The pattern of ecdysone levels during development in the desert locust, Schistocerca gregaria

A simple chemical assay, sensitive to picogram amounts (10^?12 g) of ecdysones in crude biological materials is described. By a simple, rapid purification procedure followed by conversion to their trimethylsilyl ethers, ecdysones are determined quantitatively by gas chromatography with an electron capture detector. The method has been applied to the determination of ecdysones in the fourth and fifth instars of the desert locust. 20-Hydroxyecdysone is always present in higher concentration than ecdysone and the former rises to a sharp maximum 1 to 2 days before ecdysis.     

Ecdysteroids in the developing eggs of the desert locust, Schistocerca gregaria

Ecdysteroid levels in the developing eggs of Schistocerca gregaria were determined, at daily intervals, using gas chromatography and electron capture detection of ecdysteroid derivatives. Ecdysone and 20-hydroxyecdysone were present both as the free ecdysteroid and as polar conjugates. Total ecdysteroids reached a maximum of 40 ng/egg with ecdysone contributing the greater part.     

Studies on the moulting hormones of the desert locust, Schistocerca gregaria

A chemical investigation of the desert locust (Schistocerca gregaria) using an isolated locust abdomen assay has led to the identification of 20-hydroxyecdysone as one of the hormones controlling moulting, but the evidence presented does not favour the prothoracic gland (PTG) as the site of its production. Preliminary information indicates two other active substances are present in higher concentration in the PTG which affect apolysis. Determination of 20-hydroxyecdysone titre at daily intervals in the fifth instar and adult show highest concentrations on the day of ecdysis. Ecdysone was not detected. Histological examination of cuticle suggests that PTG extracts cause growth in epidermal cells, rather than increased cell division.     

Pheromonal effects on sexual maturation,yellowing, and the vibration reaction in immature male desert locusts (Schistocerca gregaria)

Both yellow (crowded) and non-yellow (isolated) mature males of Schistocerca gregaria are equally capable of accelerating the sexual maturation and inducing integumental yellowing in immature adult males. The bioassay of a mature male extract shows conclusively the presence of the maturation-accelerating pheromone in this species. The pheromone does not appear to be effective in inducing yellowing, or in initiating the vibration reaction, in immature males.     

A re-investigation on the ecdysteroids during embryogenesis in the desert locust, Schistocerca gregaria

Comparison of gas chromatography and high-pressure liquid chromatography methods for determination of ecdysteroids in insect eggs revealed that published values for Schistocerca gregaria were very low. The error has been traced to incomplete hydrolysis of conjugates. Revised values for the levels of ecdysone, 20-hydroxy-ecdysone and 2-deoxy-ecdysone are given here. The same general pattern of falling ecdysteroid titre during early stages of embryo development followed by an increase and a final decrease is observed. Experiments have been made on the effect of phosphate, ionic strength and a specific β-glucuronidase competitive inhibitor on the hydrolysis of the polar ecdysteroid conjugates, in an attempt to provide more evidence on the chemical nature of these compounds.     

Artificial miniaturization causes eggs laid by crowd-reared (gregarious) desert locusts to produce green (solitarious) offspring in the desert locust, Schistocerca gregaria

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.     

Synthesis and release of hydrocarbons by the oenocytes of the desert locust, Schistocerca gregaria

When incubated in vitro, the oenocytes in the peripheral fat body of the desert locust incorporate Na-¹⁴C-acetate into hydrocarbons (paraffins). The presence of haemolymph in the incubation medium greatly stimulates the release of the ¹⁴C-hydrocarbons into the medium. The labelled hydrocarbons appear to be rapidly released by the cells into the incubation medium as a function of time provided that haemolymph is present. The fact that the oenocytes not only synthesize ¹⁴C-hydrocarbons but also release them into the medium supports the hypothesis that the oenocytes of the desert locust synthesize cuticle lipids.     

The embryonic apolyses of Schistocerca gregaria (Orthoptera)

The origin and differentiation of the ecdysial glands and the appearance of the cuticles were studied in embryos of Schistocerca gregaria at corresponding stages. The development of the cuticle was also studied in headless embryos explanted in vitro.     

