Morphology of antennal motoneurons in the brains of two crickets, Gryllus bimaculatus and Gryllus campestris

Active antennal movements in Drosophila can tune wind encoding

Insects use their antennae to smell odors,1,2 detect auditory cues,3,4 and sense mechanosensory stimuli such as wind5 and objects,6,7,8 frequently by combining sensory processing with active movements. Genetic access to antennal motor systems would therefore provide a powerful tool for dissecting the circuit mechanisms underlying active sensing, but little is known about how the most genetically tractable insect, Drosophila melanogaster, moves its antennae. Here, we use deep learning to measure how tethered Drosophila move their antennae in the presence of sensory stimuli and identify genetic reagents for controlling antennal movement. We find that flies perform both slow adaptive movements and fast flicking movements in response to wind-induced deflections, but not the attractive odor apple cider vinegar. Next, we describe four muscles in the first antennal segment that control antennal movements and identify genetic driver lines that provide access to two groups of antennal motor neurons and an antennal muscle. Through optogenetic inactivation, we provide evidence that antennal motor neurons contribute to active movements with different time courses. Finally, we show that activation of antennal motor neurons and muscles can adjust the gain and acuity of wind direction encoding by antennal displacement. Together, our experiments provide insight into the neural control of antennal movement and suggest that active antennal positioning in Drosophila may tune the precision of wind encoding.

The central organisation of courtship and rivalry behaviour in Gryllus bimaculatus (deGeer) as revealed by lesions of abdominal connectives

Behaviour is rooted in the organization and activity of an animal's nervous system. As male crickets use their front wings for sound production, the neural circuits underlying singing had been suggested to be housed in the thoracic ganglia. However, systematic lesion experiments of the CNS demonstrated that the abdominal nervous system is essential for their calling song behaviour. As male crickets also generate a courtship and rivalry song, we explored which parts of the abdominal central nervous system control the underlying motor patterns. A combination of systematic lesions to the abdominal nerve cord and video recording of courtship and rivalry behaviour revealed that most components of male courtship and rivalry behaviour were not affected by the lesions, except for the generation of courtship song, rivalry song, and the male's ability to copulate with the female. Any lesion to the abdominal nerve cord abolished copulations. Generation of courtship song initially failed when the connection to abdominal ganglion A6 was severed but in few males recovered after a week. For rivalry song production a central nerve cord extending to abdominal ganglion A4 was sufficient. These findings indicate that in the bispotted cricket the neural organization of courtship song is different from calling and rivalry song, while calling song and rivalry song might share a common network for generating the song patterns.

The mechanosensory-motor apparatus of antennae in the Oleander hawk moth (Daphnis nerii, Lepidoptera)

Insect antennae are sensory organs of great importance because they can sense diverse environmental stimuli. In addition to serving as primary olfactory organs of insects, antennae also sense a wide variety of mechanosensory stimuli, ranging from low-frequency airflow or gravity cues to high-frequency antennal vibrations due to sound, flight or touch. The basal segments of the antennae house multiple types of mechanosensory structures that prominently include the sensory hair plates, or Böhm's bristles, which measure the gross extent of antennal movement, and a ring of highly sensitive scolopidial neurons, collectively called the Johnston's organs, which record subtle flagellar vibrations. To fulfill their multifunctional mechanosensory role, the antennae of insects must actively move thereby enhancing their ability to sense various cues in the surrounding environment. This tight coupling between antennal mechanosensory function and antennal movements means that the underlying mechanosensory-motor apparatus constitutes a highly tuned feedback-controlled system. Our study aims to explore how the sensory and motor components of this system are configured to enable such functional versatility. We describe antennal mechanosensory neurons, their central projections in the brain relative to antennal motor neurons and the internal morphology of various antennal muscles that actuate the basal segments of the antenna. We studied these in the Oleander hawk moth (Daphnis nerii) using a combination of techniques such as neural dye fills, confocal microscopy, scanning electron microscopy and X-ray tomography. Our study thus provides a detailed anatomical picture of the antennal mechanosensory-motor apparatus, which in turn provides key insights into its multifunctional role.

Dendritic Ca2+ dynamics and multimodal processing in a cricket antennal interneuron

The integration of stimuli of different modalities is fundamental to information processing within the nervous system. A descending interneuron in the cricket brain, with prominent dendrites in the deutocerebrum, receives input from three sensory modalities: touch of the antennal flagellum, strain of the antennal base, and visual stimulation. Using calcium imaging, we demonstrate that each modality drives a Ca²⁺ increase in a different dendritic region. Moreover, touch of the flagellum is represented in a topographic map along the neuron’s dendrites. Using intracellular recording, we investigated the effects of Ca²⁺ on spike shape through the application of the Ca²⁺ channel antagonist Cd²⁺ and identified probable Ca²⁺-dependent K⁺ currents.

