Kinematic and aerodynamic aspects of ultrasound-induced negative phonotaxis inflying Australian field crickets (Teleogryllus oceanicus)

Aerodynamics of manoeuvring flight in brown long-eared bats (Plecotus auritus)

In this study, we explicitly examine the aerodynamics of manoeuvring flight in animals. We studied brown long-eared bats flying in a wind tunnel while performing basic sideways manoeuvres. We used particle image velocimetry in combination with high-speed filming to link aerodynamics and kinematics to understand the mechanistic basis of manoeuvres. We predicted that the bats would primarily use the downstroke to generate the asymmetries for the manoeuvre since it has been shown previously that the majority of forces are generated during this phase of the wingbeat. We found instead that the bats more often used the upstroke than they used the downstroke for this. We also found that the bats used both drag/thrust-based and lift-based asymmetries to perform the manoeuvre and that they even frequently switch between these within the course of a manoeuvre. We conclude that the bats used three main modes: lift asymmetries during downstroke, thrust/drag asymmetries during downstroke and thrust/drag asymmetries during upstroke. For future studies, we hypothesize that lift asymmetries are used for fast turns and thrust/drag for slow turns and that the choice between up- and downstroke depends on the timing of when the bat needs to generate asymmetries.

Signal Evolution: 'Shaky' Evidence for Sensory Bias

A study of tropical crickets suggests that a twitchy response to ultrasonic bat calls has been co-opted for mate location. The neuroethological approach picks apart some surprising evolutionary steps that could inform the widespread occurrence of complex duetting behaviour.

Sound strategies: the 65-million-year-old battle between bats and insects

The intimate details regarding the coevolution of bats and moths have been elucidated over the past 50 years. The bat-moth story began with the evolution of bat sonar, an exquisite ultrasonic system for tracking prey through the night sky. Moths countered with ears tuned to the high frequencies of bat echolocation and with evasive action through directed turns, loops, spirals, drops, and power dives. Some bat species responded by moving the frequency and intensity of their echolocation cries away from the peak sensitivity of moth ears, and the arms race was on. Tiger moths countered by producing anti-bat sounds. Do the sounds advertise moth toxicity, similar to the bright coloration of butterflies; do they startle the bat, giving the moth a momentary advantage in their aerobatic battle; or do they jam the sonar of the bat? The answer is yes. They do all and more in different situations and in different species. Any insect that flies at night must deal with bat predation. Beetles, mantids, true crickets, mole crickets, katydids, green lacewings, and locusts have anti-bat strategies, and we have just scratched the surface. In an exciting new twist, researchers are taking the technologies developed in the laboratory back into the field, where they are poised to appreciate the full richness of this remarkable predator-prey interaction.

Spatial orientation in the bushcricket Leptophyes punctatissima (Phaneropterinae; Orthoptera): II. Phonotaxis to elevated sound sources on a walking compensator

The ability of the bushcricket Leptophyes punctatissima to orient to elevated sound sources was investigated. Males were placed on a walking compensator and oriented in response to a synthetic female reply, which was broadcast via one of five loudspeakers placed at elevations of 0 degrees , 30 degrees , 60 degrees , 75 degrees and 90 degrees . Forward and backward movements were compensated, so that males remained at the same distance and elevation to the sound source. With increasing loudspeaker elevation, the males meandered more, and the ratio of the ideal path length to the actual path length decreased. The same was true for the correlation between stimulus angle and turn angle, and there were more turns to the wrong side with increasing loudspeaker elevation. Most males performed phonotaxis with a high acuity up to an elevation of 60 degrees . Individuals varied strongly in their performance especially at a source elevation of 75 degrees , where some were still very accurate in their approach, whereas the acuity of others decreased rapidly. We also describe a behaviour where males tilt their body axis to more anterior and sideward positions, both during walking and while calling on the spot. This behaviour is interpreted as a kind of directional scanning in order to actively induce changes in binaural cues.

Spatial orientation in the bushcricket Leptophyes punctatissima (Phaneropterinae; Orthoptera): I. Phonotaxis to elevated and depressed sound sources

Many species of acoustically interacting insects live in a complex, arboreal or semi-arboreal habitat. Thus mate finding by phonotaxis requires sound localization in the horizontal and vertical plane. Here we investigated the ability of the duetting bushcricket Leptophyes punctatissima to orient to one of three speakers, positioned at different levels in an artificial grid system, where each point in space could be reached by the male with almost equal probability. The system was designed analogous to a spherical calotte model of bismuth, where, once the male arrived at any nodal point had to decide between only three directions: either up or down and/or left and right. This design does not favour any phonotactic path of the males. All 12 males tested reached the three speaker positions (one in the horizontal plane, one elevated by 45 degrees , one depressed by 45 degrees relative to the starting position) with only little deviation from the shortest possible path. There was no significant difference with respect to the whole phonotactic time needed, the number of segments passed, or the number of stimuli received for the different speaker positions. This remarkable spatial orientation is achieved although the insects have no specialized external ear structures such as mammals, or some owls.

