Musculature and innervation of the neck of the desert locust,Schistocerca gregaria (Forskål)

The innervation of each of the muscles involved in mediating head movement in the desert locust Schistocerca gregaria is described in detail. The number of motor neurones to each muscle and the neutral pathway and ganglion of origin of each are deduced from both histological and electrophysiological evidence. Only two of the muscles are, on histological evidence, innervated by as few as four different neurones, while several receive more than ten, and one at least 13. Individual muscles are shown physiologically to receive, in a few cases, as many as six different motor neurones. At least six muscles are innervated by motor neurones originating in more than one ganglion. One group of four muscles consisting in total of less than 100 muscle fibres receives more than 20 different motor neurones from three different ganglia through three or four different nerve roots. In these muscles, many single muscle fibres receive innervation from at least two different ganglia. It is concluded that the segmental nature of an insect muscle can not be deduced solely from a knowledge of the ganglion of origin of the motor innervation to that muscle. The innervation patterns that exist today must reflect past evolutionary development, but changes in the peripheral distribution of motor neurones, or migration of motor neurone cell bodies from one ganglion to another, or the development of additional motor neurones, or several of these factors together, must have formed a part of that development.     

The anatomy and histology of the maxillary palp of Schistocerca gregaria (Orthoptera, Acrididae)

The general anatomy and histology of the maxillary palp are described with particular attention to the number, distribution and structure of the various sensilla.     

Qviposition of Schistocerca gregaria (Forskal) (Orthoptera: Acrididae) mated with males unable to transfer spermatophores

An account is given of the oviposition of females paired with males unable to transfer spermatophores. Various influences of the male accessory gland on the gonotrophic cycle are discussed.     

The integument and sensilla of the pregenital abdomen of the Desert locust (Schistocerca gregaria) (Orthoptera: Acrididae)

The integument and sensilla of the abdominal segments II–VII, together with the tracheal and nerve supply, are described in so far as they can be seen using whole mounts. Such a method often enables the neurons of the sensilla to be seen clearly and to be associated definitely with a certain cuticular structure. The sensilla are compared with those already known from the palps, and it is concluded that probably all those on the abdomen also occur on the palps.     

Fine structure and distribution of antennal sensilla of the desert locust, Schistocerca gregaria (Orthoptera: Acrididae)

The fine structure and distribution of various types of antennal sensilla in three nymphal stages and in adults of both solitary-reared (solitary) and crowd-reared (gregarious) phases of the desert locust, Schistocerca gregaria, were investigated by scanning and transmission electron microscopy. Four types of sensilla were identified: sensilla basiconica, s. trichodea, s. coeloconica and s. chaetica. S. basiconica contain up to 50 sensory neurons, each of which displays massive dendritic branching. The sensillar wall is penetrated by a large number of pores. In contrast, s. trichodea contain one to three sensory neurons that branch to give five or six dendrites in the sensillar lumen; the sensillum wall is penetrated by relatively few pores. The s. coeloconica are situated in spherical cuticular pits on the antennal surface. The s. coeloconica are of two types: one type contains one to three sensory neurons with double sensillar walls penetrated by slit-like pores, whereas the second type contains four sensory neurons with non-porous double sensillar walls. The s. chaetica have a flexible socket and a thick non-porous sensillum wall and contain four sensory neurons that send unbranched dendrites to a terminal pore. A fifth sensory neuron of the s. chaetica terminates in a tubular body at the base of the hair. S. basiconica and coeloconica are normally distributed over the entire antennal flagellum, with a concentration in the middle segments; s. trichodea have three areas of concentration on the 5th, 10th and 14th flagellar segments. Sensilla chaetica are most abundant on the terminal segment. Locusts raised in solitary conditions have more olfactory sensilla (s. basiconica and s. coeloconica) than crowd-reared locusts. The difference in sensillar numbers is more evident in adults than in nymphs. These results suggest that differences in the odour-mediated behaviour of nymphs and adults, and between the phases of S. gregaria, may be attributable to differences at the sensory input level.     

