Sinzo Masaki

Climatic adaptation and species status in the lawn ground cricket

SummaryThe cricket tentatively identified as Pteronemobius taprobanensis shows a saw-toothed pattern of variation in adult size along the latitudinal gradient of the Japanese Islands. A slight but abrupt increase in adult size at about 28° N indicates the replacement of the subtropical form by the temperate one. The body size of the latter slightly decreases north to about 33° N, then conspicuously increases to about 39° N and again decreases to the northern extreme. This pattern of variation seems to be related to the local adjustment of nymphal development by means of the photoperiodic response and genetic variation, since the adult size varies as a function of the duration of nymphal development. Multiple regression analysis of the variance between local populations reared at various photoperiods suggests, however, that the decrease in body size due to selection for shorter development in cooler climates is to a certain extent counteracted by selection for a higher rate of growth. The latter component of climatic selection is possibly due to greater egg production by larger-sized females within the shorter reproductive season. This hypothesis may also account for the divergence in size between the two climatic forms of this nominal species.

Hourglass and oscillator expressions of photoperiodic diapause response in the cabbage moth Mamestra brassicae

Abstract. Both oscillator and hourglass features are found in the photoperiodic response that controls the pupal winter diapause of Mamestra brassicae. The expression of oscillatory response to extended long‐night cycles is temperature dependent, i.e. circadian resonance appears at 23 and 25oC but not at 20 and 28oC. At 20oC, scanning of extended scotophases by a short light pulse does not reveal any clear circadian rhythmicity. However, a circadian feature of the photoperiodic response is indicated even at 20oC by a bistability phenomenon, i.e. either one of the two dark periods in symmetrical skeleton photoperiods determines the diapause response depending on the phase angle with the preceding (entraining) light‐dark cycles. At 20 and 25oC, the incidence of diapause increases as a function of the number of light–dark cycles regardless of the cycle length (T), if T is 24 h or 2 X 24h (with a 12 h light period). A non‐diel cycle (r=36h) is less effective, suggesting that disturbance of the circadian organization partly impairs the diapause‐inducing function. The inductive effect of a long night is largely affected by temperature, and becomes saturated with eight cycles at 20oC and 14 cycles at 25oC. Presumably, an hourglass mechanism measures the dark time, and a circadian component involved in some later sequence of the photoperiodic response may or may not be expressed depending on the mode of interaction between them.

Effect of light period on dark-time measurement for diapause induction in Mamestra brassicae

Abstract In the cabbage moth, Mamestra brassicae, the type of pupal development (winter diapause, summer diapause or non-diapause) is detrmined by photoperiod in the larval stage. Interacting effects of photophase and scotophase were examined by exposing larvae to non-24-h cycles in which the two component phases were varied independently of each other. With a photophase of 1–3 h, winter diapause was most effectively prevented by a light pulse at a fixed time (12 h) from lights-on rather than lights-off. With photophases of 4–36 h, the diapause programming was basically dependent on the scotophase length, but the critical scotophase increased as the photophase deviated from 12 h. The quantitative variation in the diapause-inducing effect of scotophases within the inductive range could interact with the available number of cycles restricted by the cycle length, and this might cause apparent shift of the “critical” scotophase. The response to interruption of 12-h scotophase was also affected by photophase length, and the effect of 1 h light pulse was more changeable in the early than in the late scotophase.

Variability in wing form of crickets

Loss of functional hindwings is observed in most subfamilies of Japanese crickets. Habitat, behaviour, body size and phylogenetic factors might be involved, but interactions among them may obscure the general trend. Wing dimorphism is common among the relatively small-sized members of Gryllinae, and the two small-sized subfamilies, Nemobiinae and Trigonidiinae. Both environmental cues (e. g. photoperiod) and genetic factors affect the wing form. InDianemobius fascipes (Nemobiinae), the percentage macroptery was drastically changed by selection for macroptery or microptery. Crossing experiments indicated polygenic control of wing form as well asX-chromosomal and maternal effects. Neither the long-winged nor short-winged line bred true after 40 generations of selection. Full-sib families revealed a large genetic variation in frequency of macropterous forms within a population. The genetic determination of the propensity for macropterism did not seem to be directly coupled to the mechanism performing the photoperiodic time-measurement.