Fluid secretion by in vitro preparations of the Malpighian tubules of the desert locust Schistocerca gregaria

In vitro preparations of locust Malpighian tubules can conveniently be made by a new technique in which the alimentary canal to which the tubules attach is removed from the insect and set up in Ringer's solution under liquid paraffin. Such Malpighian tubules will secrete a fluid iso-osmotic to the bathing fluid at a steady rate of about 1 to 2 nl min^?1 for some hours. The secreted fluid is rich in potassium ions, the lumen is at a potential positive to that of the bathing solution, and the rate of secretion can be controlled by changing the potassium concentration of the bathing fluid. It seems likely, therefore, that an active transport of potassium drives secretion ny locust Malpighian tubules. The secreted fluid contains an elevated concentration of phosphate ions. The Malpighian tubules will secrete at a high rate in a chloride-free phosphate-based solution. The rate of fluid secretion can be increased by treatment with cyclic AMP but 5-hydroxytryptamine has no such effect.     

Allatectomy and sexual receptivity in females of Schistocerca gregaria

The removal of the corpora allata from immature females of Schistocerca gregaria causes the insects to enter a phase of permanent defence towards courting males. Operated control females copulate often, although they do not appear to show an active form of sexual display or attraction. Implanted corpora allata fail to restore sexual receptivity or normal ovarial development in the allatectomized females due to the inactivity of the denervated glands. The observations are discussed in relation to findings in other locusts, grasshoppers, and crickets.     

Courtship inhibition pheromone in desert locusts,Schistocerca gregaria

Male desert locusts in the gregarious phase release phenylacetonitrile (PAN) on becoming sexually mature. It has been assumed that this chemical is responsible for aggregation of adult desert locusts. However, PAN has repellent characteristics and is involved in sexual behavior. Mature males release PAN as a volatile to serve as a kind of olfactory concealment during mating and to prevent competing males from homosexual encounters. We conclude that PAN is a courtship-inhibiting pheromone exclusively used under crowded conditions in dense populations when high sperm competition occurs among desert locust males. By chemically enhancing their mate guarding, gregarious males improve the protection of their mate from rivals and ensure their reproductive success.     

Componental analysis of the ocellar electroretinogram of the locust, Schistocerca gregaria

Ruck's componental analysis of the ocellar electroretinogram (ERG) has been reappraised using techniques of signal averaging and waveform subtraction. Components (1), (3), and (4) can readily be isolated in the locust ocellus but component (2) as recorded in the locust ocellus is probably an artefact. Component (1), produced by the receptor cells, only contributes significantly to the total ERG at higher light intensities and it is this contribution which changes most with the degree of light and dark adaptation employed in these experiments. Component (3), the response of the second-order neurones, indicates that the majority of second-order neurones hyperpolarize on illumination of the ocellus. Component (4), the afferent activity of the second-order cells, indicates that more than one afferent axon is involved in the production of off spikes in the locust ocellus.     

Histopathology of secondary infections of Malpighamoeba locustae (Protozoa,Amoebidae) in the desert locust, Schistocerca gregaria (Orthoptera,Acrididae)

Schistocerca gregaria, the desert locust, harbors the protozoan parasite Malpighamoeba locustae. Twelve to 16 days after infection, trophozoites begin to multiply rapidly inside the Malpighian tubules of the gut. Swelling and rupture of the Malpighian tubules leads to the release of large numbers of cysts and trophozoites into the hemocoel. The classic insect defense response results in these cysts and trophozoites becoming encapsulated by the hemocytes of the host. Hemocytes of the phagocytic type become attached to, and lodged between, a variety of tissues and organs of the locust and black hemocytic capsules are produced. The extent to which the different tissues are involved is graded. Some tissues are completely blackened and encapsulated by masses of hemocytes but others are so lightly affected that the small specks of blackened pigment they lay down are discernible only on close examination. The trophozoites themselves do not divide outside the Malpighian tubules. The graded response in the host tissues is related to (1) the presence of sheets and lobes of fat body and (2) the presence of phagocytic hemocytes.     

