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• Females kept in isolation did not easily change progeny characteristics upon crowding. • No evidence was obtained to show presence of a 4-day period of sensitivity to crowding. • No evidence was obtained to show that the antennae perceived crowding stimuli. • No evidence was obtained to show the importance of light in progeny phase changes. • These results contradicted the conclusions reported in previous studies. The desert locust shows conspicuous phase polyphenism of various traits in response to crowding conditions. Gregarious females lay larger eggs that produce black hatchlings, whereas solitarious females lay smaller eggs that produce green hatchlings. Previous studies have shown that changes in egg size and hatchling body color occurred easily in the laboratory upon exposure of the female parent to crowding or isolation for as few as 2 days. Based on these observations, these studies concluded that female adults perceive crowding stimuli with their antennae and require light for perception of the stimuli, with contact chemicals present on the integument of sexually mature males that are responsible for the crowding effect. We undertook this study to identify the chemical structures of the reported contact substances, which remain unknown to date. However, we could not reproduce the main results reported in the aforementioned studies and found that egg size and hatchling body color did not alter easily, even after crowding or isolation of the female parent for 2 days or longer. We were not able to observe a change in the progeny crowding characteristics by stimulating the antennae of female adults through either physical contact with male adults or using hexane extracts of their body surfaces. Similarly, the importance of light for gregarization or solitarization was also ruled out. These results indicate that some of the conclusions of previous studies should be reconsidered. [ABSTRACT FROM AUTHOR]

The present study showed that the eggs of the desert locust, Schistocerca gregaria , and the migratory locust, Locusta migratoria , responded to photoperiod by hatching when placed on sand in the laboratory. S. gregaria mainly hatched during the dark phase and L. migratoria during the light phase. The importance of light as a hatching cue depended on the magnitude of the temperature change during the thermoperiod; photoperiod played a more important role in the control of hatching time in both species when the magnitude of the temperature change was small. In addition, the eggs of the two species that were covered with sand did not respond to photoperiod and hatched during both the light and dark phases, indicating that light did not penetrate through the sand. Because locust eggs are normally laid as egg pods and a foam plug is deposited between the egg mass and the ground surface, we tested a possibility that naturally deposited eggs perceived light through the foam plug. The eggs that were deposited and left undisturbed in the sand hatched during the light and dark phases at similar frequencies. These results suggest that the eggs of both locust species responded to light and controlled their hatching timing accordingly but would not use light as a hatching cue in the field. The evolutionary significance of the ability of eggs to respond to light in these locusts was discussed. [ABSTRACT FROM AUTHOR]

Abstract: Schistocerca gregaria exhibits a phase-specific behavior in response to crowding. Nymphs occurring at low population densities (solitarious phase) tend to avoid one another, whereas those occurring at high population densities (gregarious phase) are attracted to one another. This study examined how this attraction/avoidance behavior changed after isolation or crowding. The behavior of the test nymphs was assessed by determining their positioning with respect to a stimulus cup, which contained 12 gregarious nymphs, placed at one end of an elliptical arena and an empty cup placed at the opposite end. Gregarious (crowd-reared) nymphs were most frequently observed close to the stimulus cup, whereas solitarious (isolated-reared) nymphs tended to avoid it. This tendency was easily changed by exposing the nymphs to isolation or crowding. However, contrary to a previously reported conclusion that behavioral gregarization occurs in 4–8h of crowding, the nymphs required at least 3days to achieve a significant change in the attraction/avoidance behavior in either direction, from solitarious to gregarious or the converse. The discrepancies between the present study and previous studies appear to result from the different behaviors observed. The present study focused on the attraction/avoidance behavior that is most likely the most important element in the gregarization and solitarization processes, whereas other studies used a mixture of various activity-related behaviors. No evidence was obtained for a trans-generational accumulation in the attraction/avoidance behaviors. Serotonin, a biogenic amine that is suggested by others to be involved in behavioral phase changes in S. gregaria, had no influence on this behavior or body color when injected into the solitarious nymphs. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Crowding causes many organisms to express phenotypic plasticity in various traits. Phase polyphenism in desert locusts represents one extreme example in which a solitary form (solitarious phase) turns into a gregarious form (gregarious phase) in response to crowding. Conspicuous differences in body size and colour occur even in hatchlings. The phase-specific differences in hatchling characteristics are caused by the tactile stimuli perceived by the antennae of their mother. However, the nature of the tactile stimuli and the mechanism by which the perceived stimuli are processed as a gregarizing signal remain unknown. To explore this problem, the antennae of solitarious adult females of the desert locust Schistocerca gregaria are touched with the bodies of conspecific locusts at different physiological stages and those of other species. The results suggest that a cuticular chemical factor at a specific developmental stage of conspecific locusts causes the solitarious females to produce large eggs that give rise to black hatchlings characteristic of gregarious forms (progeny gregarization), and that this or a similar compound occurs in other acridids, crickets and cockroaches but not in beetles. The involvement of a chemical substance is also supported by hexane extracts of cuticular surfaces of locusts that induce the same effects. Interestingly, crowding induces such gregarizing effects only when the female receives the appropriate stimulus in the presence of light. Solitarious female S. gregaria with their head capsule coated with phosphorescent paint exhibit progeny gregarization in response to crowding and light pulses in darkness, whereas those treated in the same way without light pulses fail to do so. [ABSTRACT FROM AUTHOR]

