Effect of topical application of a juvenile hormone mimic on the duration of the moulting cycle in fifth instar Rhodnius

Fifth instar Rhodnius prolixus given a single topical application of synthetic juvenile hormone (JH) on one of the first 12 days after feeding showed a sigmoid relationship between day of treatment and duration of the moulting cycle. A similar relationship was observed between day of treatment and mean ‘juvenile score’ in subsequently emerging insects. The linear relationship between mean ‘juvenile score’ and duration of the moulting cycle suggests that the increased time from feeding to ecdysis for late-treated insects is a result of the morphogenetic effects of the JH treatment. Some of the partially juvenilized insects obtained by treatment after day 5 produced an odour characteristically detected only from adult insects. The possible significance of this observation is discussed.     

Effects of azadirachtin on the moulting cycle,endocrine system,and ovaries in last-instar larvae of the milkweed bug,Oncopeltus fasciatus

Azadirachtin, injected into newly moulted last-instar larvae of Oncopeltus fasciatus, induces a variety of effects, which are dose-dependent. It can be used as a tool for studying some processes of the moulting cycle and their endocrine control, as well as regulation of egg maturation, since azadirachtin-induced permanent larvae exhibit adult ovarian development.Low doses of azadirachtin merely prolong the intermoult stage, apparently due to a delayed ecdysteroid peak. Medium and high doses suppress adult ecdysis, and the larvae become permanent larvae, the longevity of which increases with rising doses. Although medium doses prevent ecdysis, apolysis and secretion of adult cuticle takes place. The ecdysteroid peak is further delayed in these larvae and is somewhat lower than in controls. Permanent larvae induced by high azadirachtin doses show neither ecdysis nor apolysis. However, the epidermis engages in secretory activity which may correspond to adult procuticle secretion. These larvae also show an ecdysteroid peak, which is considerably delayed and distinctly lower than in the controls. Thus, treatment with different azadirachtin doses allows some dissection of the moulting cycle into different steps, in which the hormonal regulation can be studied independently.Adult ovarian development begins in a number of female permanent larvae induced with high azadirachtin doses, in some cases leading to chorionated eggs. The corpora allata are enlarged in a number of permanent larvae. It is suggested that the last-larval ecdysteroid peak sets a clock for activation of the corpora allata, i.e. its gonadotropic function, regardless of whether the adult moult takes place or not.Neurosecretion appears to be affected by azadirachtin, however, the exact mode of action remains to be determined.     

Moulting and ecdysone release in response to electrical stimulation of protocerebral neurosecretory cells in Locusta migratoria

Locusta migratoria larvae were submitted to electrical stimulation of the protocerebral neurosecretory cells (median neurosecretory cells of the pars intercerebralis and lateral neurosecretory cells), during the last larval instar. The effects of the treatment were observed both on the duration of the stage and on the variations in haemolymph ecdysone levels. In untreated larvae, there was an initial ecdysone peak at the beginning of day 5, which was followed by 4 larger peaks between days 6 and 8. Stimulation of the median neurosecretory cells at the beginning of the instar resulted in the formation of one very large hormonal peak at the end of day 3: a day and a half earlier than in the control groups. Moulting was likewise accelerated. Stimulation also increased the size of the peaks, as compared with the controls. Stimulation of the lateral neurosecretory cells had a weaker ecdysiotropic effect; neither the number nor the size of the peaks were changed, though, like ecdysis, they occurred earlier. Stimulation of the deutocerebrum had no effect on either ecdysone titres or moulting. Electrical stimulation of the median neurosecretory cells at the end of day 5, that is after the occurrence of the first ecdysone peak, shortened the larval stage while having no significant effect on ecdysone levels in the haemolymph. The neuroendocrine control of ecdysis in Locusta is discussed.     

