Tonic effects of stationary luminous slits on the discharge rates of some locust visual interneurones

The presence of an illuminated slit in the visual field of a locust compound eye produced changes in the tonic discharge rate of the DCMD and three other visual interneurones, recorded in a connective. The DCMD discharge peaked initially in the range of low slit subtenses, but over a period of minutes of exposure its character changed so that there was a rise at high subtenses also. When the luminance of a slit of fixed subtense was increased in steps, there was an initial rise then a sharp fall in discharge, indicating an abrupt onset of inhibition. Lateral spread of inhibition could account for the peak in response to slits, at a subtense falling well within the acceptance angle of a single ommatidium. The results show the ability of some visual interneurones to maintain a changed level of discharge in the presence of a stationary object in the visual field of the eye.     

Responses of locust visual interneurones to transiently-illuminated slits,paired spots,annuli and disc arrays

One compound eye of an immobilised locust viewed a large screen on to which could be projected a slit of variable width, parallel thin slits of variable separation, large discs or annuli, pairs of small discs or large arrays of small discs. The on-response in the contralateral cervical connective, for both the DCMD and a smaller axon group (M-spikes), was recorded over a period of 2 sec coinciding with the period of illumination. The normal spontaneous discharge of the connective is described. For a single slit the DCMD response peaked in the range 0.02–0.7° of width subtense, the most common result being 0.05°. The M-axon group showed a negative peak at a corresponding subtense. Comparing the responses to single bars and paired slits of the same overall subtense, it was found that below about 1° these were not discriminated by either type of neurone. At higher subtenses the DCMD showed preference for paired slits, the M-axons for solid bars. Comparing responses to arrays of spots with those to uniformly illuminated areas, neither the DCMD nor the M-axon group showed discrimination at angles less than 1°, but at higher angles the DCMD showed preference for arrays and the M-axons for solid areas. Annuli were more effective stimuli than were discs of the same size. Near the edge of large-area targets it was possible to detect spatial transients in excitability. Two small spots produced interaction over a wide range of separations, affording evidence for wide-range mutual inhibition affecting both types of interneurone, and for short-range facilitation for the M-axon group only.     

Control of flight by means of lateral visual stimuli in gregarious desert locusts, Schistocerca gregaria

Abstract. Tethered flying locusts were stimulated either by a periodic grating or by a spotted 'swarm-simulating' pattern moving horizontally, parallel to their longitudinal body axis within their lateral visual fields. The direction of movement of the pattern was changed periodically from progressive to regressive and vice versa.
Both kinds of patterns induced a correlated modulation of yaw-torque and thrust. The two measured flight parameters were modulated independently of each other. Each parameter either increased with progressive and decreased with regressive pattern motion or vice versa. The characteristic curves of thrust and yaw-torque responses - i.e. response amplitude versus contrast frequency resp. angular velocity – measured upon stimulation with the periodic grating between 2 and 70 Hz were at a maximum at 10 Hz and decreased at higher and lower contrast frequencies. The shape of the curves was nearly identical. The characteristic curves measured upon stimulation with the 'swarm-simulating' pattern between 60 and 1500^o s^-1 could be simulated using the spatial wavelength content of the pattern and the characteristic curves for periodic gratings.
Therefore, we suggest that the speed and direction of locusts' flight result from the optomotor effectiveness of the pattern image formed by the neighbouring individuals under free flight. The measured responses would thus contribute to the common orientation of groups of locusts within a migrating swarm and thus to swarm cohesion.     

Peaking of response in locust visual interneurones to objects of small angular subtense

One compound eye of an immobilised locust viewed a large screen on to which were projected discs of light for periods of 2 sec every 40 sec. The spike response was counted concurrently in the DCMD and the next largest axon in the contralateral nerve cord connective. The average score for 10 trials, after correction for background was plotted for a series of discs subtending a range of angles from 0.05 to 84°.It was found that the response of the DCMD peaked sharply and consistently at a subtense of 0.3°, and fell away to a low plateau or to zero over the range 2° to 84°. The response could exceed background down to subtenses as low as 0.05° (3′ of arc). The response of the next largest axon also showed an early peak, but it was inhibitory and resembled a mirror image of that of the DCMD, although it did not always coincide, ranging from 0.2 to 0.3°. The response, by contrast with the DCMD, rose to a high level at large subtenses, forming a flat peak.No explanation in optical terms could be found for this peaking at small subtenses, and a scheme is proposed by which a peak response could develop by the interaction of excitatory and inhibitory processes in the optic lobe.The peak value of 0.3° corresponds with the resolution limit for moving periodic patterns repeatedly demonstrated by Burtt and Catton (e.g. 1962, 1969). Such peaking behaviour would serve to lift the response curve of the whole visual system at high spatial frequencies, and thus extend the resolution limit.With stepwise reduction in intensity of a small luminous target there was a steep fall in the DCMD response, but a similar reduction for a large target had only a small effect. This could be explained by assuming that excitatory processes were prevalent for small targets, subtending about 0.3°, whereas for larger targets the excitatory and inhibitory processes came into balance over a wide range of intensities, thereby stabilising the response, at a low level.     