NEW & NOTEWORTHY Different dendritic regions of the cricket brain neuron DBNi1-2 showed localized Ca²⁺ increases when three modalities of stimulation (touch of the flagellum, strain at antennal base, and visual input) were given. Touch stimulation induces localized Ca²⁺ increases according to a topographic map of the antenna. Ca²⁺ appears to activate K⁺ currents in DBNi1-2.

Single perturbations cause sustained changes in searching behavior in stick insects

Stick insects (Cuniculina impigra) possessing only a single front leg perform untargeted stereotypical cyclic searching movements with that leg when it loses contact with the ground. When encountering an object, the animals grasp it. We hypothesized that removal of the object immediately after contact with the leg's tibia would result in a change in searching strategy, i.e. searching movements confined to the former location of the object to regain contact. In our experimental setup, searching movements were restricted to upward and downward movements. After removal of the object, searching movements were continued. However, in post-contact searching, two movement parameters were usually changed: (1) average positions of searching movements were shifted towards the former position of the object; and (2) movement amplitudes were considerably smaller and accompanied by a decrease in cycle period. This confinement of searching movements to the location of contact was interpreted as targeting behavior. All parameters regained initial values after approximately 6 s. Changes in position and amplitudes were independently controlled. Neither of the changes was under visual control, but rather depended on the presence of the trochanteral hairplate, a sensory organ that measures the coxa-trochanter joint position. Changes in average leg position were linked to changes in the ratio of electrical activity in the levator and depressor trochanteris muscles, which were based on altered activity in both or either one of the muscles. Our data demonstrate a switch in a simple behavior that is under local sensory control and may utilize a form of short-term memory.

Active touch in orthopteroid insects: behaviours, multisensory substrates and evolution

Orthopteroid insects (cockroaches, crickets, locusts and related species) allow examination of active sensory processing in a comparative framework. Some orthopteroids possess long, mobile antennae endowed with many chemo- and mechanoreceptors. When the antennae are touched, an animal's response depends upon the identity of the stimulus. For example, contact with a predator may lead to escape, but contact with a conspecific may usually not. Active touch of an approaching object influences the likelihood that a discrimination of identity will be made. Using cockroaches, we have identified specific descending mechanosensory interneurons that trigger antennal-mediated escape. Crucial sensory input to these cells comes from chordotonal organs within the antennal base. However, information from other receptors on the base or the long antennal flagellum allows active touch to modulate escape probability based on stimulus identity. This is conveyed, at least to some extent, by textural information. Guidance of the antennae in active exploration depends on visual information. Some of the visual interneurons and the motor neurons necessary for visuomotor control have been identified. Comparisons across Orthoptera suggest an evolutionary model where subtle changes in the architecture of interneurons, and of sensorimotor control loops, may explain differing levels of vision-touch interaction in the active guidance of behaviour.

Behavioral response to antennal tactile stimulation in the field cricket Gryllus bimaculatus

We examined behavioral responses of the field cricket Gryllus bimaculatus to tactile stimuli to the antennae. Three stimulants of similar shape and size but different textures were used: a tibia from the hunting spider Heteropoda venatoria (potential predator), a tibia from the orb-web spider Argiope bruennichi (less likely predator), and a glass rod. Each stimulus session comprised a first gentle contact and a second strong contact. The evoked behavioral responses were classified into four categories: aversion, aggression, antennal search, and no response. Regardless of the stimulants, the crickets exhibited antennal search and aversion most frequently in response to the first and second stimuli, respectively. The frequency of aversion was significantly higher to the tibia of H. venatoria than to other stimulants. The most striking observation was that aggressive responses were exclusive to the H. venatoria tibia. To specify the hair type that induced aggression, we manipulated two types of common hairs (bristle and fine) on the tibia of the predatory spider. When bristle hairs were removed from the H. venatoria tibia, aggression was significantly reduced. These results suggest that antennae can discriminate the tactile texture of external objects and elicit adaptive behavioral responses.

