelity (98%) for containment within a specific glomerulus or glomerular subdivision, and the neuropil targeted seemed to be related to the specificity of a neurone to a particular female-emitted sex pheromone component (27-12:Ac, Z7-14:Ac, Z9-14:Ac, 12:Ac, 11–12:Ac, Z5-12:Ac), or to a behavioural antagonist (Z7-12:OH). Double (twenty-one) and multiple stainings (three) showed axons projecting into two or more glomeruli, respectively, with 100% fidelity for the component-specific glomerulus or glomerular subdivision to be targeted. We suggest that the potential for a single minor component to cross-stimulate two or more neurones within a sensillum may enable partial blends to continue to provide sensory input into all of the pheromone-processing glomeruli of the complex. Our interpretation is that redundancy occurs at the receptor level on neighbouring dendrites, and thus allows various four-component partial blends to evoke full pheromone-mediated behaviour.     

The tracheal system of the developing primary olfactory pathway of Manduca sexta: tracheae do not play a guidance or targeting role for ingrowing receptor axons

Axons navigate to their targets by detecting signals within the environment through which they are growing. The surfaces of tracheae, which are prominent features of the insect body plan, could be detected as favorable pathways for sensory axons growing toward the brain. The pattern of the tracheal investment of the adult antennal lobe of the moth Manduca sexta suggested two specific possibilities for interaction between tracheae and axons during development: that tracheae might be involved in guiding olfactory receptor axons to their target region of the brain, the antennal lobe; and that tracheae could provide an address system within the lobe that defines the sites of glomeruli, which are olfactory-axon target areas within the lobe. To determine whether tracheae contribute to development of the primary olfactory pathway, the distribution of tracheae in the adult and developing antennal lobes was examined with both confocal and electron microscopes. During the major stages in which axons are growing into the antennal lobe and in which glomeruli are forming, the tracheal investment of the nerve and lobe was found to be minimal. Tracheae thus cannot serve as axon guides or as local address sites for newly forming glomeruli during the initial targeting of receptors onto the antennal lobe.     

Pheromone-sensitive glomeruli in the primary olfactory centre of ants

Tremendous evolutional success and the ecological dominance of social insects, including ants, termites and social bees, are due to their efficient social organizations and their underlying communication systems. Functional division into reproductive and sterile castes, cooperation in defending the nest, rearing the young and gathering food are all regulated by communication by means of various kinds of pheromones. No brain structures specifically involved in the processing of non-sexual pheromone have been physiologically identified in any social insects. By use of intracellular recording and staining techniques, we studied responses of projection neurons of the antennal lobe (primary olfactory centre) of ants to alarm pheromone, which plays predominant roles in colony defence. Among 23 alarm pheromone-sensitive projection neurons recorded and stained in this study, eight were uniglomerular projection neurons with dendrites in one glomerulus, a structural unit of the antennal lobe, and the remaining 15 were multiglomerular projection neurons with dendrites in multiple glomeruli. Notably, all alarm pheromone-sensitive uniglomerular projection neurons had dendrites in one of five 'alarm pheromone-sensitive (AS)' glomeruli that form a cluster in the dorsalmost part of the antennal lobe. All alarm pheromone-sensitive multiglomerular projection neurons had dendrites in some of the AS glomeruli as well as in glomeruli in the anterodorsal area of the antennal lobe. The results suggest that components of alarm pheromone are processed in a specific cluster of glomeruli in the antennal lobe of ants.     

Physiological and morphological characterization of honeybee olfactory neurons combining electrophysiology,calcium imaging and confocal microscopy