Ability of the desert locust (Schistocerca gregaria) to compensate for visual deprivation during the final pre‐adult and adult stages

In the desert locust (Schistocerca gregaria Forsskål), vision is a seemingly indispensable prerequisite for many behaviour patterns. The question arises as to whether and to what extent other senses can compensate for the loss of vision, and whether this can take place in the adult stage. To answer this question, both of the compound eyes of nymphs in the final pre‐adult stage are blinded (but not the ocelli), resulting in permanent visual deprivation during adult life. The results are somewhat unexpected: under laboratory conditions, in comparison with sighted controls, the blinded locusts do not exhibit any noticeable change in overall agility relative to daily activity, nor any detriment in terms of the final moult, reproductive rate or longevity. In their search for a specific food source in an experimental arena with a narrow passage between the compartment where the animals are released and the compartment containing food, in the first trial, visually deprived females need significantly more time than the sighted controls. However with an increasing number of trials, each performed after 1 day of food deprivation, the food finding latency of blinded locusts (S. gregaria) approaches or even surpasses that of normally sighted locusts. The blinded locusts use their antennae, mouthparts and tarsi more extensively, suggesting a more frequent use of tactile and chemical cues, which they gradually learn to use more efficiently. The results indicate that pre‐adult and adult S. gregaria can respond to abrupt, permanent changes in their sensory inputs, and have a significant capacity for adaptation.     

Oxygen consumption of flying locusts

ABSTRACT. Desert locusts ( Schistocerca gregaria Forskal.) were flown into an airstream from a wind tunnel while tethered to a balance. Rate of oxygen consumption and lift were recorded. The mean rate of oxygen consumption of male locusts in level flight at airspeeds between 2.5 and 3m/s was found to be 12.6ml/g/h.     

Purification and sequence determination of a yellow protein from sexually mature males of the desert locust, Schistocerca gregaria

A yellow protein from abdominal cuticle of the desert locust, Schistocerca gregaria, has been purified and its amino acid sequence determined. The yellow color comes from bound carotene, the protein is only deposited in the epidermis and cuticle of male locusts during their sexual maturation, and the deposition is dependent upon a sufficiently high titer of juvenile hormone. The sequence of the protein is atypical for a cuticular protein, but it has some similarity to a putative juvenile hormone binding protein from Manduca sexta. It is suggested that the protein is involved in the transport of carotenes from internal tissues to epidermis and cuticle of the locust.     

Central nervous processing of behaviourally relevant odours in solitary and gregarious fifth instar locusts, Schistocerca gregaria

Physiological and morphological characteristics of antennal lobe neurons of solitary and gregarious fifth-instar nymphs of the desert locust, Schistocerca gregaria, were studied using intracellular recording and staining techniques. Physiological characteristics of antennal lobe neurons of both locust phases responding to stage-dependent aggregation pheromones, egg-laying attractants, a putative sex pheromone and plant-associated volatiles are described. Antennal lobe neurons showed excitatory, inhibitory, combined excitatory and inhibitory and delayed responses. In addition, one neuron␣showing an initial inhibition followed by an excitation and inhibition response was found. Pheromone-specific-, plant-specific- and pheromone-plant-generalist neurons were found in both locust phases. Antennal lobe neurons displayed stage- and phase-dependent differences in the processing of aggregation pheromone component input. Nymphal antennal lobe neurons showed stage-dependent response characteristics highly correlated with the preferential behavioural attraction to the nymphal aggregation pheromone. Phase-dependent differences were found in the response spectra and the sensitivity of the same neuron types. Neurons of solitary locusts responded significantly more frequently to some of the tested components than neurons of gregarious locusts. Furthermore, antennal lobe neurons of solitary locusts showed a higher sensitivity to most of the tested compounds. Accepted: 4 July 1998     