The neural mechanisms of antennal positioning in flying moths

In diverse insects, the forward positioning of the antenna is often among the first behavioral indicators of the onset of flight. This behavior may be important for the proper acquisition of the mechanosensory and olfactory inputs by the antennae during flight. Here, we describe the neural mechanisms of antennal positioning in hawk moths from behavioral, neuroanatomical and neurophysiological perspectives. The behavioral experiments indicated that a set of sensory bristles called Böhm's bristles (or hair plates) mediate antennal positioning during flight. When these sensory structures were ablated from the basal segments of their antenna, moths were unable to bring their antennae in flight position causing frequent collisions with the flapping wing. Fluorescent dye-fills of the underlying sensory and motor neurons revealed that the axonal arbors of the mechanosensory bristle neurons spatially overlapped with the dendritic arbors of the antennal motor neurons. Moreover, the latency between the activation of antennal muscles following stimulation of sensory bristles was also very short (< 10 ms), indicating that the sensory-motor connections may be direct. Together, these data show that Böhm's bristles control antennal positioning in moths via a reflex mechanism. Because the sensory structures and motor organization is conserved across most Neoptera, the mechanisms underlying antennal positioning, as described here, is likely to be conserved in these diverse insects.

Active and passive antennal movements during visually guided steering in flying Drosophila

Insects use feedback from a variety of sensory modalities, including mechanoreceptors on their antennae, to stabilize the direction and speed of flight. Like all arthropod appendages, antennae not only supply sensory information but may also be actively positioned by control muscles. However, how flying insects move their antennae during active turns and how such movements might influence steering responses are currently unknown. Here we examined the antennal movements of flying Drosophila during visually induced turns in a tethered flight arena. In response to both rotational and translational patterns of visual motion, Drosophila actively moved their antennae in a direction opposite to that of the visual motion. We also observed two types of passive antennal movements: small tonic deflections of the antenna and rapid oscillations at wing beat frequency. These passive movements are likely the result of wing-induced airflow and increased in magnitude when the angular distance between the wing and the antenna decreased. In response to rotational visual motion, increases in passive antennal movements appear to trigger a reflex that reduces the stroke amplitude of the contralateral wing, thereby enhancing the visually induced turn. Although the active antennal movements significantly increased antennal oscillation by bringing the arista closer to the wings, it did not significantly affect the turning response in our head-fixed, tethered flies. These results are consistent with the hypothesis that flying Drosophila use mechanosensory feedback to detect changes in the wing induced airflow during visually induced turns and that this feedback plays a role in regulating the magnitude of steering responses.

Innervation pattern of suboesophageal ventral unpaired median neurones in the honeybee brain

In honeybees (Apis mellifera), the biogenic amine octopamine has been shown to play a role in associative and non-associative learning and in the division of labour in the hive. Immunohistochemical studies indicate that the ventral unpaired median (VUM) neurones in the suboesophageal ganglion (SOG) are putatively octopaminergic and therefore might be involved in the octopaminergic modulation of behaviour. In contrast to our knowledge about the behavioural effects of octopamine, only one neurone (VUMmx1) has been related to a behavioural effect (the reward function during olfactory learning). In this study, we have investigated suboesophageal VUM neurones with fluorescent dye-tracing techniques and intracellular recordings combined with intracellular staining. Ten different VUM neurones have been found including six VUM neurones innervating neuropile regions of the brain and the SOG exclusively (central VUM neurones) and four VUM neurones with axons in peripheral nerves (peripheral VUM neurones). The central VUM neurones innervate the antennal lobes, the protocerebral lobes (including the lateral horn) and the mushroom body calyces. Of these, a novel mandibular VUM neurone, VUMmd1, exhibits the same branching pattern in the brain as VUMmx1 and responds to sucrose and odours in a similar way. The peripheral VUM neurones innervate the antennal and the mandibular nerves. In addition, we describe one labial unpaired median neurone with a dorsal cell body, DUMlb1. The possible homology between the honeybee VUM neurones and the unpaired median neurones in other insects is discussed.

Physiological characterisation of antennal mechanosensory descending interneurons in an insect (Gryllus bimaculatus, Gryllus campestris) brain

We investigated five different descending brain interneurons with dendritic arborizations in the deutocerebrum in the crickets Gryllus bimaculatus and G. campestris. These interneurones convey specific antennal mechanosensory information to the ventral nerve cord and all responded to forced antennal movements. These interneurones coded for velocity and showed preferences for distinct sectors of the total range of antennal movements. Their axons descended into the posterior connective either ipsilateral or contalateral to the cell body. Electrical stimulation of sensory nerves indicated that the interneurons received input from different afferents of the two antennal base segments. One interneuron had a particularly large axon with a conduction velocity of 4.4ms−1. This was the only one of the five interneurons that also received visual input. Its activity was reduced during voluntary antennal movements. The reduction in activity occurred even after de-efferentation of the antenna, indicating that it had a central origin. Although we do not have experimental evidence for behavioural roles for the descending antennal mechanosensory interneurons, the properties described here suggest an involvement in the perception of objects in the path of the cricket.