Effects of a juvenile hormone analogue on the duration of the fifth instar in the milkweed bug Oncopeltus fasciatus

Topical application of JHA to fifth instar nymphs of Oncopeltus fasciatus, immediately following ecdysis from the fourth instar, decreases the duration of the fifth instar by approximately 36 hr in addition to inducing a supernumerary larval moult. JHA appears to accelerate the time of subsequent ecdysis in two ways: first, the onset of ecdysone secretion is accelerated, and is accompanied by a similarly premature initiation of mitotic activity in epidermal cells. This is the classical prothoracicotropic action of JH. Second, the period between the onset of mitotic activity and the time of ecdysis itself is shortened. That is, once cellular activities associated with the moulting cycle are triggered by ecdysone, such activities are completed more rapidly in the presence of JHA. It appears that the larval-larval moult induced by JHA requires intrinsically less time to accomplish than a normal metamorphic moult.     

Implications of Chaoborus pupation and ecdysis in cold water

1. Cold water acted differently to delay and lengthen the pupation period for the larvae of two species of the zooplankton predator Chaoborus (Diptera: Chaoboridae). During Chaoborus pupation, the zooplankton community is released from predation, while the dark-coloured Chaoborus pupae are more susceptible to their own predators.
2. Fourth instar larvae of Chaoborus americanus and C. trivittatus , collected from an oligotrophic lake, were reared individually at 5 °C in the dark. Chaoborus americanus was also reared at 9 and 12 °C under spring photoperiod conditions (L : D, 16 : 8 h). Individuals were observed through pupation to emergence (ecdysis) or death.
3.  Chaoborus americanus pupated at 5, 9 and 12 °C with substantial emergence only at 12 °C. In comparison, C. trivittatus emerged at 5 °C. Light was not a necessary cue for pupation and ecdysis, contrary to previous reports. Cold water delayed the onset and lengthened and increased the variability of the duration of pupation.
4. In Shirley Lake, C. americanus pupated in late June–early July while C. trivittatus pupated first in April and again in June–July.
5.  Chaoborus americanus pupae needed a temperature cue to complete ecdysis. The ecdysis temperature threshold helps to explain the difference in pupation timing, and the geographical distribution, of C. americanus and its relatively inflexible life history contrasted with C. trivittatus . Delayed predator pupation in years with low spring temperature can affect the community dynamics of the prey.     

Juvenile hormone regulation of the larval diapause of the Southwestern corn borer,Diatraea grandiosella

The hormonal mechanism which controls the larval diapause of the southwestern corn borer was examined. The onset of this facultative mature larval diapause is marked by a transition from a spotted to an immaculate larval form, and during diapause individuals may undergo one or more stationary larval ecdyses. Experiments were designed to uncover the nature of the humoral mechanism regulating this diapause state. The finding that injecting diapause larvae with 20-hydroxyecdysone only brought about a stationary larval ecdysis suggests that diapause was not maintained by the lack of ecdysone. Neck ligations performed on larvae which had just entered diapause resulted in a premature termination of diapause, and larval-pupal ecdysis occurred in the thoraco-abdominal section, suggesting that a cephalic factor was necessary for the maintenance of diapause. This finding was further supported by the discovery that injecting 20-hydroxyecdysone into the thoraco-abdominal section of previously ligated diapause larvae also resulted in a premature termination of diapause and larval-pupal ecdysis, indicating that ecdysone only initiated the pupal moulting cycle when the cephalic factor was absent.Further experiments led to the conclusion that the juvenile hormone is the cephalic factor. Topical treatment with a juvenile hormone mimic caused non-diapause mature larvae to become immaculate and enter diapause. Periodical topical application of this mimic to diapause larvae prolonged diapause and increased the number of stationary larval ecdyses. These findings suggest that the initiation and maintenance of diapause are regulated by juvenile hormone titre. Results indicate that larvae retain a high titre of juvenile hormone until the last stages of diapause. Injection of 20-hydroxyecdysone into early or middiapause larvae only caused stationary larval ecdyses, while the same injection into larvae in the late stages of diapause caused some of them to pupate. Histological studies of the neurosecretory cells, corpus cardiacum-allatum complex, and prothoracic glands showed that the endocrine system was not inactive during diapause. A new hypothesis is therefore proposed which recognizes the existence of hormonal activity during larval diapause and emphasizes the principal regulatory rôle of juvenile hormone.     