Complex dynamics of desert locust plagues

Abstract. 1. Spectral analysis of 66 years of locust swarm abundance data failed to reveal any significant cycles although the dominant cycle detected in a de-trended series, adjusted to take account of a significant partial autocorrelation for a lag of 1 year, had a periodicity of 16 years.
2. Although some estimates of the intrinsic rate of increase (r) of desert locusts are indicative of chaos, reconstructions of locust dynamics using response surface methodology (RSM) suggested exponential stability. This was also true for data for the West African Region alone and inclusion of rainfall data from the Sahel improved the significance of an RSM model for West Africa.
3. The observed positive relationship between locust abundance and rainfall in West Africa confirmed the importance of rain; but the variance of the locust abundance also increased with rainfall, making rainfall alone a poor predictor. However, this heteroscedastic pattern was reproducible by a simple logistic model with a chaotic r and a variable K. This was not the case when a stable value for r was used.
4. The available data and current methodologies are insufficient to provide unequivocal conclusions on locust dynamics, which are complicated by phase changes and associated switches in r values.     

The time-course of behavioural phase change in nymphs of the desert locust, Schistocerca gregaria

Abstract. The time-course of behavioural phase change was investigated in nymphs of Schistocerca gregaria , using logistic regression analysis of behaviour recorded in a standard assay. Gregarization occurred very rapidly. Solitary-reared nymphs became markedly gregarious in behaviour within 1-4h of being placed in a crowd. These insects re-solitarized equally quickly if removed from the crowd. Crowd-reared locusts also solitarized within l-4h, but this effect was not complete. Results indicate that, while behavioural gregarization is maximal within a few hours of crowding, solitarization is a two-stage process, changing rapidly at first, then more slowly as a function of the period of previous crowding.     

The optomotor yaw response of the desert locust, Schistocerca gregaria

Abstract The optomotor yaw response of the desert locust, Schistocerca gregaria (Forsk.), was investigated under open- and closed-loop conditions. When flying tethered in the centre of a vertically striped hollow sphere, the polarity of response of the locust was always the same as the stimulus. The response, therefore, appears suitable to stabilize body posture against passive rotations around the yaw-axis in free flight. Responses were induced by contrast frequencies up to 150 Hz with a maximum of amplitude at about 20 Hz. The characteristic curve, measured between 0.3 and 160 Hz, is widened up towards higher frequencies as compared with those of bees and flies.
Variability was the most striking feature in the locust's yaw response. The amplitude of modulation not only varied greatly between individuals but also changed with the same visual stimulus in the course of an experiment. We therefore suppose that the locust's turning behaviour is subject to gain control mechanisms and that spontaneous gain modulations are responsible for the observed variability in the stimulus-response conversion.     

Diurnal rhythms of locomotory activity in isolated desert locusts (Schistocerca gregaria (forsk.))

Abstract

The results of aktograph experiments suggest that S. gregaria is primarily day‐active although its circadian clock can, to some extent, be synchronized by changes in both light and temperature. Low temperatures exert a threshold effect on activity. The nycthemeral rhythm of the desert locust is feeble in LD 12:12, even with fluctuating temperatures. This, is probably of adaptive significance. It certainly accords with the plastic and opportune nature of locust behaviour, upon which survival depends under the unpredictable conditions of the desert climate. For the insect must always be ready to exploit temporary and irregular amelorations of its harsh, arid environment. The desert locust, therefore, cannot afford to adopt rigid rhythms or behaviour patterns that might cause it to lose the benefits of a chance shower or to miss the temporary appearance of green grass. It is argued that the persistence of a rhythm fora short while could be due to an endogenous ‘clock’ or to an exogenous periodicity that continues briefly in constant conditions. It has been shown experimentally, however, that although locusts do not exhibit a marked periodicity under field conditions they, nevertheless, possess good circadian ‘clocks’ whose disclosure can be elicited by subjecting the insects to unnatural light‐dark régimes.     

