Responses of high-frequency movement detector neurons in dragonfly larvae to motion of complex patterns

Neurons of the class described respond by a discharge with a frequency of 150–200/sec to the appearance and motion of single dark objects in a new part of the large receptive field. Movement of a complex pattern induces a weak initial response. Repeated movements of a complex pattern evoke habituation and do not affect the firing rate but they suppress the response to movement of a single object at distances of up to 15–30° away from the zone of habituation. The features of optic stimuli evoking maximal responses in high-frequency movement detector neurons correspond in many cases to the features of objects evoking fright and flight in the larvae.     

Responses of medulla neurons to illumination and movement stimuli in the tiger beetle larvae

Intracellular responses of medulla neurons (second-order visual interneurons) have been examined in the tiger beetle larva. The larva possesses six stemmata on either side of the head, two of which are much larger than the remaining four. Beneath the cuticle housing the stemmata an optic neuropil complex occurs consisting of lamina and medulla neuropils. Response patterns of medulla neurons to illumination and moving objects varied from neurons to neurons. For movement stimuli black discs and a black bar were moved in the rostro-caudal direction above the larva. Comparison of responses to the discs and the bar suggested a spatial summation of responses in some neurons, and tuning to small objects in some neurons. The majority of neurons responded to objects moving at heights of 10 mm and 50 mm with the same discharge pattern. A few neurons, however, showed distance sensitivities responding with an increase of spike discharges to moving objects only at either of the two heights. Such distance sensitivities still remained in one-stemma larvae, three of the four stemmata being occluded. These data are discussed in relation to distinct visual behavior of the larva and with special reference to perception of the hunting range.     

Jet-propulsion in anisopteran dragonfly larvae

Summary Jet-propulsion in dragonfly larvae is achieved by the rapid ejection of water from a specialised rectal chamber via the anus, at a frequency of up to 2.2 cycles/s. Movement, forward thrust and muscular activity have been recorded in restrained and free-swimming larvae. Forward thrusts of up to 1.5 g wt result from the expiratory phases of cycles lasting 0.1 to 0.4 s. Swimming velocities are in the order of 10 cm/s. The following muscles are shown to be active during expiratory phases of jetting: anterior, posterior and respiratory dorso-ventrals; primary and secondary longitudinal tergals; lateral primary longitudinal sternopleural; dorso-ventral oblique; ventral adductors of the anal appendages. The sub-intestinal muscle is active during the inspiratory phases of jetting. Activity recorded is compared with that found during normal ventilation. The larval jet-propulsive mechanism is compared with that of certain cephalopods and found to be very effective for the larva's relatively small size.R. S. P. wishes to thank the Science Research Council for a studentship during the tenure of which the above work was carried out.     

Intraspecific interference among larvae in a semivoltine dragonfly population

Summary This study focuses on ways that the size distribution of individuals influences the types and intensities of competitive interactions within a population of aquatic arthropod predators. Three field experiments and one laboratory experiment were designed to test for feeding interference, interference mortality, and dispersal effects within and between larval size classes of the primarily semivoltine dragonfly Tetragoneuria cynosura in Bays Mountain Lake. One field experiment documented the temporal pattern of colonization of large-mesh cylinders by the small, first-year-class larvae during a 30-day period; the results are consistent with passive (density-independent) colonization. A second field experiment examined the effect of large, second-year-class larvae at densities of 1 or 3 per cylinder (14 or 42 m^-2) on colonization by small larvae; this colonization was inhibited at the high density of large larvae. In the laboratory experiment, when larvae of the two size-classes were together in the same aquarium, small larvae moved around less than when by themselves (dispersal inhibition). Thus the inhibition of colonization observed in the field may result from interference mortality, rather than from a flight response to the presence of larger conspecifics.To evaluate this interpretation, the third field experiment measured the in-situ functional response of large larvae to each other and to their small conspecific prey. Results suggest a type 1 (linear) functional response, with feeding inteference among large larvae. Moreover, the interference mortality inflicted by larger larvae on smaller conspecifics was apparently more intense on larger individuals within the small size-class. Taken together, the three field experiments and a statistical power analysis show how colonization and interference interact to determine the local density of small larvae, and why such interference effects are difficult to detect experimentally in the field.     