Circadian clock and prothoracicotropic hormone secretion in relation to the larval-larval ecdysis rhythm of the saturniid Samia cynthia ricini

Observations on the timing of ecdysis and neck ligation experiments on larvae of Samia cynthia ricini under various light-dark conditions show that an endogenous circadian clock controls the timing of larval ecdysis and prothoracicotropic hormone (PTTH) secretion preceding it. The clock, upon reaching a specific phase point, causes the brain to secrete PTTH provided that the brain has acquired the secretory competence. This time may vary, in relation to a previous ecdysis, according to the light-dark conditions by which the clock phase is specifically determined, but is fixed relative to a subsequent ecdysis. Thus, in the case of the ecdysis to the 5th instar, PTTH is secreted [15+nτ] hr (τ: free-running period, slightly less than 24 hr) after the clock has started when the rhythm is free-running, and in the second and third nights of the 4th instar under a photoperiod of 12 hr light and 12 hr dark. Full secretion of ecdysone occurs 6 hr after PTTH secretion and ecdysis ensues 34 hr thereafter to complete the ultimate sequence of ecdysis.     

In vitro stimulation of cell death in the moulting glands of Oncopeltus fasciatus by 20-hydroxyecdysone

The moulting glands of the milkweed bug, Oncopeltus fasciatus, normally degenerate just before the time of ecdysis to an adult (day 7 of the fifth instar). Morphologically normal cell death can be prematurely stimulated in vitro by 20-hydroxyecdysone. Breakdown is triggered by a 24-hr period of exposure to 20-hydroxyecdysone, but an additional incubation period is required before clear signs of degeneration are manifested. Glands removed after the onset of endogenous ecdysteroid secretion degenerate in vitro in the absence of added hormones. Thus, in the moulting glands of Oncopeltus, ecdysteroids appear to act as an important trigger for metamorphic cell death.     

Evidence for the presence of a threshold weight for entering diapause in the yellow-spotted longicorn beetle, Psacothea hilaris

Under long-day conditions larvae of Psacothea hilaris (Coleoptera: Cerambycidae) pupate after the 4th or 5th instar, while under short-day conditions they undergo 2-4 nonstationary supernumerary molts and eventually enter diapause. To explore the possibility of a threshold weight for entering diapause, P. hilaris larvae were deprived of food on days 0 (day of ecdysis), 4 or 8 of the 4th, 5th and 6th instars under short-day conditions. Within the first 40 days of starvation, 60% of the larvae starved starting on day 0 of the 4th instar died, but all the larvae starved at later stages survived. The incidence of diapause in these survivors was determined by the occurrence of pupation after a temporary chilling at 15 degrees C for 15 days. Diapause incidence increased as the onset of starvation was delayed; from 11% in the larvae starved on day 0 of the 5th instar to 100% in the larvae starved on day 4 and day 8 of the 6th instar. Analysis of the relationship between the initial weight of a respective larva at the onset of starvation and its pupation success revealed that none of the larvae weighing 690 mg did. This finding suggests the presence of a threshold weight (about 600 mg), below which larvae are incapable of entering diapause. We discuss these findings with reference to the life history of P. hilaris.     