The influence of environmental microstructure on the behavioural phase state and distribution of the desert locust Schistocerca gregaria

Abstract. In previous studies we used logistic regression analysis to quantify the change in behavioural phase state of Schistocerca gregaria (Forskål) nymphs subjected to variations in population density. Such work involved restricting insects in small containers either alone or in a crowd. In the present paper we have shown that the fine-scale distribution of food plants, perches and favourable microclimatic sites influences the spatial distribution of locusts, both in the laboratory and under semi-field conditions. When multiple resource sites were provided, solitarious locusts tended to disperse and behavioural gregarization was inhibited. However, provision of only a single site promoted congregation, overcoming the tendency of solitarious insects to avoid each other, and led to behavioural gregarization. The time-course and extent of this response was fully consistent with our earlier experiments using enforced crowding. We suggest that such quantitative, experimental studies of the effects of environmental microstructure on behaviour may yield fundamental insights into the dynamics of plague formation in the desert locust.     

Processing of mechanosensory information from gustatory receptors on a hind leg of the locust

Gustatory receptors (basiconic sensilla) on the legs of the desert locust, Schistocerca gregaria, are innervated by chemosensory afferents and by a mechanosensory afferent. We show, for the first time, that these mechanosensory afferents form an elaborate detector system with the following properties: 1) they have low threshold displacement angles that decrease with increasing stimulus frequency in the range 0.05–1 Hz, 2) they respond phasically to deflections of the receptor shaft and adapt rapidly to repetitive stimulation, 3) they encode the velocity of the stimulus in their spike frequency and have velocity thresholds lower than 1°/s, and 4) they are directionally sensitive, so that stimuli moving proximally towards the coxa elicit the greatest response.The mechanosensory afferents, but not the chemosensory afferents, make apparently monosynaptic connections with spiking local interneurones in a population with somata at the ventral midline of the metathoracic ganglion. They evoke excitatory synaptic potentials that can sum to produce spikes in the spiking local interneurones. Stimulation of the single mechanosensory afferent of a gustatory receptor can also give rise to long lasting depolarizations, or to bursts of excitatory postsynaptic potentials in the interneurones that can persist for several seconds after the afferent spikes. These interneurones are part of the local circuitry involved in the production of local movements of a leg. The mechanosensory afferents from gustatory receptors must, therefore, be considered as part of the complex array of exteroceptors that provide mechanosensory information to these local circuits for use in adjusting, or controlling locomotion.     

Diversity of locust gut bacteria protects against pathogen invasion

Diversity–invasibility relationships were explored in the novel context of the colonization resistance provided by gut bacteria of the desert locust Schistocerca gregaria against pathogenic bacteria. Germ‐free insects were associated with various combinations of one to three species of locust gut bacteria and then fed an inoculum of the pathogenic bacterium Serratia marcescens. There was a significant negative relationship between the resulting density of Serratia marcescens and the number of symbiotic gut bacterial species present. Likewise there was a significant inverse relationship between community diversity and the proportion of locusts that harboured Serratia. Host mortality was not negatively correlated with resistance to gut‐invasion by Serratia marcescens, although there were significantly more deaths among pathogen fed germ‐free insects than tri‐associated gnotobiotes. The outcome is consistent with the predictions of community ecology theory that species‐rich communities are more resistant to invasion than species‐poor communities.     

Small-scale vegetation patterns in the parental environment influence the phase state of hatchlings of the desert locust

Desert locusts (Schistocerca gregaria Forskål (Orthoptera: Acrididae)) change phase in response to population density. Solitarious insects avoid one another; when crowded, they shift to the gregarious phase and aggregate. Laboratory experiments and individual‐based modelling have shown that small‐scale resource distribution can affect locust phase state via an influence on crowding. Laboratory work has also shown that parental phase state is transmitted to offspring via maternal inheritance. These effects had not been investigated in the field previously. We maintained small populations of adult desert locusts in semi‐field enclosures with different distribution patterns of a single plant species (Hyoscyamus muticus L. (Solanaceae)). The offspring of locusts exposed to more clumped patterns of vegetation exhibited more gregarious behaviour when tested in a behavioural phase assay than did progeny from parents left in enclosures with more scattered vegetation. These effects on nymphal behaviour appeared to be mediated by influences of resource distribution on adult phase state. Phase state in small semi‐field populations was influenced by small‐scale vegetation distribution. Phase differences engendered by environmental structure were maintained in time and transmitted to progeny.     