Mechanics and aerodynamics of insect flight control

Insects have evolved sophisticated fight control mechanisms permitting a remarkable range of manoeuvres. Here, I present a qualitative analysis of insect flight control from the perspective of flight mechanics, drawing upon both the neurophysiology and biomechanics literatures. The current literature does not permit a formal, quantitative analysis of flight control, because the aerodynamic force systems that biologists have measured have rarely been complete and the position of the centre of gravity has only been recorded in a few studies. Treating the two best-known insect orders (Diptera and Orthoptera) separately from other insects, I discuss the control mechanisms of different insects in detail. Recent experimental studies suggest that the helicopter model of flight control proposed for Drosophila spp. may be better thought of as a facultative strategy for flight control, rather than the fixed (albeit selected) constraint that it is usually interpreted to be. On the other hand, the so-called 'constant-lift reaction' of locusts appears not to be a reflex for maintaining constant lift at varying angles of attack, as is usually assumed, but rather a mechanism to restore the insect to pitch equilibrium following a disturbance. Differences in the kinematic control mechanisms used by the various insect orders are related to differences in the arrangement of the wings, the construction of the flight motor and the unsteady mechanisms of lift production that are used. Since the evolution of insect flight control is likely to have paralleled the evolutionary refinement of these unsteady aerodynamic mechanisms, taxonomic differences in the kinematics of control could provide an assay of the relative importance of different unsteady mechanisms. Although the control kinematics vary widely between orders, the number of degrees of freedom that different insects can control will always be limited by the number of independent control inputs that they use. Control of the moments about all three axes (as used by most conventional aircraft) has only been proven for larger flies and dragonflies, but is likely to be widespread in insects given the number of independent control inputs available to them. Unlike in conventional aircraft, however, insects' control inputs are likely to be highly non-orthogonal, and this will tend to complicate the neural processing required to separate the various motions.

Categorical perception of sound frequency by crickets

Partitioning continuously varying stimuli into categories is a fundamental problem of perception. One solution to this problem, categorical perception, is known primarily from human speech, but also occurs in other modalities and in some mammals and birds. Categorical perception was tested in crickets by using two paradigms of human psychophysics, labeling and habituation-dishabituation. The results show that crickets divide sound frequency categorically between attractive (<16 kilohertz) and repulsive (>16 kilohertz) sounds. There is sharp discrimination between these categories but no discrimination between different frequencies of ultrasound. This demonstration of categorical perception in an invertebrate suggests that categorical perception may be a basic and widespread feature of sensory systems, from humans to invertebrates.

Ultrasound sensitive neurons in the cricket brain

1. The aim of this study was to identify neurons in the brain of the cricket, Teleogryllus oceanicus, that are tuned to high frequencies and to determine if these neurons are involved in the pathway controlling negative phonotaxis. In this paper we describe, both morphologically and physiologically, 20 neurons in the cricket brain which are preferentially tuned to high frequencies. 2. These neurons can be divided into two morphological classes: descending brain interneurons (DBINs) which have a posteriorly projecting axon in the circumesophageal connective and local brain neurons (LBNs) whose processes reside entirely within the brain. All the DBINs and LBNs have processes which project into one common area of the brain, the ventral brain region at the border of the protocerebrum and deutocerebrum. Some of the terminal arborizations of Int-1, an ascending ultrasound sensitive interneuron which initiates negative phonotaxis, also extend into this region. 3. Physiologically, ultrasonic sound pulses produce 3 types of responses in the DBINs and LBNs. (1) Seven DBINs and 6 LBNs are excited by ultrasound. (2) Ongoing activity in one DBIN and 5 LBNs is inhibited by ultrasound, and (3) one cell, (LBN-ei), is either excited or inhibited by ultrasound depending on the direction of the stimulus. 4. Many of the response properties of both the DBINs and LBNs to auditory stimuli are similar to those of Int-1. Specifically, the strength of the response, either excitation or inhibition, to 20 kHz sound pulses increases with increasing stimulus intensity, while the response latency generally decreases. Moreover, the thresholds to high frequencies are much lower than to low frequencies. These observations suggest that the DBINs and LBNs receive a majority of their auditory input from Int-1. However, the response latencies and directional sensitivity of only LBN-ei suggest that it is directly connected to Int-1. 5. The response of only one identified brain neuron, DBIN8, which is inhibited by 20 kHz sound pulses, is facilitated during flight compared to its response at rest. This suggests that suppression of activity in DBIN8 may be associated with ultrasound-induced negative phonotactic steering responses in flying crickets. The other DBINs and LBNs identified in this paper may also play a role in negative phonotaxis, and possibly in other cricket auditory behaviors influenced by ultrasonic frequencies.