Abstract: This study re-examines the effects of environmental factors including substrate color, humidity, food quality, light intensity and temperature on the green–brown polyphenism, black patterning and background body color of solitarious (isolated-reared) nymphs of Schistocerca gregaria. All individuals reared in yellow–green or yellow containers became green morphs, whereas those reared in white, ivory-colored, blue, grey, brown, zinc-colored and black containers produced brown morphs in similar proportions. The intensity of black patterns was negatively correlated with the brightness of the substrate color of the containers. Humidity, which previous studies claimed controls green–brown polyphenism in this species, exerted no significant influence on either the green–brown polyphenism or the black patterning. Food quality also had little effect on body color. High temperature tended to inhibit darkening. The background body color on the thorax was greatly influenced by the substrate color of rearing containers and a close correlation was found between these two variables, indicating that, in contrast to what has been suggested by others, this species exhibits homochromy to match the body color to the substrate color of its habitat. Similar responses were observed in another strain, although some quantitative differences occurred between the two strains examined. Based on these results, a new model explaining the control of body-color polyphenism in this locust is proposed and the ecological significance of black patterns in solitarious nymphs is discussed. [Copyright &y& Elsevier]

Abstract: Solitarious female adults are known to produce smaller hatchlings than those produced by gregarious adults of the desert locust, Schistocerca gregaria. This study investigated developmental, morphological and reproductive responses to different qualities of food in hatchlings of different phases. Mortality was higher, the duration of nymphal development longer and adult body weight lighter with a low-quality food than a high-quality food. Gregarious hatchlings showed better survivorship, grew faster and became larger adults than did solitarious ones. The incidence of locusts exhibiting extra molting, which was typically observed in the solitarious phase, was dramatically increased when a low-quality food was given to the solitarious hatchlings. Low-quality food caused locusts to shift morphometric ratios toward the values typical of gregarious forms; smaller F/C (hind femur length/maximum head width) and larger E/F (elytra length/hind femur length). Solitarious hatchlings grown at either high- or low-quality foods and then given high-quality food after adult emergence revealed that food qualities during the nymphal stage influence their progeny quality and quantity via adult body size that influenced reproductive performance. Female adults showed an overshooting response to a shift from low- to high-quality food by increasing egg production that was specific to body size. This study may suggest that gregarious hatchlings are better adapted to adverse food conditions than solitarious counterparts and extra molting is induced even among gregarious hatchlings under poor food conditions. [Copyright &y& Elsevier]

Abstract: Maternal determination of progeny body size and coloration in the desert locust, Schistocerca gregaria, depends on the crowding conditions experienced during the short sensitive period that occurs two to six days before the deposition of the egg pod. Solitarious (isolated-reared) females produce relatively small eggs that yield solitarious green hatchlings but, females that are exposed to crowded conditions during the sensitive period, produce larger eggs that yield the dark-colored hatchlings characteristic of gregarious forms. The present study aimed to determine the stimuli influencing the maternal determination of progeny characteristics as well as the site at which such stimuli are perceived. By exposing isolated female adults to various combinations of visual, olfactory and tactile stimuli from a crowd of other adults, we found that no crowding effects could be elicited without tactile stimulation. Coating of various body surfaces with nail polish followed by exposure to crowding stimulation suggested that female adults perceive crowding stimuli with their antennae. This finding was supported by another experiment in which the antennae were either removed or covered with wax before the isolated females were exposed to crowded conditions. Neither serotonin nor an antagonist of its receptor affected the density-dependent maternal determination of progeny characteristics when injected into isolated or crowded female adults. [Copyright &y& Elsevier]