New methods of dye application for staining motor neurons in an insect

Motor neurons innervating tiny muscles in the antennal system of the cockroach, Periplaneta americana, were stained by two new approaches to backfilling motor axons and somata. Both methods are easier, and produce less leakage of dye, than conventional methods that rely on application of dye to the distal cut end of a motor nerve or immersion of a whole muscle in dye. One approach used here was to pressure-inject nickel (or cobalt) chloride directly into a muscle from a micropipette. The other was to place filter paper soaked in nickel chloride on the muscle surface. The first approach enabled staining of motor neurons most reliably and with preservation of stain to the specific muscle of interest. The second approach had different advantages: it was easier to perform, resulted in more intense staining and frequently more complete staining of dendritic structure than the first approach. Taken together, these two approaches should prove useful for tracing motor circuits in a number of model systems.

Anatomy of the antennal motoneurons in the brain of the honeybee (Apis mellifera)

This paper describes the morphology and location of the cerebral motoneurons that control the movement of the antennae in the honeybee. The position of each antenna is controlled by two muscle systems; the basal segment (scape) is moved by four muscles within the head capsule, and two muscles within the scape control the distal segments (flagellum) of the antenna. The motor system of the scape is controlled by nine motoneurons, and that of the flagellum by six motoneurons. All of these motoneurons share the dorsal lobe as a common projection area where their dendritic fields overlap extensively. These motoneurons do not have contralateral projections. The cell bodies of the antennal motoneurons are located in the soma layer lateral to the dorsal lobe. The somata for each muscle system are arranged in three clusters; two clusters are located in a region of the cortex dorsal to the dorsal lobe and one cluster is located in the cortex ventral to the dorsal lobe. In the cortex dorsal to the dorsal lobe, one cluster of each muscle system shares the same region. Altogether five groups of cell bodies can be distinguished. Double labeling of the motoneurons and presumptive mechanosensory primary antennal afferents with fluorescent dyes has shown that there is an extensive overlap of axonal projections of antennal mechanosensory afferents with dendritic fields of antennal motoneurons.

The antennal motor system of crickets: proctolin in slow and fast motoneurons as revealed by double labelling

This study describes proctolin-like immunoreactivity (PLI) of identified antennal motoneurons in the brain of adult crickets (Gryllus bimaculatus). The motoneurons were first backfilled with the fluorescent dye Lucifer Yellow and then immunohistochemically labelled with an antibody against proctolin. Altogether 14 of the 17 excitatory antennal motoneurons, including physiologically fast and slow types, showed PLI. The only common inhibitor consistently demonstrated a weak positive PLI. PLI was also present in the dendritic arborizations and varicosities of motor axons in the intrinsic antennal muscles. Densitometric measurements of motoneuron somata showed significant differences in the intensity of PLI in different types of antennal motoneurons, suggesting that antennal motoneurons produce different amounts of proctolin. Identical motoneuron somata display a large variance of PLI intensities in different brains. This observation may indicate up- and down-regulation of proctolin in individual crickets.

Antennular projections to the midbrain of the spiny lobster. III. Central arborizations of motoneurons

The central organization of antennular motoneurons in the brain of the spiny lobster, Panulirus argus, was analyzed by combining biocytin backfills with serial reconstructions of the antennular nerves and the brain. Eighty-nine to 99 antennular motoneurons occur in each hemibrain. The somata of the motoneurons are distributed in a consistent pattern in two complex soma clusters, the ventral paired mediolateral cluster of the deutocerebrum and the dorsal unpaired median cluster of the tritocerebrum. The motoneurons arborize ipsilaterally in the lateral and median antennular neuropils and the tegumentary neuropil. The backfills indicate a minimum of five morphological types of motoneurons with different arborization patterns. The innervation pattern of the motoneurons, together with previously reported innervation patterns of antennular sensory afferents, suggest that the lateral antennular neuropil is a lower motor center driving local antennular reflexes in response to chemical and mechanical stimulation of the antennule, whereas the median antennular neuropil is a lower motor center for equilibrium responses.