Larval-pupal transformation of the prothoracic glands of Mamestra brassicae

The sensitivity of the prothoracic glands to juvenile hormone and prothoracicotropic hormone (PTTH) of penultimate (5th)-instar larvae of Mamestra brassicae was compared with that of the same-instar larvae destined for pupal ecdysis by allatectomy. The activity of the prothoracic glands was assessed using either moulting of isolated abdomens or ecdysone radioimmunoassay. Juvenile hormone application immediately after neck-ligation (which removes brain-corpora cardiaca-corpora allata complex) prevented prothoracic gland function in larvae at all stages. When larvae were allatectomized 12 hr after ecdysis, followed by neck-ligation at different times and given juvenile hormone immediately, the hormone inhibited the prothoracic glands of young larvae, but activated the prothoracic glands from day-5 or older larvae. Juvenile hormone I, juvenile hormone II and methoprene activated the prothoracic glands, but juvenile hormone III was relatively ineffective. Brain implantation instead of juvenile hormone application led to activation of the prothoracic glands at all stages.Allatectomy thus caused changes leading to metamorphosis including a transformation of the prothoracic glands from ‘larval’ to ‘pupal’ type. After this change these prothoracic glands were able to respond not only to PTTH but also to juvenile hormone just as in last-instar larvae.     

Daily rhythms of clock gene expression and feeding behavior during the larval development in gilthead seabream,Sparus aurata

Light is the main environmental time cue which synchronizes daily rhythms and the molecular clock of vertebrates. Indeed, alterations in photoperiod have profound physiological effects in fish (e.g. reproduction and early development). In order to identify the changes in clock genes expression in gilthead seabream larvae during ontogeny, three different photoperiods were tested: a regular 12L:12D cycle (LD), a continuous light 24L:0D (LL) and a two-phases photoperiod (LL?+?LD) in which the photoperiod changed from LL to LD on day 15 after hatching (dph). Larvae were sampled on 10, 18, 30 and 60 days post-hatch (dph) during a 24?h cycle. In addition to the expression of clock genes (clock, bmal1, cry1 and per3), food intake was measured. Under LD photoperiod, larvae feed intake and clock genes expression showed a rhythmic pattern with a strong light synchronization, with the acrophases occurring at the same hour in all tested ages. Under LL photoperiod, the larvae also showed a rhythmic pattern but the acrophases occurred at different times depending on the age, although at the end of the experiment (60 dph) clock genes expression and feed intake rhythms were similar to those larvae exposed to LD photoperiod. Moreover, the expression levels of bmal1 and cry1 were much lower than in LD photoperiod. Under the LL?+?LD photoperiod, the 10 dph larvae showed the same patterns as LL treatment while 18 and 30 dph larvae showed the same patterns as LD treatment. These results revealed the presence of internal factors driving rhythmic physiological responses during larvae development under constant environmental conditions. The LL?+?LD treatment demonstrates the plasticity of the clock genes expression and the strong effect of light as synchronizer in developing fish larvae.     

Effects of ingested phytoecdysteroids on the growth and development of two Lepidopterous larvae

The effects of ingested or injected 20-hydroxyecdysone on silkworm larvae (Bombyx mori) including death without moulting, death following completion of promoted moulting, death during promoted moulting (ecdysis inhibition) and inhibition in growth with and without effects on moulting, are dependent upon the concentration of exogenous hormone, the precise developmental stage of the treated larvae, and the duration of exposure to the exogenous ecdysteroid. Comparisons of 20-hydroxyecdysone with other phytoecdysteroids in the silkworm and pink bollworm, Pectinophora gossypiella, show a similar but more potent effect induced by ponasterone A, while cyasterone causes an ‘antiecdysone’ effect.     

Lipid metabolism of the female crab Pachygrapsus marmoratus during the moulting cycle

The moult induces important variations in the concentration and fatty acids composition of lipid classes during the moulting cycle of the female crab Pachygrapsus marmoratus regardless of the maturational degree of ovaries. Sexual maturity is characterized by a rise in lipids. Juveniles contain high levels of polyunsaturated fatty acids except in the ovary, whereas adults are composed mainly of monoethylenic acids. The moulting decrease of saturated fatty acids shows their importance during this crucial period, owing to their utilization as energy sources at the time of ecdysis. The late premoult fall of hepatopancreatic lipids results at once from inanition during the period before the exuviation and transfer of lipids through the hemolymph to the periphery in order to realize numerous changes occurring during the moulting process. The relative stability of lipid composition of ovaries in sexual pause during the moulting cycle agrees with a reduced metabolism of ovarian cells.     

