An overview is given of photoperiodic research performed with English and Scottish clones of the vetch aphid, Megoura vicine, and the black bean aphid, Aphis fabae, during the last decade, with emphasis on the photoperiodic photoreceptor and the clock-counter mechanisms.Photoperiodic photoreceptor. The photoperiodic photoreceptors of the vetch aphid, M. viciae, are located in the brain. Immunocy tochemical techniques have recently indicated that an anterior dorsal region of the protocerebrum possesses antigenic sites that are consistently labelled with a number of antibodies raised to invertebrate and vertebrate opsins and phototransduction proteins.Counter. In two clones of both M. viciae and A. fabae, long-night accumulation appears to be temperature compensated, whereas short-night accumulation is temperature sensitive.Clock. (1) The clocks of the English and Scottish clones of M. viciae can be modelled by a slowly damping long-night (LN) system and a rapidly damping short-night (SN) system. The LN system of the Scottish clone damps more slowly and has a shorter period than that of the English clone. The critical night length (CNL) of the English clone is highly temperature compensated, that of the Scottish clone is less so. (2) The clock;in presumptive: gynoparae of an English clone of A. fabae can be modelled by rapidly damping LN and SN systems; the SN system's damping rate being temperature dependent, but that of the LN system being temperature compensated. (3) The clocks in gynopara and male producers of the English clone of A. fabae are mimicked by rapidly damping LN systems and self-sustained SN systems, while in a Scottish clone they are assumed to consist of slowly damping LN systems and self-sustained SN systems. (4) The CNLs for gynopara and male production are temperature compensated in both the English and the Scottish clones of A. fabae. (C) 2001 Elsevier Science Ltd. All rights reserved.

The honeybee is used as a model system to study memory stages at the behavioral, neural, and cellular levels. Five memory stages are distinguished: early and late short-term memory (e and lSTM), mid-term (e memory (MTM), early and late long-term memory and lLTM). Memory consolidation converts eSTM to lSTM by a time-dependent process after a single learning trial. This consolidation process leads to stable memory with different contents and different distributions in the brain. At the neural level, various forms of plasticity are found which reflect the differences between e and lSTM, and at the cellular level eSTM correlates with the transient enhancement of the cAMP/PKA pathway. The transition to longer lasting memory requires multiple learning trials. These lead to memories which incorporate contents accessible only by repetitive learning, e.g., massed/spaced effects, context dependence, rule learning. Enhanced and prolonged activation of the cAMP/PKA pathway is required for LTM formation, and two different forms of consecutively active PKC are a characteristic for MTM and eLTM. The transition from e and lLTM requires protein synthesis. The dynamics of memory stages is interpreted as an adaptation to sequential events during natural foraging.

SummaryPhotoperiodic response curves were determined for two clones of the black bean aphid, Aphis fabae Scopoli, at three temperatures, 12.5, 15 and 17.5°C. Critical night lengths for the induction of winged females in an English clone (52° N) were 10.5, 11 and 11.5 h, respectively, and 10, 10.5 and 11 h in a Scottish clone (57° N). Critical night lengths for male induction were 10.5, 11 and 11 h at 12.5, 15 and 17.5°C in the English clone, and 10, 10.5 and 10.5 h, respectively, in the Scottish clone. High incidences of winged females and males were observed at all scotophases longer than the critical night length in both clones. In addition, in the English clone, the incidences of winged female and male producers in continuous darkness were 0% at 15 and 17.5°C, and 6% at 12.5°C. In the Scottish clone, however, continuous darkness resulted in high incidences of both winged female and male producers at 12.5 and 15°C, but 0% winged female producers and 6% male producers at 17.5°C. In scotophases shorter than the critical night length, including continuous light, no males or winged females were observed in either clone under the non‐crowded rearing conditions used. The results are discussed in terms of the ‘double circadian oscillator model’ for photoperiodic induction.

Photoperiodic response curves were determined for a Scottish clone of the vetch aphid, Megoura viciae Buckton, at three temperatures: 12.5, 15, and 17.5 degrees C. Critical night lengths (CNLs) for ovipara (sexual female) induction were 6 h, 7 h and 8 h, respectively. High incidences of ovipara production were observed in all night lengths longer than the CNL including continuous darkness (DD), as well as in continuous light (LL) at 12.5 and 15 degrees C. At the same three temperatures, the number of long- or short-night cycles required for half of the experimental aphids to be ovipara producers (i.e. the required day number, RDN) was determined. The RDN for long-night cycles (LD12:12) could not be determined at 12.5 degrees C, but was temperature compensated between 15 and 17.5 degrees C. The RDN for short-night cycles (LD20:4) could not be determined at any temperature. However, as induction of oviparae was always 100% in 12.5 degrees C, 94-100% in 15 degrees C and dropped from 100% to between 47 and 71% in 17.5 degrees C, it seems that short-night accumulation was temperature dependent. When fourth-stadium larvae were transferred from LD20:4 at 20 degrees C to the same light-dark cycle at 15 degrees C, the aphids, when adult, switched to the production of oviparae after about 4 weeks. First-born progeny kept in LD20:4 and 15 degrees C switched to the production of oviparae about 7 days after the moult to adult. Thus, the photoperiodic response can be directly affected by temperature, irrespective of photoperiod. Model-generated response curves using the 'double circadian oscillator model' for photoperiodic time measurement (Vaz Nunes, M., 1998. A double circadian oscillator model for quantitative photoperiodic time measurement in insects and mites. Journal of Theoretical Biology 194, 299-311) closely resembled the observations. Differences between these data and the results of previous experiments with an English clone of M. viciae could be accounted for by differences in the photoperiodic clocks (damping rate and period) as well as the photoperiodic counters.