The insect antennal lobe is the first brain structure to process olfactory information. Like the vertebrate olfactory bulb the antennal lobe is substructured in olfactory glomeruli. In insects, glomeruli can be morphologically identified, and have characteristic olfactory response profiles. Local neurons interconnect glomeruli, and output (projection) neurons project to higher-order brain centres. The relationship between their elaborate morphology and their physiology is not understood. We recorded electrophysiologically from antennal lobe neurons, and iontophoretically injected a calcium-sensitive dye. We then measured their spatio-temporal calcium responses to a variety of odours. Finally, we confocally reconstructed the neurons, and identified the innervated glomeruli. An increase or decrease in spiking frequency corresponded to an intracellular calcium increase or decrease in the cell. While intracellular recordings generally lasted between 10 and 30 min, calcium imaging was stable for up to 2 h, allowing a more detailed physiological analysis. The responses indicate that heterogeneous local neurons get input in the glomerulus in which they branch most strongly. In many cases, the physiological response properties of the cells corresponded to the known response profile of the innervated glomerulus. In other words, the large variety of response profiles generally found when comparing antennal lobe neurons is reduced to a more predictable response profile when the innervated glomerulus is known.Abbreviations ACT antenno-cerebralis-tract - AL antennal lobe - AP action potential - l-ACT lateral ACT - LN local neuron - LPL lateral protocerebral lobe - m-ACT medial ACT - MB mushroom body - OSN olfactory sensory neuron - PN projection neuron - T1 tract 1 of the antennal nerve     

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     

Distinct Projections of Two Populations of Olfactory Receptor Axons in the Antennal Lobe of the Sphinx Moth Manduca sexta

The central projections of olfactory receptor cells associatedwith two distinct types of antennal sensilla in the sphinx mothManduca sexta were revealed by anterograde staining. In bothsexes, receptor axons that arise from sexually isomorphic, type-IItrichoid sensilla (and possibly some basiconic sensilla) projectto the spheroidal glomeruli in the ipsilateral antennal lobe.Each axon terminates in one glomerulus. Axons from a limitedregion of the antenna project to glomeruli throughout the lobe,arguing against strict topographic mapping of antennal receptorcells onto the array of glomeruli. Axons of sex-pheromone-selectivereceptor cells in the male-specific type-I trichoid sensillaproject exclusively to the sexually dimorphic macroglomerularcomplex (MGC). Axons from sensilla on the dorsal surface ofthe antenna are biased toward the medial MGC and those fromventral sensilla, toward the lateral MGC. Some receptor-cellaxons branch before reaching the MGC, but their terminals arealways confined to one of the two main glomerular divisionsof the MGC, the cumulus and toroid. These findings confirm thatprimary-afferent information about pheromonal and non-pheromonalodors is segregated in the antennal lobe and suggest that thereis a functional correspondence between particular olfactoryreceptor cells and specific glomeruli. Chem. Senses 20: 313–323,1995.     

Structure and development of the insect antennodeutocerebral system

The structure and postembryonic development of antennae and deutocerebrum in various insect orders are reviewed. First, the number and/or size of system components, i.e. antennal sensilla, neuroreceptors, deutocerebral neurons and synaptic complexes (glomeruli), are compared in adult insects. Second, the neuronal organization of the system is examined. Evidence of projection of the neuroreceptors from the 2 basal antennal segments (scape and pedicel) in the antennal mechanosensory and motor center and from the distal segments (flagellum) in the antennal lobe is discussed. Third, the types of cerebral neurons found in the antennal lobe are described. Fourth, all synaptic contacts between neurons in the antennal lobe take place in discrete glomeruli whose ultrastructure and neurotransmitters are examined. Evidence of individual identifiability of glomeruli is given. Fifth, various sexual dimorphisms present in the antenna and antennal lobe, related or not to sex pheromone perception, are described at sensillar, neuronal, and glomerular levels. Sixth, the postembryonic development of neurons and glomeruli is analyzed in holo- and hemimetabolous insects. In conclusion, the columnar organization of the system is emphasized, alternative models of antennal-neuron projection into glomeruli are considered, and the functional significance of identified glomeruli is discussed.     