Seeing objects in motion

This paper reports estimates of the conjoint spatiotemporal tuning functions of the neural mechanisms of the human vision system which detect image motion. The functions were derived from measurements of the minimum contrast necessary to detect the direction of drift of a sinusoidal grating, in the presence of phase-reversed masking gratings of various spatial and temporal frequencies. A mask of similar spatial and temporal frequencies to the test grating reduces sensitivity considerably, whereas one differing greatly in spatial or temporal frequency has little or no effect. The results show that for test gratings drifting at 8 Hz, the tuning function is bandpass in both space and time, peaked at the temporal and spatial frequency (SF) of the test (SFs were 0.1, 1 or 5 c deg-1; c represents cycles throughout). For a grating of 5 c deg-1 drifting at 0.3 Hz, the function is bandpass in space but lowpass in time. Fourier transform of the frequency results yields a function in space-time which we term the 'spatiotemporal receptive field'. For movement detectors (bandpass in space and time) the fields comprise alternating ridges of opposing polarity, elongated in space-time along the preferred velocity axis of the detector. We suggest that this organization explains how detectors analyse form and motion concurrently and accounts, at least in part, for a variety of perceptual phenomena, including summation, reduction of motion smear, metacontrast, stroboscopic motion and spatiotemporal interpolation.     

Activity of descending contralateral movement detector neurons and collision avoidance behaviour in response to head-on visual stimuli in locusts

We recorded the activity of the right and left descending contralateral movement detectors responding to 10-cm (small) or 20-cm (large) computer-generated spheres approaching along different trajectories in the locust's frontal field of view. In separate experiments we examined the steering responses of tethered flying locusts to identical stimuli. The descending contralateral movement detectors were more sensitive to variations in target trajectory in the horizontal plane than in the vertical plane. Descending contralateral movement detector activity was related to target trajectory and to target size and was most sensitive to small objects converging on a direct collision course from above and to one side. Small objects failed to induce collision avoidance manoeuvres whereas large objects produced reliable collision avoidance responses. Large targets approaching along a converging trajectory produced steering responses that were either away from or toward the side of approach of the object, whereas targets approaching along trajectories that were offset from the locust's mid-longitudinal body axis primarily evoked responses away from the target. We detected no differences in the discharge properties of the descending contralateral movement detector pair that could account for the different collision avoidance behaviours evoked by varying the target size and trajectories. We suggest that descending contralateral movement detector properties are better suited to predator evasion than collision avoidance.     

On the existence of 'fast' and 'slow' directionally sensitive motion detector neurons in insects.

In a fly, butterfly, locust and dragonfly we examined the responses of a variety of directional motion-sensitive neurons which run from the brain down the ventral cord. The stimulus was a sinusoidally modulated moving pattern of regular stripes presented at a range of velocities in random order for either 0.1 s or 2.0 s. The response was measured as the total number of spikes to each stimulus. The neurons fall into two groups, 'fast' and 'slow'. The responses of the fast type rise progressively to a peak contrast frequency at 15-20 Hz for all four insects, and decline at higher contrast frequencies. The responses of slow neurons rise rapidly to a peak at 1-10 Hz and then decline more slowly across the range where the fast neurons are at their peak. The existence of two groups of neurons with overlapping response ranges to different velocities of the same pattern, presented in exactly the same way, provides the insect with a means of measuring angular velocity irrespective of contrast, spatial frequency or intensity. As an input mechanism it is proposed that there are two types of unit motion detector, fast and slow, the latter being the main input to the optomotor system. It is also argued that even these inputs are not sufficient to provide a mechanism for the whole repertoire of normal insect vision.     