The "double circadian oscillator model" for the photoperiodic clock has been used to simulate thermoperiodic responses in insects and mites. Two assumptions have been made: (1) the clock measures cryophase in a similar way to scotophase, and (2) temperature cycles are able to entrain the clock in a similar way to LD cycles. Simulations showed that Assumption 1 causes the "critical cryophase" to be of about equal duration as the critical night length. Assumption 2 is not always needed if diapause incidences in DD are high at low temperatures but low or zero at high temperatures. The latter assumption is needed, however, if high diapause occurs in thermoperiodic cycles in DD, whereas nondiapause occurs in DD with both high and low constant temperatures. The model accounts for the observation that the amplitude of the temperature cycle is important in some insects, whereas the temperature of the cryophase is crucial in others.

From studies on photoperiodic time measurement in insects it is known that different night lengths at the same side of the critical night length can have different inductive strengths. This means that nights of different length, and either longer or shorter than the critical night length, can have qualitatively different values. Nevertheless, few photoperiodic-clock models have been developed that are based on quantitative night-length measurement. In this paper a model is proposed that consists of two independent, circadian mechanisms. Both mechanisms determine the length of a night and give it a quantitative value, which is either zero or positive. One of the mechanisms (LN system) generally gives a scotophase a positive value when it is "long" (i.e. lights-on occurs when the LN oscillator is in its descending phase), whereas the other (SN system) gives a scotophase a positive value when it is "short" (i.e. lights-on occurs when the SN oscillator is in its ascending phase). In this particular context, therefore, "long" and "short" do not necessarily mean longer or shorter than the critical night length. The reasons for two time-measuring systems instead of one are: first, in some insects only long nights are accumulated, not short nights, or vice versa. Second, long nights are less sensitive to temperature than short nights. Third, in some cases it seems that long and short nights are determined in a different manner. These observations indicate that long and short nights could be determined by separate mechanisms. Responses generated by the proposed model parallel those observed experimentally with the spider mite, Tetranchus urticae and the aphid, Megoura viciae. General properties of the model are discussed and compared with Zaslavski's quantitative clock model which shares some features. Copyright 1998 Academic Press Limited

Entomologia Experimentalis et ApplicataVolume 84, Issue 2 p. 195-197 'Bistability' experiments and the photoperiodic clock in the spider mite Tetranychus urticae Marlies Vaz Nunes, Marlies Vaz Nunes Aphid Biology Group, Department of Biology, Imperial College at Silwood Park, Ascot, Berks. SL5 7PY, UKSearch for more papers by this authorAlfred Veerman, Corresponding Author Alfred Veerman Institute for Systematics and Population Biology, Section Population Biology, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The NetherlandsSearch for more papers by this author Marlies Vaz Nunes, Marlies Vaz Nunes Aphid Biology Group, Department of Biology, Imperial College at Silwood Park, Ascot, Berks. SL5 7PY, UKSearch for more papers by this authorAlfred Veerman, Corresponding Author Alfred Veerman Institute for Systematics and Population Biology, Section Population Biology, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The NetherlandsSearch for more papers by this author First published: 20 October 2003 https://doi.org/10.1046/j.1570-7458.1997.00215.xCitations: 4 Author for correspondence AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume84, Issue2August 1997Pages 195-197 RelatedInformation

Winged and wingless individuals of a pink clone of the pea aphid, Acyrthosiphon pisum (Harris), showed differences in the response curves for photoperiodic induction of both males and sexual females (oviparae). The critical night length (CNL) for ovipara induction in winged aphids was 0.75 h shorter than in wingless aphids, whereas the CNL for male induction in winged aphids was 1.0 h longer than in wingless aphids. This means that in winged aphids the CNL for male induction in winged aphids was 0.5 h longer than that for ovipara induction, while in wingless aphids the CNL for male induction was 1.0-1.5 h shorter than that for ovipara induction, and also the shapes of the curves differed.Winged aphids were produced by wingless mothers which were crowded as young adults. However, when young adults were crowded in long nights, winged aphids were not produced, and the CNL for wing inhibition was between 9.5 and 10 h. This effect of photoperiod on wing induction was maternal.