Antennal movements induced by odour and central projection of the antennal neurones in the honey-bee

Movements of the antennae induced by odour were investigated. Odour presented to the antenna of one side induced both antennae to move to that side. The EMGs recorded from the flexor muscles of both scapes showed that the latency of the movement of the ipsilateral flagellum when induced by odour was about 71 msec shorter than that of the contralateral flagellum. Recording electrical activities from the antennal nerve showed that there are more than 14 neurones in the antenno-motor externus.The distribution of the antennal nerve in the brain was investigated histologically by the injection of fluorescent dye. Antennal sensory neurones terminated at the glomeruli in the antennal lobe, in the dorsal lobe, in the protocerebrum, and in the commissural part of the suboesophageal ganglion. Injection of the fluorescent dye into the antennal nerve after degeneration of the antennal sensory neurones showed that the antennal motoneurones run in the ventral side of the antennal and dorsal lobes, and terminate in the marginal region of the ipsilateral oesophageal connective.The difference in latency of odour-induced flagellar movements is discussed in relation to the histological results and the unitary responses in the brain reported previously.     

Encoding of plant odour information in insects: peripheral and central mechanisms

Insects are suitable model organisms for studying mechanisms underlying olfactory coding and olfactory learning, by their unique adaptation to host plants in which the chemical senses are essential. Recent molecular biological studies have shown that a large number of genes in insects and other organisms are coding for olfactory receptor proteins. In general, one receptor type seems to be expressed in each neurone. The functional characterisations of olfactory receptor neurones have been extensive in certain insect species, demonstrating a fine-tuning of single neurones to biologically relevant odourants; both insect and plant produced volatiles. Stained neurones of the same functional type have been shown to project in one and the same glomerular unit in the primary olfactory centre, the antennal lobe. This corresponds to molecular biological studies, showing projections in one glomerulus by neurones expressing the same receptor type. Comparison of these findings with physiological and morphological characterisations of antennal lobe neurones has indicated correspondence between input and output of the glomerular units. Examples are presented from studies of heliothine moths. From the antennal lobe, the olfactory information is further conveyed to the mushroom bodies, particularly important for learning, and the lateral protocerebrum, a premotoric area. The three brain areas are regions of synaptic plasticity important in learning of odours, which is well studied in the honeybee but also in species of moths.     

蚊虫嗅觉识别的神经机制

蚊虫主要依赖嗅觉系统与外界环境进行化学信息交流。蚊虫通过嗅觉感受系统寻找食物、 配偶和产卵场所, 进而做出相应的行为反应。本文综述了近年来蚊虫嗅觉系统对气味信号神经传导机制的研究进展。蚊虫的嗅觉感器主要位于触角和下颚须, 触角上的毛形感器和锥形感器感受氨水、 乳酸、 羧酸类化合物等人体和其他动物释放的微量气味物质, 下颚须上的锥形感器则感受呼出的二氧化碳以及一些其他的挥发性物质; 蚊虫嗅觉感器内部有受体神经细胞, 其上分布有嗅觉受体蛋白, 蚊虫对外界环境的化学感受就是通过气味物质与这些受体蛋白互作而得以实现; 根据对不同气味物质的反应谱差异, 嗅觉神经细胞被分为不同的功能类型; 来自嗅觉神经细胞的神经信号进一步从外周传导至中枢神经中脑触角叶内的神经小球, 在此对信息进行初步的处理, 通过评估嗅觉神经细胞的反应和触角叶内的神经小球相应被激活的区域, 不同小球被分别命名; 最后, 神经信号继续整合, 由投射神经传向前脑, 最终引发一系列昆虫行为反应。这些研究从理论上剖析了气味信号在蚊虫嗅觉系统中的神经转导通路, 对于我们深刻理解蚊虫的嗅觉系统具有重要意义, 同时也有助于进一步理解其他昆虫甚至人类的气味识别机制及进行更深层次神经科学的探索。     

Development of the labial pit organ glomerulus in the antennal lobe of the moth Manduca sexta: the role of afferent projections in the formation of identifiable olfactory glomeruli.

Iterated neuropil modules called glomeruli are characteristic of primary olfactory centers in both vertebrates and invertebrates. To gain insight into the developmental mechanisms underlying the formation of such structured, organized neuropil, we have examined the development of an identified glomerulus in the olfactory (antennal) lobe of the moth Manduca sexta. The labial pit organ glomerulus (LPOG) receives bilateral sensory projections from the labial pit organs in the labial palps of the mouthparts, while other glomeruli in the antennal lobe receive unilateral projections from the antenna. Here, we chronicle the development of the LPOG under normal and perturbed conditions. Our findings suggest that the sensory axons of the labial pit organ, like those of the antenna, induce and shape growth of interneuronal arborizations, but specific features of interneuronal arborizations such as the relative position of glomerular arborizations within the antennal lobe are independent of both classes of afferent innervation. Labial pit organ axons and antennal axons exhibit a high degree of specificity for their respective target regions, independent of the presence or absence of the other class of afferent axon or the route taken to the antennal lobe. Specification of glomerular position is intrinsic to the antennal lobe rather than a consequence of competition between afferent axons.     