Phase-independent responses to phase-specific aggregation pheromone in adult desert locusts, Schistocerca gregaria (Orthoptera: Acrididae)

Abstract. Volatiles from solitary-reared (solitarious) and crowd-reared (gregarious) adult male desert locusts, Schistocerca gregaria (Forskal) (Orthoptera: Acrididae), were quantitatively and qualitatively different.In particular, solitarious males did not emit phenylacetonitrile, a key component of the aggregation pheromone produced by gregarious adult males.In laboratory bioassays, solitarious and gregarious adults of both sexes responded similarly to the natural aggregation pheromone blend, the major pheromone component phenylacetonitrile, and a synthetic pheromone blend comprising benzaldehyde, guaiacol, phenylacetonitrile and phenol.EAG measurements showed significant differences in the responsiveness of adults of the two phases to the four synthetic components at high doses; however, the general response patterns were similar.These results suggest that the gregarious adult male aggregation pheromone may play a role in the arrestment and subsequent recruitment of solitarious individuals into gregarious or gregarizing groups during the early stages of a locust outbreak.     

Effects of juvenile hormone treatment on phase changes and pheromone production in the desert locust,Schistocerca gregaria (Forskal) (Orthoptera: Acrididae)

The roles of juvenile hormone III (JH III) on phase changes and pheromone production were examined in laboratory-reared gregarious desert locust, Schistocerca gregaria (Forskal). The hormone was applied to 5th instar nymphs and newly emerged adult locusts. Generally, the 5th instar nymphs exhibited a higher sensitivity to hormone treatments than the adults. Hormone applications inhibited pheromone production (as measured by the amounts of phenylacetonitrile released). In addition, JH III had a significant effect on the external colouration and absorbance ratios of the haemolymph pigments. It is concluded that the effects of exogenous JH III on gregarious locusts represent a shift towards the solitarious phase.     

Take-off activity in caged desert locusts,Schistocerca gregaria (Forsk.) (Orthoptera,Acrididae) in relation to meteorological disturbances

Take-off activity of the desert locust,Schistocerca gregaria (Forsk.), phasegregaria, was studied in a temperature-controlled laboratory environment and possible relationships with certain meteorological factors were analysed. A statistically significant relationship between the passage of cold fronts and an increased take-off rate was found. The reaction of the locusts could not be correlated to changes in VLF electromagnetic radiation or to the atmospheric pressure.     

Physiological responses and central nervous projections of antennal olfactory receptor neurons in the adult desert locust, Schistocerca gregaria (Orthoptera: Acrididae)

Olfactory receptor neurons present in two morphological sensillum types on the male Schistocerca gregaria antenna were for the first time investigated physiologically when stimulated with behaviourally relevant odours. Neurons present in trichoid/basiconic sensilla showed clear excitatory responses to compounds present in the male-produced aggregation pheromone and also to a plant produced compound. Sensilla could be categorised physiologically according to the responses of their receptor neurons to the tested stimuli. Also receptor neurons present in sensilla coeloconica responded to aggregation pheromone components, but always in an inhibitory fashion. These neurons could, however, be excited by a plant produced compound and by some acids present in the nymphal odour. The antennal lobe of the male S. gregaria was observed to contain about 1000 very small glomerular structures. Single receptor neurons were stained from the antenna to the antennal lobe using a cobalt lysine technique. These stainings revealed a multi glomerular axonal branching pattern of antennal receptor neurons.Abbreviations AN antennal nerve - AL antennal lobe - RN receptor neuron     

Influence of male presence on clutch size in Schistocerca vaga Scudder (Orthoptera: Acrididae)

Male Schistocerca vaga produce a pheromonal factor that causes increase in clutch size. Females exposed to males, but prevented from physically contacting or copulating with them, produce significantly more eggs (P < 0.01) than virgins not exposed to any males at all. There is no significant difference between the clutch sizes of such exposed females and those of females which are allowed both physical contact and copulation. Castration does not abolish this effect of male presence on clutch size. Visual exposure alone does not enhance clutch size, whereas olfactory exposure, with or without concomitant physical contact, leads to increased clutch size.     

Phase-related morphological changes induced by [His7]-corazonin in two species of locusts, Schistocerca gregaria and Locusta migratoria (Orthoptera: Acrididae)

The effects of a neurohormone, [His(7)]-corazonin, on phase-related morphological traits (F/C and E/F ratios; F = length of the hind femur, C = maximum width of the head; E = length of fore wing) were re-examined in the desert locust, Schistocerca gregaria Forsk?l. The F/C ratio was significantly different between adults with five and six nymphal instars, respectively, indicating that they need to be analysed separately. Injections of the synthesized peptide (1 nmol) into individually-reared (solitary) nymphs at the second and third instars caused a shift in classical morphometric ratio towards the value typical for crowded (gregarious) individuals in both sexes. The E/F ratio, which is smaller in solitary locusts than in gregarious ones, was also influenced significantly by injections of [His(7)]-corazonin into individually-reared locusts. The effect of [His(7)]-corazonin on E/F ratios was shown more clearly when the nymphs were injected at a higher dose (2 nmol) at the beginning of the third instar. Single injections of the peptide into individually-reared nymphs at different instars revealed that the earlier the injection the larger the 'gregarizing' effects of the peptide on F/C and E/F ratios. The same tendency was also detected in Locusta migratoria Linnaeus. These results supported the hypothesis that [His(7)]-corazonin plays an important role in the control of phase polymorphism in locusts.     