Abstract: Desert locust female adults respond to crowded conditions by changing progeny characteristics such as egg size, clutch size (no. of eggs per pod), hatchling body size and coloration. This study was conducted to determine the stage sensitive to crowding in this locust. Reproductively active females reared in isolation increased egg size and decreased clutch size and the proportion of green hatchlings after exposure to crowded conditions (in which each female was kept with four male adults). These changes depended not only on the timing of exposure to crowded conditions during the reproductive cycle but also on the length of the exposure. By varying the time and length of the exposure, it was found that crowding had no influence on progeny characteristics during the last two days of egg development at 31°C and that there was a four-day sensitive stage before this period. The sensitive stage coincided with the time when the affected oocytes were 1.5–4mm long, while the sensitivity to crowding appeared to be constant over the sensitive stage. The larger the magnitude of the increase in egg size after exposure to crowding, the smaller the proportion of green hatchlings (and the larger the proportion of gregarized dark hatchlings); there was a sigmoidal relationship between the two variables. Based on these results, we propose a model for determining the stage sensitive to crowding in both the female parent and the oocytes. [ABSTRACT FROM AUTHOR]

Abstract: Hatchlings of the desert locust, Schistocerca gregaria, display phase polyphenism in body coloration and size. This phenomenon has been found to be maternally controlled and two different mechanisms have been proposed to explain the underlying process. One mechanism claims that a water-soluble pheromonal factor secreted by gregarious female adults into the foam plugs of egg pods induces darkening in their progeny. The other mechanism states that hatchling body coloration is pre-determined in the ovaries and that no foam factor is involved in this phenomenon. This mechanism was supported by the observation that hatchling melanization was not prevented by the early washing or separation of eggs, which should have removed the pheromonal factor from the eggs and produced green hatchlings according to the other mechanism. This paper reviews the latest findings related to this phenomenon with special reference to reproductive cycles and genetic differences. The close relationships between egg size and the degree of melanization in hatchlings may provide strong evidence against the possible involvement of the pheromonal factor, because egg size is determined in the ovarioles. Furthermore, the absence of “solitarizing” effects of early washing and separation on hatchling melanization was also confirmed in different genetic strains. A hypothesis proposed by others that such effects occur only in eggs from the first reproductive cycle was tested and rejected. Based on these and other results, a model to explain the mechanisms underlying the maternal control of progeny characteristics and embryonic control of melanization in the hatchling was proposed. [Copyright &y& Elsevier]

The genetic and hormonal control of body colouration is investigated using two recessive genetic mutant strains, the reddish–brown (RB) mutant and an albino mutant, as well as a normal (pigmented) strain of the desert locust Schistocerca gregaria. The colour patterns of the RB nymphs are similar to those of a normal strain, although the intensity of the melanization is weaker in the former. Reciprocal crosses between the RB and albino mutants produce only normal phenotypes in the F1 generation. In the F2 generation, the normal, RB and albino phenotypes appear in a ratio of 9 : 3 : 4, indicating that two Mendelian units might determine the appearance of dark body colour and the intensity of melanization, respectively. In other words, at least two steps of regulation might be involved in the expression of body colour. Injections of [His7]-corazonin, a neuropeptide inducing dark colour in this locust, fail to induce dark colour in albino nymphs but show a dose-dependent darkening in RB nymphs in the range, 10 pmol to 1 nmol. Some RB nymphs become indistinguishable from normal individuals after injection of the peptide. Implantation of corpora cardiaca (CC) taken from RB mutants into other RB individuals induces darkening in the latter and CC from RB, albino and normal strains have similar dark colour-inducing activity when implanted into albino Locusta migratoria. These results suggest the possibility that the RB mutant gene regulates the intensity of melanization, possibly through controlling the pathway of pigment biosynthesis associated with [His7]-corazonin. [ABSTRACT FROM AUTHOR]