ASYMMETRIC PERCEPTION OF TWILIGHT AFFECTING DIAPAUSE INDUCTION BY THE FALL WEBWORM, HYPHANTRIA CUNEA

In order to assess the natural photophase effective for controlling the pupal diapause of Hyphantria cunea, larvae were exposed in the long‐day season to “natural” conditions of light (through a window) for a period of 14 hr, 50 min. This photophase included different portions of either the dawn or dusk twilight period. Since the critical photophase was found to be 14 hr 35 min under natural daylight as well as under conditions of artificial light, 50% diapause was expected when the twilight intensity reached the threshold level 15 min after the onset (dawn) or before the end (dusk) of the exposure. The threshold intensities of twilight thus determined showed a significant asymmetry, being about 1 and 10 lux at dawn and dusk, respectively. From this, it was inferred that the photophase under natural conditions would begin about 40 min before sunrise and end about 20 min after sunset. This asymmetry in sensitivity seems to be caused by the conditions (light or dark) to which the larval photo‐receptive system has been exposed. The larvae that had been kept under artificial light of 180–200 lux for 14 hr were sensitive to a subsequent 1 hr exposure to 0.5 lux or greater and averted diapause, whereas those held under 9,000 lux failed to avert diapause even when the photophase was supplemented by light of 7.5 lux for 1 hr.

Photoperiodic control of larval development in the semivoltine cockroachPeriplaneta japonica (Blattidae: Dictyoptera)

Periplaneta japonica is semivoltine, entering early diapause in any (except the first) larval instar before the last, and late diapause in the last instar. Early diapause was induced under a short day of 13 h or less at 28°C, and under both short and long daylength (12–16 h) at 20°C. The shorter the daylength and the lower the temperature, the younger the instar was entering early diapause. Early diapause was terminated by a long day (16 h) or a high temperature (28°C), after which larvae grew faster in short days than in long days until the last instar, when they again entered diapause, always in short days and frequently in long days. This late diapause was terminated also by an increase in daylength and was always followed by adult emergence. In this case, 13 and 14 h daylengths after exposure to 12 h daylength were as effective as 16 h daylength. Ourdoor samples collected in late autumn, winter and early spring at Hirosaki (40.5°N) comprised two distinct size groups, corresponding with the early and late diapause instars. Based on these results, the seasonal development strategy and intriguing aspects of the photoperiodic response in this cockroach are discussed.

Injury causes microptery in the ground cricket, Dianemobius fascipes

Abstract Dianemobius fascipes is a wing-dimorphic cricket, developing long wings under long-day and crowded conditions at 28°C. Even under these conditions, however, severance of appendages in the nymphal stage exerted a micropterizing effect. A similar, though less pronounced, effect was obtained by exposure to heat (40°C) or cold (0°C) for 1 day. The period sensitive to such stresses ranged from the 3rd to the last (6th) nymphal instar 5 days before adult eclosion. The antepenultimate (4th) and penultimate (5th) instars were the most sensitive. Micropterous adults obtained by leg severance could be classified into 3 categories defined by discontinuous variation in wing length. These represent 3 descending steps of micropterization with increased age at the time of surgery during the sensitive period. It is inferred that the macropterous and micropterous morphogeneses bifurcate before the 2nd day of the penultimate instar, after which the surgery produced intermediate wing forms. Severance of the leg just before or after adult eclosion accelerated shedding of hindwings. Thus, injury or other stresses can modify a presumptive disperser into a non-disperser during development.