Abstract. Day‐length changes, as well as periods of twilight, that occur in the course of each natural day‐night cycle, were recreated for two chosen latitudes in a computer‐controlled ‘natural‐day‐length simulator’.The photoperiodic responses of two aphid species, Aphis fabae and Megoura viciae, were examined in conditions which mimicked late summer to autumn at two simulated latidudes, 51.5d̀N (e.g.Ascot, Southern Britain) and 60°N (e.g.southern tip of Shetland Islands, Northern Britain), with temperatures between 16°C (‘night’) and 18°C (‘day’).The responses under simulated natural photoperiodic conditions were similar to those observed under conventional experimental conditions of squarewave light‐dark cycles (with abrupt lights‐on and lights‐off and constant light intensities during the light phase): both aphid species responded to civil twilight as light, and the critical day lengths (including civil twilight) for the induction of sexual morphs by the two aphid species observed in the simulator were the same as those found in squarewave light‐dark cycles.

The photoperiodic counter mechanism in the black bean aphid, Aphis fabae, was analysed by exposing newly-born, presumptive gynoparae to a variety of photoperiodic regimes at 15°C, and by the development of a model. It was shown that the counter accumulated long as well as short scotophases and that the “inductive strength”, or “value”, of long and short scotophases depended on photophase duration. Whilst the value of a long, 12-h, scotophase increased when accopanied by long photophases (>32 h), the value of a short, 8-h, scotophase decreased when accompanied by short photophases (<14 h). There was no detectable change in sensitivity to either long or short nights during the 4-day postnatal sensitive period. The counter mechanism appeared similar to that of the vetch aphid Megoura viciae, except that in M. viciae the value of a 12-h scotophase decreased with long photophases, whilst it increased in A. fabae.

Recent investigations into the photoperiodic clock of Megoura viciae indicated that night-length measurement in this aphid is accomplished by a circadian-based oscillator mechanism, despite its typical “hourglass” responses in Nanda-Hamner and Bünsow experiments. To reconcile these apparently contradicting observations, it is suggested here that although long-night determination is executed in a repetitive manner, this is not true for short-night determination. To test this hypothesis, the previously developed “coupled oscillator model” has been modified such that once a scotophase has exceeded the critical night length (as in the Nanda-Hamner experiments) it will always be determined as “long”, no matter whether a subsequent “photo-inducible phase” coincides with darkness or with light. In this respect the modified model differs from all known circadian-clock models. Model-generated responses are compared with a variety of experimental results previously observed with Megoura and the validity of the model as a description of the photoperiodic clock-counter mechanism in Megoura is discussed.

A mathematical model is described that simulates the sequences of behaviour that make up the marching behaviour of the African migratory locust, Locusta migratoria migratorioides. The “model locust” was developed using the results from behavioural experiments in which locusts walked in continuous light and the predictive value of the model was tested against results of behavioural experiments obtained under a variety of other conditions. The model locust consists of two reciprocally inhibitory systems—one controlling walking, the other controlling all non-walking activities—that form a switch mechanism that prevents both systems from being active at the same time. Real locusts switch spontaneously between different behavioural activities, they will also switch from walking behaviour to non-walking activities in response to pulses of 3-kHz sound. In the model locust the spontaneous alternations in behaviour occur because the inhibitory input from the active system to the suppressed system “habituates” over time, whilst the switch from walking to non-walking induced by sound stimuli occurs when the excitatory input to non-walking is increased by a stimulus to a level higher than the inhibitory input it receives from the walking activity. The stopping threshold to sounds does not remain constant however, but falls exponentially through a bout of continuous walking. In the model this is simulated by an exponential fall in the rate at which the inhibition imposed on the non-walking activity by walking habituates. During marching the strength of the component behaviours changes over time. The observed build-up of walking activity of locusts placed in continuous light was simulated accurately by the model locust by the mechanism of post-inhibitory rebound, for when the non-walking system is active it not only inhibits the walking system, but as an after-effect it also increases its excitatory input as well. The behaviour of the model locust is consistent with these results.

A control systems model for the photoperiodic clock is developed, which is an extended version of the earlier published “damped circadian oscillator model”, and which consists of two feedback circadian oscillators, a “pacemaker” and a “slave”, both entrainable by light and by temperature. The pacemaker is self-sustained or slightly damping, and the slave, which is coupled to the pacemaker, is strongly damped. The coupling is such, that with stronger coupling the slave's amplitude increases and the oscillation becomes less damped. In the first paper we demonstrate that the model is capable of describing many features of circadian rhythms, such as the occurrence of transients in constant darkness after a single light pulse, temperature-compensated shapes of phase response curves of both type 1 (weak resetting) and type 0 (strong resetting), and changes in pacemaker-slave phase relations when entrained by light-dark cycles with increasing photophases.