Separate distribution of deutocerebral projection neurons in the mushroom bodies of the cricket brain

Deutocerebral projection neurones in the brain of the cricket (Gryllus bimaculatus) have been investigated by experimental dextran staining, viewed by light and electron microscopy. These neurones of two separate somata clusters innervate two separate primary glomerular neuropils of the deutocerebral segment, either the antennal lobe receiving only antennal nerve sensory input, or the glomerular lobe, receiving input from sensory neurones of lower segmental origin, including chemosensory fibres from mouth parts. Projection neurones of the antennal lobe only invade the anterior calyx of the mushroom body neuropil via the inner antenno glomerular tract, while glomerular relay neurones of the glomerular lobe innervate only the posterior calyx via the tritocerebral tract. All types of projection neurones give rise to presynaptic boutons. forming the central core of microglomeruli with patterned distribution. These projection neurons are cholinergic. The results are discussed in view of maintained segregated modal information, first processed in the separated primary deutocerebral neuropiles and further on in the second order input neuropils of the mushroom bodies. The large posterior calyces are proposed as a compartment for gustatory information.     

Serotonin-immunoreactive neurones in the brain of Locusta migratoria innervating the corpus cardiacum

Summary The serotoninergic innervation of the corpus cardiacum (CC) of Locusta migratoria was investigated using two antisera against serotonin. A dense network of immunoreactive nerve fibres was present in the storage lobe of the CC. Immunopositive fibres only sporadically crossed the border between the storage lobe and the glandular lobe of the CC. Immunopositive fibres entered the storage lobe of the CC via the nervus corporis cardiaci I (NCCI); NCCII was immunonegative. Unilateral retrograde fillings of the NCCI with the fluorescent tracer Lucifer yellow, followed by antiserotonin immunocytochemistry, revealed about 20 double-labelled neurones in the anterior part of the pars intercerebralis. The double-labelled neurones were scattered between fluorescent non-immunoreactive neurones. Additionally, 5–7 neurones labelled only with Lucifer yellow were found at the ventrolateral side of the tritocerebrum. No immunopositive neurones were observed in the hypocerebral ganglion. Immunopositive fibres from neurones in the frontal ganglion ran via the recurrent nerve and the neuropile of the hypocerebral ganglion into the paired oesophageal nerve. At most, a few immunopositive nerve fibres occurred in the cardiostomatogastric nerves II, which connect the storage lobe of the CC with the paired oesophageal nerve at the caudal end of the hypocerebral ganglion.     

Sexual dimorphism in the antennal lobe of the ant Camponotus japonicus

The carpenter ant, a social hymenopteran, has a highly elaborated antennal chemosensory system that is used for chemical communication in social life. The glomeruli in the antennal lobe are the first relay stations where sensory neurons synapse onto interneurons. The system is functionally and structurally similar to the olfactory bulbs of vertebrates. Using three-dimensional reconstruction of glomeruli and subsequent morphometric analyses, we found sexual dimorphism of the antennal lobe glomeruli in carpenter ants, Camponotus japonicus. Female workers and unmated queens had about 430 glomeruli, the highest number reported so far in ants. Males had a sexually dimorphic macroglomerulus and about 215 ordinary glomeruli. This appeared to result from a greatly reduced number of glomeruli in the postero-medial region of the antennal lobe compared with that in females. On the other hand, sexually isomorphic glomeruli were identifiable in the dorsal region of the antennal lobe. For example, large, uniquely shaped glomeruli located at the dorso-central margin of the antennal lobe were detected in all society members. The great sexual dimorphism seen in the ordinary glomeruli of the antennal lobe may reflect gender-specific tasks in chemical communications rather than different reproductive roles.     