Interacting effects of photophase and scotophase on the diapause response of the Indian meal moth, Plodia interpunctella

Abstract The diapause-programming response to photoperiod in Plodia interpunctella was analyzed by exposing larvae to various 24-h and non-24-h regimes of light and darkness. The response to 24-h regimes indicated three photoperiodic parameters—a critical scotophase, a minimal photophase, and a minimal scotophase for a full expression of the response. The critical response was based on dark-time measurement, because disruption of the scotophase abolished the response and the diapause incidence varied as a function of scotophase in non-24-h regimes. The critical scotophase varied with the duration of the preceding photophase. Prevention of diapause by single or double-night interruptions of long scotophases could be explained by resetting of the dark-time measurement. The effect of a light pulse was modified by the quantitative interaction of light and dark reactions. The sensitivity to resetting by a light pulse seemed to be decreased in the early scotophase with an increasing duration of the preceding light period. Therefore, the significance of light in the photoperiodic response was something more than delimiting scotophase for the time measurement.

Response to nigh interruption in photoperiodic determination of wing form of the ground cricket Dianemobius fascipes

ABSTRACT. In the ground cricket Dianemobius fascipes (Walker), the occurrence of long‐ or short‐winged form is determined by photoperiod during larval development. The pattern of this response is similar to that of diapause induction in other insects with a clearly defined critical nightlength (11.5 h). In symmetric skeleton regimes formed by a pair of 1 h light pulses, the critical nightlength is about 2 h longer than in the complete photoperiod. The long‐night (short‐wing) effect is eliminated by an interrupting light pulse. This response to night interruption shows only one peak in scotophases of various lengths and can be related to the critical nightlength before or after interruption. The light‐reaction time required for the interruption effect changes during the night. In the early scotophase, 1 h of light is enough but 15 min fails to reverse the long‐night effect, while a light pulse of only 1 min is effective about 1 h before the critical nightlength is reached. Several possible interpretations of this change are discussed.

Tropical, subtropical and temperate life cycles in ground crickets

The band-legged ground cricket group comprises a pair of very closely related species; Dianemobius fascipes occurs widely over tropical and subtropical regions of Southeast Asia, while D. nigrofasciatus is restricted to temperate regions. A similar situation is found in the lawn ground cricket group; the tropical/subtropical species D. taprobanensis is replaced by D. mikado in temperate regions. In both southern species, tropical strains were almost homodynamic, but they produced a few delayed eggs independently of photoperiod and a variable proportion of macropters in response to photoperiod; subtropical strains showed clearer responses. Both northern species controlled egg diapause, nymphal development and wing form by various photoperiodic responses. The two species groups showed parallel geographical profiles of size variation, and the adults and eggs were smaller in the southern than in the northern species. In the absence of strong climatic selection for larger growth rate, size increase in tropical regions would be counteracted by predation pressure. The optimal adult size thus seems to be smaller in tropical than in temperate regions, being determined by the geographically variable balance between counteracting selecting forces.RésuméLe groupe de grillon des champs qui a les pattes rayés contient une paire d’espèces très étroitement apparantées; Dianemobius fascipes occupe largement les régions tropicales et subtropicales du Sud-Est asiatique, tandis que D. nigrofasciatus est restreint aux régions temperées. Une situation semblable se voit dans le groupe de grillon des champs qui habite dans le gazon; les espèces tropicales subtropicales D. taprobanensis est remplacé par D. mikado dans les région temperées. Dans tous les deux espèces du sud, les races tropicales etaient presque homodynamiques, mais elles produisaiet quelques oeufs indépendamment de photopériodisme et une proportion variable de macroptères selon l’influence du photopériodisme; les races subtropicales montraient les influences plus claires. Tous les deux espèces du nord réglaient la diapause des oeufs, le development nymphal et la forme d’ailes selon les influences diverses du photopériodisme. Les deux espèces montraient les parallèles profils géographiques de la variation de grandeur, et les imagos et les oeufs étaient plus petits dans les espèces du sud que dans celles du nord. En l’absence de la séléction puissante climatique pour le taux plus grand de croissance, l’accroissement de grandeur dans les régions tropicales serait contrecarré par la pression de prédation. Déterminée par la balance variable de géographie d’entre les puissances d’opposition et de séléction, la grandeur optimale de l’imago donc semble plus petie dans la région tropicale que dans la région temperée.