Behavioral responses of Schistocerca americana (Orthoptera: Acrididae) to Azadirex (neem)-treated host plants

Azadirex (azadirachtin and other biologically active extracts from neem trees) has been shown to have considerable potential to be used in integrated pest management systems based on its growth regulator/insecticide properties. Less well known are the antifeedant properties. The feeding-deterrent properties of a commercial azadirex formulation (Azatrol EC) were evaluated using both no-choice and choice tests, the American grasshopper, Schistocerca americana (Drury), and four host plants [savoy cabbage, Brassica oleracea variety capitata L.; cos (romaine) lettuce, Lactuca sativa variety longifolia Lam.; sweet orange, Citrus sinensis variety Hamlin L.; and peregrina, Jatropha integerrima Jacq.]. These studies demonstrated that azadirex application can significantly affect the feeding behavior of grasshoppers. Some degree of protection can be afforded to plants that differ markedly in their innate attractiveness to the insect, although the level of protection varies among hosts. The tendency of grasshoppers to sometimes feed on azadirex-treated foliage suggests that it will be difficult to prevent damage from occurring at all times, on all hosts. No evidence of rapid habituation to azadirex was detected. Rapid loss of efficacy was observed under field conditions, suggesting that daily retreatment might be necessary to maintain protection of plants from feeding.     

Embryonic development of the central projection of auditory afferents (Schistocerca gregaria, Orthoptera, Insecta)

The auditory system of Schistocerca gregaria is a well investigated sensory network in the adult grasshopper. Here we present a first study on the embryonic development of this neuronal network. Focussing on the auditory receptor cells we show that they differentiate axonal processes at around 45% of embryonic development. These axons fasciculate with the intersegmental nerve and enter the central nervous system by 45-50% of development. First collaterals sprout into the major arborization area, the frontal auditory projection area of the metathoracic ganglion by 60%. This projection increases in density until an adult-like morphology is established by 90% of development. Furthermore, by the end of embryogenesis all three types of receptor fiber projections can be distinguished. This development is independent of a hearing ability, which develops much later during postembryonic life. The auditory projection co-develops with the fusion of neuromeres to the metathoracic ganglion, the formation of the target neuropile areas and the expression of the synapse associated molecule synapsin. Fasciclin I and Lachesin, both potential axon-guidance molecules, are expressed strongly on both, peripheral and central auditory pathways and, although much weaker, within the synaptic target area.     

Laboratory evidence showing the separate species status of Schistocerca gregaria, americana and cancellata (Acrididae, Crytacanthacridinae)

Abstract. Results are given of experimental crosses of Schistocerca species from the New World and the Old World Desert Locust. It is concluded that S. cancellata (Serville), S. gregaria (Forskal) and S. americana (Drury) are separate species in their own right not subspecies of americana. The African Desert Locust should thus be called S. gregaria gregaria and its S. W. African race S. gregaria flaviventris (Burmeister). The name S. vicaria (Walker) is proposed for the Central American Locust. The relationship of S. paranensis to Sxancellata is discussed.     

Lectin binding to the egg envelopes in Eyprepocnemis plorans (Charp.) (Orthoptera, Acrididae)

The distribution of glycoconjugates in the egg envelopes of Eyprepocnemis plorans was investigated using various FITC-conjugated lectins. In the epichorion, the lectins ConA, SBA and WGA each have particular binding patterns, while TPA binding is confined to its deepest regions only. The glycoconjugates of the micropylar wall present different characteristics from those of the surrounding chorion. The vitelline coat shows a marked binding for WGA and TPA only; below the inner micropylar openings, this binding pattern is uniform over the whole extent of the coat and therefore it is not possible to identify specific binding sites for these two lectins. Contrary to what has been observed in some other insect species, the vitelline coat does not seem to be involved in the structural organization of the mycropyles.