Abstract: The role of juvenile hormone (JH) in the maternal regulation of progeny characteristics was examined in the desert locust, Schistocerca gregaria. Female adults of this species are known to produce smaller but more eggs when reared in isolation than do those reared in a group. Eggs laid by isolated females develop green hatchlings typical of solitarious forms, whereas those laid by the latter produce black hatchlings typical of gregarious forms. Topical application of a juvenile hormone analog (JHA), fenoxycarb, or implantation of corpora allata (CA) taken from the migratory locust, Locusta migratoria, caused crowded S. gregaria females to deposit smaller eggs, but did not have a significant effect on the number of eggs per egg pod except at high doses of JHA. The production of smaller eggs by treated and untreated crowded females was closely associated with earlier deposition of the egg pods and shorter oviposition intervals. However, neither JHA application nor CA implantation influenced the progeny characteristics in actively reproducing aged females under crowded conditions, while untreated control females started producing smaller and more eggs upon transfer to isolated conditions. These results may suggest that JH is not directly involved in the maternal regulation of phase-dependent progeny characteristics. [Copyright &y& Elsevier]

Abstract: To understand the underlying trans-generational phase accumulation, a classical morphometric characteristic, the F/C ratio (F, hind femur length; C, maximum head width), of adult desert locusts (Schistocerca gregaria) was monitored over eight consecutive generations. Adult F/C ratios, which are larger in solitarious locusts than in gregarious ones, were negatively correlated to the darkness of body color at hatching. Two successive generations were required for a complete shift from the gregarious (crowd-reared) to the solitarious (isolated-reared) phase and vice versa in the laboratory. That is (1) female adults needed to be exposed to crowded (or isolated) conditions so that their hatchlings would become large (or small) and dark (or green) in color, and (2) the hatchlings then needed to be exposed to crowded (or isolated) conditions for their entire nymphal stage. Solitarious locusts exhibited extra molting that influenced the F/C ratio in the adult stage, but did not exert significant influences on the trans-generational changes in this trait because the incidence was low. The incidence of extra molting was negatively correlated with nymphal survival rates. The morphometric trans-generational changes may be explained without assuming any accumulating internal factor. [Copyright &y& Elsevier]

Abstract: The mechanism underlying the phase-dependent polyphenism in hatchling body coloration was studied by testing for a possible causal relationship with egg size in the desert locust, Schistocerca gregaria. Crowd-reared (gregarious) females typically produce large, black offspring, whereas females reared in isolation (solitarious) deposit small, green offspring. We first tested for possible genetic differences in the role of egg foam by washing or separating eggs from two strains of locust. No solitarizing effect was found in either of the strains tested, supporting a previous finding, using another laboratory strain, to show that the hatchling body coloration and size are pre-determined in the ovary of the mother and no egg foam factor is involved in the control of the hatchling body coloration. Topical application of fenoxycarb, a juvenile hormone analog (JHA), and implantation of extra corpora allata (CA), taken from Locusta migratoria, caused gregarious female adults of S. gregaria to produce small eggs. Some eggs laid by CA-implanted females produced green hatchlings. All large eggs chosen among those deposited by gregarious females produced black hatchlings. When eggs were either kept on dry filter paper at nearly saturated relative humidity during embryogenesis or pricked with a needle so that some egg yolk was squeezed out, some produced small, green hatchlings. These results suggested that the amount of egg yolk or the availability of yolk material may determine the body coloration of hatchlings. [Copyright &y& Elsevier]

Abstract: The effects of rearing density and maternal age on the progeny size, number and coloration of the desert locust, Schistocerca gregaria, were investigated. Isolated-reared females deposited smaller, but more eggs than crowd-reared females. The former produced smaller and more eggs with age, whereas the latter showed a tendency to produce larger and fewer eggs over time. A similar tendency was also observed with virgin females, indicating that mating or the presence of males was not important. The first egg pod produced by each mated crowd-reared female contained significantly smaller and more eggs than did the subsequent egg pods. The former often produced many green hatchlings (0–100%) characteristic of solitarious forms, whereas the egg pods deposited after the first pod produced predominantly black hatchlings typical of gregarious forms. Adults were highly sensitive to a shift in rearing density and quickly modified the quality and quantity of their progeny depending on the density encountered. The number of eggs per pod was influenced not only by the mother''s rearing density but also by rearing density of the grandmother. The present results demonstrated that the characteristics of progeny are influenced not only by the crowding conditions experienced by the mother and grandmother but also by the mother''s reproductive cycle. [Copyright &y& Elsevier]