A model is described for the photoperiodic clock in insects and mites based on a circadian "pacemaker-slave" system, in which both the pacemaker and the slave are entrainable by light and by temperature. The pacemaker is self-sustained or slightly damped, whereas the slave is heavily damped and coupled to the pacemaker. It is proposed that the actual nightlength measurement is performed by the slave. Under certain circumstances the pacemaker may disturb the accumulation of photoperiodic information contained in subsequent light-dark cycles. Model-generated response curves parallel those found in a diversity of photoperiodic experiments performed with insects and mites as known from the literature.

The photoperiodic response of 10 strains of the two-spotted spider mite (Tetranychus urticae), originating between 40.5° and 60°N in Western and Central Europe, was found to be highly variable. The critical nightlength for photoperiodic induction of diapause was strongly correlated with latitude for the lowland populations and varied from 7.75 hr in the north to 13.25 hr in the south. The length of the circadian period, taken as the peak-to-peak interval in response curves of resonance experiments done with T. urticae, varied between 17.75 and 21.5 hr and appeared weakly correlated with latitude. Only a very weak correlation was observed between critical nightlength and circadian period. These results do not provide evidence in favor of a circadian-based photoperiodic clock in T. urticae. On the other hand, they also do not refute this possibility, as there may be other circadian or noncircadian factors affecting the critical nightlength, which could mask the influence of circadian period.

Existing models for the accumulation of photoperiodic information (the photoperiodic "counter") cannot explain satisfactorily the various effects of temperature on diapause induction in insects and mites, nor the observation that in several insect species the so-called "required day number" has a high degree of temperature compensation. In the model presented in this paper the effect of temperature has been incorporated in such a way that both the rate of increase of the "induction sum" (or "diapause titre") and the level of the "minimally required induction sum" (or "threshold level") are temperature dependent. The proposed model is in accordance with observed temperature effects, and gives an explanation for a temperature-compensated "required day number".

The blowfly, Calliphora vicina Meigen (Diptera: Calliphoridae), displays a maternally induced larval diapause. The critical nightlength for adult flies at 23.5°C and larvae derived from eggs deposited on days 10–14 and maintained in darkness at 11°C, is about 10 h 45 min and thus about 1 h longer than at 20°C.

Abstract. Newly born larvae (presumptive gynoparae) of the aphid Aphis fabae were exposed to ‘symmetrical skeleton’ photoperiods, consisting of two equal pulses of light per 24 h. The general form of the photoperiods tested was LDLD 1:10:1:12 h. This regime is open to two ‘interpretations’, LDLD 1:10:1:12 h (PPs12) and LDLD 1:12:1:10 h (PPs14). As both light pulses are close to 12 h apart, it is expected that the interpretations result in two distinct steady state phase relationships, i.e. this regime would show bistability. If this is true and both interpretations could be adopted, PPs12 is expected to result in a low and PPs14 in a high percentage of ‘apterized’ insects.Experiments undertaken with the aphid showed that bistability did not occur either at 15d̀C or 20d̀C, and the results could be entirely explained on the basis of an ‘instantly’ damping circadian oscillator clock.

ABSTRACT. The blowfly, Calliphora vicina Meigen (Diptera: Calliphoridae), displays a maternally induced larval diapause. Progeny of adults exposed to short days enter diapause if the larval temperature is 15d̀C or below; exposing adults to long days, or larvae to temperatures above 15d̀C, results in non‐diapause. By keeping progeny from short‐day parents at the parental temperature of 23.5d̀C for various lengths of time before transfer to 11d̀C, it could be shown that the diapause‐averting effect of high temperature operates in the late wandering stage of larva, possibly at the moment of the decision for the brain‐ring gland complex to release PTTH/ecdysone at the diapause/non‐diapause stage. Although photoperiodic induction in the blowfly is maternal, previous work had shown that the larvae were also slightly sensitive to photoperiod. From the present study it is evident that also the eggs are photo‐sensitive.

Mites which experienced a sequence of longnight cycles during their entire sensitive period showed 100% diapause; no diapause was observed in continuous darkness. When an increasing number of long-night cycles was applied to the mites against a ‘background’ of continuous darkness, diapause incidence was found to rise steadily: only 3 cycles sufficed to induce diapause in about half the population, whereas a minimal number of 6 cycles was required for 90–100% diapause to be attained. At the test temperature of 18.5°C the sensitive period lasted 11–12 days, comprising the complete post-embryonic developmental period, up to the final moult. Photoperiodic sensitivity was found to vary slightly over the whole sensitive period of the mites, the highest sensitivity being observed around days 3–6.