Responses of neurons to extreme osmomechanical stress

Neurons are often regarded as fragile cells, easily destroyed by mechanical and osmotic insult. The results presented here demonstrate that this perception needs revision. Using extreme osmotic swelling, we show that molluscan neurons are astonishingly robust. In distilled water, a heterogeneous population of Lymnaea stagnalis CNS neurons swelled to several times their initial volume, yet had a ST₅₀ (survival time for 50% of cells) >60 min. Cells that were initially bigger survived longer. On return to normal medium, survivors were able, over the next 24 hr, to rearborize.Reversible membrane capacitance changes corresponding to about 0.7 F/cm² of apparent surface area accompanied neuronal swelling and shrinking in hypo- and hyperosmotic solutions; reversible changes in cell surface area evidently contributed to the neurons' ability to accommodate hydrostatic pressures then recover. The reversible membrane area/capacitance changes were not dependent on extracellular Ca²⁺.Neurons were monitored for potassium currents during direct mechanical inflation and during osmotically driven inflation. The latter but not the former stimulus routinely elicited small potassium currents, suggesting that tension increases activate the currents only if additional disruption of the cortex has occurred.Under stress in distilled water, a third of the neurons displayed a quite unexpected behavior: prolonged writhing of peripheral regions of the soma. This suggested that a plasma membrane-linked contractile machinery (presumably actomyosin) might contribute to the neurons' mechano-osmotic robustness by restricting water influx. Consistent with this possibility, 1 mM, N-ethylmaleimide, which inhibits myosin ATPase, decreased the ST₅₀ to 18 min, rendered the survival time independent of initial size, and abolished writhing activity.For neurons, active mechanical resistance of the submembranous cortex, along with the mechanical compliance supplied by insertion or eversion of membrane stores may account for the ability to withstand diverse mechanical stresses. Mechanical robustness such as that displayed here could be an asset during neuronal out-growth or regeneration.This work was supported by a NSERC Canada research grant to CEM.     

Responses of neurons in somatosensory cortical area II of cats to high-frequency vibratory stimuli during iontophoresis of a GABA antagonist and glutamate

Areas in the second somatic sensory cortex (SII) of cats that responded vigorously to low-amplitude, high-frequency vibratory stimulation were mapped with respect to the surrounding somatotopic organization. Neurons with these properties were found in the posterior and medial parts of the distal forelimb zone and were judged as receiving input from Pacinian mechanoreceptors. The responses of these neurons to sinusoidal vibrotactile stimulation were studied during iontophoretic administration of glutamate or bicuculline methiodide (BMI) to determine if the temporal fidelity of these cortical neurons was controlled by inhibitory circuits that used gamma-aminobutyric acid (GABA) as a neurotransmitter. The data from 19 Pacinian-sensitive neurons were analyzed for changes in the mean firing rate, the percentage of entrainment, and the pattern of periodicity as revealed by autocorrelograms and interval histograms. Iontophoresis of BMI or glutamate caused significant increases in mean firing rates during low- and high-frequency vibratory stimulation. The pattern of increased activity produced by BMI was characterized by a small, yet significant, reduction in the percentage of entrainment, whereas glutamate caused smaller and fewer significant changes in this measure. Analysis of autocorrelation and interval histograms suggested that BMI increased the probability of firing on consecutive stimulus cycles in small segments of the stimulus duration.     

Morphological and behavioral defenses in dragonfly larvae: trait compensation and cospecialization

Many animals have two basic traits for avoiding being killedby a predator: behavioral modification and morphological defense.We examined the relationship between antipredator behavior andmorphological defense in larvae of three closely related dragonflyspecies within the genus Leucorrhinia. The three species differwith regard to their morphological defense as expressed in thelength of the larval abdominal spines. Results showed that longerabdominal spines provided protection against an attacking fishpredator (perch) because the probability of being rejected afteran attack was significantly higher in the species with the longestabdominal spines. In contrast to other studies, the specieswith the strongest morphological defense did not show the leastbehavioral predator avoidance. Instead, the species with intermediatemorphological defense showed the least predator behavioral avoidance.The results suggest that the Leucorrhinia system is a mixtureof trait cospecialization (a positive correlation between antipredatorbehavior and morphological defense) and trait compensation (anegative correlation between antipredator behavior and morphologicaldefense). Differences in the relationship between morphologicaland behavioral defense between species might be related to abundancepatterns of the three species in lakes with and without fishpredators.