SERINE proteinase like activity in apolipophorin III from the hemolymph of desert locust, Schistocerca gregaria

Apolipophorin III (apoLp-III) has been known as a lipid transport protein of insects. Recent studies indicated the involvement of apoLp-III in immune reactions and in the control of cell destruction, but no enzymatic activity has so far been detected. In the present study, a protease from the hemolymph of Schistocerca gregaria was purified to homogeneity and its enzymatic activity was examined. Identity as chymotrypsin-like proteinase was established by its high affinity toward bulky aromatic substrates and its catalytic specificity for amide or ester bonds on the synthetic substrates, Suc-Ala-Ala-Pro-Xaa-AMC (where Xaa was Phe, Tyr, Trp, and Lys, and AMC is 7-amino-4-methyl-coumarin) and thiolbenzyl ester substrate Suc-Ala-Ala-Pro-Phe-SBzl. The sensitivity for serine protease and chymotrypsin-specific covalent inhibitors, PMSF, TPCK, and noncovalent inhibitors SGCI, showed that it is a chymotrypsin-like proteinase. It showed its maximum activity at pH 8.0 and 55°C for the hydrolysis of Suc-Ala-Ala-Pro-Tyr-AMC. According to similarities in the amino terminal sequence, molar mass (19 kDa) and retention on reversed-phase analytical high-performance liquid chromatography (HPLC) column, this protein is S. gregaria homologue of Locusta migratoria apoLp-III. Our data suggest that apoLp-III also has an inherent proteolytic activity. Results indicated that S. gregaria apoLp-III is a good catalyst and could be used as a biotechnological tool in food processing and in agricultural biotechnology.     

PCR primers for polymorphic microsatellite loci in the desert locust,Schistocerca gregaria (Orthoptera: Acrididae)

Nine pairs of polymerase chain reaction (PCR) primers that amplify polymorphic microsatellite loci in the desert locust, Schistocerca gregaria (Forskal), were developed using a magnetic bead‐based enrichment protocol. A sample of 48 locusts collected during the 1993 and 1995 upsurge periods in Eritrea, East Africa, were genotyped. The number of alleles per locus ranged from six to 20; the average was 12.67. Allelic distributions were significantly different between samples from different localities.     

Yellowing,morphology and behaviour in sexually mature gynandromorphs of the desert locust Schistocerca gregaria

The morphology, colouration and sexual behaviour of two gynandromorph adults of the desert locust Schistocerca gregaria Forskål are described. In each of these specimens, the left half of the external genitalia has male characteristics and the right half has female characteristics. Yellowing of the epidermis occurs to different degrees in normal sexually mature male and female adult S. gregaria under crowded conditions. Two hypotheses are known to explain this phenomenon. One suggests involvement of a sex‐hormone/receptor complex, whereas the other proposes different sensitivities of the epidermis to the same hormonal environment. To examine which hypothesis is more likely, one gynandromorph adult is kept under crowded conditions and the other is maintained under isolated conditions. In the former, the left half of the body turns bright yellow, as in a sexually mature crowded male, and the right half is brownish with little yellowing, as in a sexually mature crowded female of the same age. The bilaterally divided body colouration may support the second hypothesis. In the second gynandromorph, which is isolated after adult emergence, the body colour remains brownish. Upon sexual maturity, this individual is kept together with normal male or female adults for behavioural observations. It attempts to mount a female but is approached and mounted by males.     

Morphological and behavioural characteristics of a gynandromorph of the desert locust, Schistocerca gregaria

Abstract.  Morphological and behavioural characteristics are investigated for a gynandromorph of the desert locust, Schistocerca gregaria , appearing under isolated rearing conditions in the laboratory. It has both male and female external reproductive organs bilaterally. The body size and dimensions are similar to a normal male. Morphometric traits (fore wing length/maximum head width ratio and fore wing length/hind femur length ratio) of the gynandromorph are typical for the values of solitarious locusts. When the gynandromorph is placed into an arena holding ten sexually mature gregarious females, it shows a distinct male behaviour: it jumps on a female and tries to mate with her. When kept together with males, males recognize this gynandromorph as a female because some of them try to mount, although no successful copulation is observed. The results suggest that the gynandromorph might have had a female-specific pheromone. Dissection reveals that the gynandromorph has no testis but abnormal ovaries containing vitellogenic oocytes. These observations indicate that the gynandromorph obtained has a mixture of male and female morphological characteristics and behaves like a male but is recognized as a female by conspecific males.