Abstract: Hatchling body color and size of the desert locust, Schistocerca gregaria, are determined by the population density of the mothers during their reproductive period. Smaller green hatchlings are produced by adults at low population density (solitarious conditions) and larger dark hatchlings at high population density (gregarious conditions). One claim states that a pheromonal factor secreted by gregarious mothers into foam plugs of egg pods induces darkening in hatchlings. Previous research suggests that the foam factor can be removed by separating eggs individually within 1h of deposition, causing presumptive gregarious eggs to hatch without darkening. The present study re-examined this claim and possible factors that have been proposed which could account for the difference between our results and those reported earlier. Early separation was performed on eggs with a low mortality rate. The results showed that the egg separation did not increase the incidence of green hatchlings. Once chorionated in the ovary, eggs remained unchanged in size until the second day after oviposition in either isolated or crowded locusts. This and other results suggest that the phase-dependent differences in body size and color of hatchlings are established in the ovary and that modifications by the accessory gland factor either in the oviduct or after deposition are unlikely. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The influences of hatchling character and rearing density on body colour at the last-nymphal stadium are investigated for the desert locust, Schistocerca gregaria. Hatchlings are divided into five groups based on the darkness of the body colour and reared either under isolated or crowded conditions. Two types of body colour variation at the last-nymphal stadium are separately analysed (i.e. the background colour and black patterns). Under isolated conditions, the background body colour is either greenish or brownish. Most individuals are greenish and the highest percentage of brownish insects is obtained from hatchlings with the darkest body colour. Under crowded conditions, the background colour is yellow or orange and the percentage of yellowish nymphs tends to decrease when they are darker at hatching. The intensity of black patterns differs depending on the body colour at hatching and subsequent rearing density. Most isolated-reared nymphs exhibit few or no black patterns but nymphs with some black patterns also appear, particularly among those that had been dark at hatching. Under crowded conditions, the black patterns become more intense when they are darker at hatching. Therefore, last-stadium nymphs with typical solitarious or gregarious body colouration appear when they have the phase-specific body colouration at hatching as well. The present results demonstrate that both body colour at hatching and rearing density during nymphal development influence body colouration at the last-nymphal stadium. [ABSTRACT FROM AUTHOR]

Abstract: The mechanism controlling the body color of hatchlings was studied for the desert locust, Schistocerca gregaria. A pheromonal factor secreted by gregarious female adults into the foam plugs of egg pods has been suggested to cause darkening in their progeny. We re-examined the role of this maternal factor by washing or separating eggs at deposition. Eggs produced by crowd-reared female adults were washed with saline or separated individually without being washed immediately after deposition and the body color of the hatchlings from them was compared with that from the eggs unwashed and kept in the egg pod until hatching. Most hatchlings were dark and no significant difference was found in the proportions of dark- and light-colored hatchlings between the treatments and controls. Likewise, eggs separated before the foam plug deposition produced dark-colored hatchlings as in the un-separated controls. These results demonstrated that neither washing nor separation of eggs at deposition affected the hatchling body coloration. The variation in hatchling body color was correlated closely to the body weight at hatching, indicating that hatchling body color had been determined maternally. Green hatchlings reared under crowded conditions remained green until the second stadium at which black patterns were induced. It was concluded that body color at hatching has been determined maternally and crowding during the first nymphal stadium influences nymphal body color but its effect is not manifested until the second stadium. The present study casts doubts on the presence of a recently suggested pheromonal factor on the color of the hatchlings. [Copyright &y& Elsevier]

The effect of [His7]-corazonin on the abundance of antennal sensilla in the desert locust, Schistocerca gregaria, was investigated to test the hypothesis that injection of this neuropeptide would mimic a crowding effect. Solitarious locusts (reared in isolation) were injected with [His7]-corazonin at the 3rd nymphal instar and the numbers of sensilla on the 2nd, 8th and 14th antennal segments in the adult stage were compared with those for oil-injected solitarious controls or un-injected gregarious locusts (reared in group). The numbers of sensilla on these antennal segments were all reduced significantly after [His7]-corazonin injection compared with those for oil-injected controls, but similar to the values for gregarious individuals. Among the four major types of olfactory sensilla, coeloconic, trichoid, basiconic type A and basiconic type B, [His7]-corazonin injection influenced the abundance of all but the last type. The effect of [His7]-corazonin injection varied with the time of injection; the earlier the injection the larger the effects on the abundance of total antennal sensilla on the 8th segment, although the way in which the injection affected the abundance varied with the sensillum type. A hypothesis explaining how crowding affects the abundance of antennal sensilla and other phase-related characteristics through changes in [His7]-corazonin concentrations was proposed. [Copyright &y& Elsevier]