The crucial role of cyclic GMP in the eclosion hormone mediated signal transduction in the silkworm metamorphoses.

The signal transduction of the peptide, eclosion hormone, in the silkworm Bombyx mori appears to be mediated via the second messenger cyclic GMP throughout their life cycle. Injection of 8-bromo-cGMP induced the ecdysis behavior in pharate adults with similar latency to eclosion hormone-induced ecdysis; the moulting occurred 50-70 min after the injection. The potency of 8Br-cGMP was 10(2) fold higher than that of cGMP and the efficacy was increased by the co-injection of the phosphodiesterase inhibitor IBMX. On the other hand, in the silkworm pupal ecdysis the eclosion hormone and also 8Br-cGMP induced the moulting behavior in a dose-dependent manner. The adult development of the ability to respond to 8Br-cGMP took place concomitantly with the response to the eclosion hormone. Both the developmental time courses were shifted by a shift of light and dark cycles. Accordingly, the sensitivities to the peptide and cyclic nucleotide developed correspondently under the light and dark circadian rhythm. Thus throughout the silkworm life cycle, eclosion hormone is effective to trigger the ecdysis behavior and cGMP plays a crucial role as the second messenger in the eclosion hormone-mediated signal transduction.     

Action de l'ecdystérone sur l'apolysis et l'ecdysis de divers crustacés isopodes

Ecdysterone acts differently according to the period of the moulting cycle when it is applied. In period C it induces apolysis, while at the beginning of apolysis it delays ecdysis. Experiments show that the exoskeleton cannot be rejected in the presence of ecdysterone. The possibility of stopping exuviation or ecdysis with ecdysterone seems to prove that this phenomenon is controlled by a particular factor called the factor of exuviation or the ecdysis factor. Ecdysterone would control only the release of this specific factor but would not prevent its action.     

白蛾周氏啮小蜂滞育诱导及滞育后发育

本研究针对人工繁育白蛾周氏啮小蜂Chouioia cunea Yang过程中出现的小蜂滞育现象, 对其滞育诱导的光周期反应及敏感光照虫态进行了调查。结果表明: 沈阳地区的白蛾周氏啮小蜂属长日照型昆虫, 以老熟幼虫进入滞育状态, 但在不同的温度条件下诱导滞育的临界光周期不同, 在18℃时诱导滞育的临界光周期处于13L∶11D和14L∶10D之间; 在21℃和24℃时诱导滞育的临界光周期变短, 处于12L∶12D和13L∶11D之间。白蛾周氏啮小蜂滞育诱导的敏感光照虫态为幼虫期, 且以幼虫的后期最为敏感, 但整个幼虫期接受短光照对滞育的形成更为有利。通过观察白蛾周氏啮小蜂滞育后在18℃, 21℃, 24℃和30℃的恒温条件下的发育历期, 由最小二乘法计算出白蛾周氏啮小蜂老熟幼虫滞育后发育起点温度和有效积温分别为14.60±0.31℃和209.38±8.72日·度。这些结果可为进一步研究白蛾周氏啮小蜂的种蜂长期保存技术和指导商品蜂生产, 正确把握放蜂时机提供理论依据。     

Stimulation of metamorphosis in an estuarine crab, Chasmagnathus granulata (Dana, 1851): temporal window of cue receptivity