Control of melanization in first instar larvae of Schistocerca gregaria

Melanization in first-instar larvae of Schistocerca is controlled by a hormone released from neurosecretory axon terminals of the fine nerves posterior to the metathoracic ganglion. The hormone is not detectable in the haemolymph before the embryonic ecdysis but is present within seconds after the ecdysis has started. It is suggested that horizontal displacement of the embryonic cuticle is the trigger for the release of the hormone and that the prothoracic ganglion forms part of the neural pathway between the sensory input caused by ecdysis and the release of the hormone.     

The Comstock-Kellogg glands in female Schistocerca gregaria: Their eversion and haemocoelic pressure

The glandular pouches of Comstock and Kellogg on the genital chamber of females of Schistocerca gregaria can be everted and displayed by exerting firm pressure on the tip of the abdomen. The glands evert when the ovipositor values are extended and the gland apertures pass the posterior border of the sub-genital plate. This process has not been observed to occur spontaneously in females in this colony. Increasing the pressure in the haemocoel by means of a saline reservoir is effective in causing gland eversion only when pressures in excess of 75 mm mercury are used: these pressures cause abnormal distortion in the insect. Haemolymph pressure monitored in digging and egg-laying females varies only slightly from atmospheric pressure and never approaches the pressures required to enforce gland eversion. Gland eversion is not observed in digging females, but is difficult to observe in females in the act of laying, due to the presence of egg-pod secretion.These observations are discussed in relation to the structure of the pouches and other information concerning their possible functions in the female desert locust.     

The need for high levels of phenylalanine in the diet of Schistocerca gregaria nymphs

It has been demonstrated that an apparently good, nutrient-rich plant can be relatively short of phenylalanine for fifth-instar nymphs of Schistocerca gregaria. Supplementary phenylalanine invariably increased the efficiency of conversion of ingested food to body substance, showing that this was the limiting nutrient. Relatively large quantities of phenylalanine are required for cuticle protein over a few days in the first half of the instar, which is when this amino acid is at a premium.     

Comparison between the sclerotization of adult and larval cuticle in Schistocerca gregaria

Femur cuticle from fifth instar larvae of the desert locust, Schistocerca gregaria, has been characterized with respect to composition, rate of deposition, and rate of sclerotization. The results are compared with those from adult cuticle of the same species. The protein compositions of the two types of cuticle are very similar, but the rates of deposition of both protein and chitin are different. The main difference is, however, that sclerotization is restricted to the first day after ecdysis in larval cuticle, whereas in adult cuticle sclerotization continues for at least a couple of weeks. The result is that the endocuticle remains untanned in the larvae, whereas in the adults the whole cuticle becomes tanned.     

Juvenile hormone activity in larvae and adult females of the locust, Schistocerca gregaria

The isolation and purification of fractions with juvenile hormone activity from whole animal extracts of larvae, and from extracts of haemolymph from larvae and adults is described. Using the Galleria bioassay three such fractions were isolated from both third and fourth instar hoppers and from adult females. The chromatographic behaviour of these fractions indicates that one contains JH III, but the other two contain unknown juvenilizing compounds.     

Novel cross-stage solitarising effect of gregarious-phase adult desert locust (Schistocerca gregaria (Forskål)) pheromone on hoppers

Previous studies had demonstrated stage differentiation in the cohesion (aggregation) pheromone systems of the desert locust, Schistocerca gregaria. In laboratory arena, the nymphal and adult stages responded aggregatively to their own pheromone, but dispersed evenly within the arena in the presence of the other. In the present study, we explored the effects of longer-term contact of field gregarious hopper bands and laboratory crowd-reared nymphs with the major constituent of the adult pheromone. During the first few days, hoppers in treated bands became relatively hyperactive. Over the next few days, their movements became random and they stopped marching as coherent groups, they started to roost for longer periods on vegetations, and they fragmented into smaller and smaller groupings and individuals. When attacked by birds, they demonstrated subdued levels of collective defensive behaviour compared to normal hoppers, and there were clear signs of increased predation and cannibalism at the roosting sites. In cage experiments, crowd-reared nymphs treated with the pheromone component became hyperactive, showed abnormal diel patterns and reduced feeding on plants but increased cannibalism. Our observations show that the major adult pheromone constituent has a solitarising effect on gregarious hoppers. The mechanism underlying this effect and the potential of the agent in desert locust control are discussed.