Segregation of calcium signalling mechanisms in magnocellular neurones and terminals

Every cell or neuronal type utilizes its own specific organization of its Ca(2+) homeostasis depending on its specific function and its physiological needs. The magnocellular neurones, with their somata situated in the supraoptic and paraventricular nuclei of the hypothalamus and their nerve terminals populating the posterior hypophysis (neural lobe) are a typical and classical example of a neuroendocrine system, and an important experimental model for attempting to understand the characteristics of the neuronal organization of Ca(2+) homeostasis. The magnocellular neurones synthesize, in a cell specific manner, two neurohormones: arginine-vasopressin (AVP) and oxytocin (OT), which can be released, in a strict Ca(2+)-dependent manner, both at the axonal terminals, in the neural lobe, and at the somatodendritic level. The two types of neurones show also distinct type of bioelectrical activity, associated with specific secretory patterns. In these neurones, the Ca(2+) homeostatic pathways such as the Na(+)/Ca(2+) exchanger (NCX), the endoplasmic reticulum (ER) Ca(2+) pump, the plasmalemmal Ca(2+) pump (PMCA) and the mitochondria are acting in a complementary fashion in clearing Ca(2+) loads that follow neuronal stimulation. The somatodendritic AVP and OT release closely correlates with intracellular Ca(2+) dynamics. More importantly, the ER Ca(2+) stores play a major role in Ca(2+) homeostatic mechanism in identified OT neurones. The balance between the Ca(2+) homeostatic systems that are in the supraoptic neurones differ from those active in the terminals, in which mainly Ca(2+) extrusion through the Ca(2+) pump in the plasma membrane and uptake by mitochondria are active. In both AVP and OT nerve terminals, no functional ER Ca(2+) stores can be evidenced experimentally. We conclude that the physiological significance of the complexity of Ca(2+) homeostatic mechanisms in the somatodendritic region of supraoptic neurones and their terminals can be multifaceted, attributable, in major part, to their specialized electrical activity and Ca(2+)-dependent neurohormone release.     

昆虫触角叶的结构

触角叶是昆虫脑内初级嗅觉中心,通过触角神经与触角联系。触角叶主要由嗅觉受体神经元、局域中间神经元、投射神经元和远心神经元构成。这些神经元的形态多样,其形态变化与其功能和昆虫嗅觉行为相关。这些神经元在触角叶内交织形成神经纤维网,在突触联系紧密的地方形成纤维球,纤维球通常排列在触角叶外周。通常,昆虫触角叶内纤维球的数量、大小和位置相对固定,并且几乎每个小球都可以被识别和命名。不同种类、性别和品级的昆虫中,纤维球的数量、大小和排列方式各不相同。触角叶结构神经元组成和纤维球的多样性,与各种昆虫嗅觉行为的特异性相对应。     

Development of the labial pit organ glomerulus in the antennal lobe of the moth Manduca sexta: The role of afferent projections in the formation of identifiable olfactory glomeruli

Iterated neuropil modules called glomeruli are characteristic of primary olfactory centers in both vertebrates and invertebrates. To gain insight into the developmental mechanisms underlying the formation of such structured, organized neuropil, we have examined the development of an identified glomerulus in the olfactory (antennal) lobe of the moth Manduca sexta. The labial pit organ glomerulus (LPOG) receives bilateral sensory projections from the labial pit organs in the labial palps of the mouthparts, while other glomeruli in the antennal lobe receive unilateral projections from the antenna. Here, we chronicle the development of the LPOG under normal and perturbed conditions. Our findings suggest that the sensory axons of the labial pit organ, like those of the antenna, induce and shape growth of interneuronal arborizations, but specific features of interneuronal arborizations such as the relative position of glomerular arborizations within the antennal lobe are independent of both classes of afferent innervation. Labial pit organ axons and antennal axons exhibit a high degree of specificity for their respective target regions, independent of the presence or absence of the other class of afferent axon or the route taken to the antennal lobe. Specification of glomerular position is intrinsic to the antennal lobe rather than a consequence of competition between afferent axons. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 28–44, 1999