The larvae of most benthic marine invertebrate species must develop for a minimum of time in the plankton before they become competent for settlement and metamorphosis in response to stimulating external cues. In an experimental laboratory study, we identified the temporal window of cue receptivity within the moulting cycle of the megalopa stage of an estuarine crab, Chasmagnathus granulata Dana. This species shows an export strategy including an early larval transport to coastal marine waters where zoeal development takes place, followed by the return of the megalopa stage to brackish habitats where the adults live. In two series of experiments (A, B), megalopae were exposed for differential periods to a combination of metamorphosis-stimulating cues which had previously been found effective (seawater conditioned with adult odor and presence of mud). In experimental series A, these cues were added on successively later days of the moulting cycle, while series B comprised treatments in which the cues were provided from the first day (postmoult) and removed on successively later days of the moulting cycle. Each series of experiments was repeated with larvae originating from two different females (F1, F2). The average development time of megalopae kept continuously in the presence of cues (control experiments, C1) ranged in the two hatches from 9.3 to 9.6 days. In the inverse controls where no cue was added at any time (C2), megalopal development to metamorphosis took on average 11.2-12.0 days. In series A, development duration in treatments with exposure to the cues commencing within 3-4 days after moulting was not significantly different from that in the permanently exposed C1 controls. Later beginning of the exposure, by contrast, had no stimulating effect (significant delay compared to C1, no significant difference from unexposed control, C2). In series B, no significant differences in development time were observed between the C1 controls and treatments with an initial exposure for a minimum of 4 or 6 days of the moulting cycle (F1, F2, respectively). Shorter initial periods of exposure had no metamorphosis-stimulating effects (no significant difference from C2). In conclusion, our results from both experiments suggest that the megalopa stage of C. granulata is most receptive of stimulating cues during a period lasting from ca. one third to one half of the moulting cycle, which coincides with the transition between stages C (intermoult) and D₀ (early premoult) of Drach's classification system. This suggests an interaction of extrinsic stimulating cues with intrinsic (hormonal) factors involved in the control of the moulting cycle.     

Responses to stepwise photoperiodic changes for the larval diapause of the Indian meal moth Plodia interpunctella

Abstract The Indian meal moth Plodia interpunctella Hübner (Lepidoptera: Pyralidae) diapauses as a last‐instar (fifth) larva. At 30 °C, no larvae enter diapause under any photoperiodic conditions; at 25 °C, the photoperiodic response curve is a long‐day type with a critical length of approximately 13 h light; at 20 °C, diapause is induced moderately even under long days (> 13 h). Cumulative effects of short days or long days on diapause induction are determined by alternate, stepwise and gradually changing regimes of photoperiod at 25 °C. When the larvae are repeatedly exposed to LD 16 : 8 h and LD 12 : 12 h photoperiods every other day, the incidence of diapause is 37%. When the larvae are placed under an LD 16 : 8 h photoperiod for 2 days and then under an LD 12 : 12 h photoperiod for 1 day, it is 38 %. Exposure to an LD 16 : 8 h photoperiod for 1 day and then to an LD 12 : 12 h photoperiod for 2 days induces only 15% diapause. This may indicate that the photoperiodic information is not accumulated in a simple fashion despite the generally accepted hypothesis (i.e. photoperiodic counter). Larvae exposed to an LD 16 : 8 h photoperiod for 5 days after oviposition express a very high incidence of diapause even under short days between an LD 2 : 22 h and LD 12 : 12 h photoperiod. After 10 days exposure to an LD 16 : 8 h photoperiod, however, the short day does not induce diapause strongly. On the other hand, an LD 12 : 12 h photoperiod in the early larval life is highly effective in the induction of diapause. A gradual increase or decrease of photoperiod (2 min day^?1) shows that the direction of photoperiodic change does not affect the diapause determination.     

Evidence of hormonal control of the nucleolar activity in Sciara ocellaris.

The nucleolus organizing region in the salivary gland of Sciara ocellaris becomes gradually less active as the larvae approach pupation. The cytological visible condensation of the nucleolar puff is paralleled by a decrease in the incorporation of [3H]-uridine into the ribosomal RNA. Alterations in the nucleolar behavior were brought about by extirpation of the neuroendocrine glands at six stages of development. There is a 'critical stage' during the 7th day of the 4th instar when the hormonal stimulus for pupation occurs. If the ligature is performed before this stage the NOR remains expanded and the larval-pupal ecdysis is inhibited; if done later on the NOR condenses at a normal rate and the ligated larvae mount to head-less pupae. These results suggest that inhibition of rRNA synthesis and consequent condensation of the NOR is under the control of the moulting hormone.