营养对雄性短翅鸣螽繁殖投资的影响

螽斯交配时产生精包,精包由精护与精荚组成。交配后雌性咬食精护。吃完精护后将精荚吃掉。精护是螽斯的一种繁殖投资方式。我们研究营养对雄性螽斯投资的影响。结果表明：⑴螽斯首次交配后产生的精包、精护、精夹的鲜重以及精荚、精护干重组间差异显著,高营养组雄性投资显著高于其它组;⑵营养缺乏时,有效性比偏雌。本研究支持性差异理论以及Ｇｗｙｎｎｅ（１９９０）和Ｓｉｍｍｏｎｓ（１９９５）的工作;⑶精荚和精护重量与雄性     

Remating and male-derived nutrients in Drosophila melanogaster

Laboratory natural selection and environmental manipulations were used to investigate the importance of male-derived nutrients to female Drosophila melanogaster. No evidence for the importance of such nutrients was found. Females from the same wild type base stock exposed as adults to low quality food did not show elevated fecundity or survival when they remated more frequently, and on high quality food the females showed a ‘cost of mating’ in reduced survival. Laboratory evolution on low quality food did not lead to elevated rates of remating by females; females from each selection regimen remated more frequently than one another when kept on the food type to which they had been exposed for the previous 5 years, on which they also showed higher fecundity than one another. Even under conditions of extreme nutritional stress, when females were exposed to short term (4-day) cycle of exposure to very low and high quality food, they remated more frequently immediately after exposure to high quality food. The results of this last experiment suggested that, under these circumstances, current nutrition, fecundity or rate of sperm usage was more important than number of sperm in store or cumulative fecundity in determining the probability that a female would remate.     

The effect of developmental nutrition on life span and fecundity depends on the adult reproductive environment in Drosophila melanogaster

Both developmental nutrition and adult nutrition affect life‐history traits; however, little is known about whether the effect of developmental nutrition depends on the adult environment experienced. We used the fruit fly to determine whether life‐history traits, particularly life span and fecundity, are affected by developmental nutrition, and whether this depends on the extent to which the adult environment allows females to realize their full reproductive potential. We raised flies on three different developmental food levels containing increasing amounts of yeast and sugar: poor, control, and rich. We found that development on poor or rich larval food resulted in several life‐history phenotypes indicative of suboptimal conditions, including increased developmental time, and, for poor food, decreased adult weight. However, development on poor larval food actually increased adult virgin life span. In addition, we manipulated the reproductive potential of the adult environment by adding yeast or yeast and a male. This manipulation interacted with larval food to determine adult fecundity. Specifically, under two adult conditions, flies raised on poor larval food had higher reproduction at certain ages – when singly mated this occurred early in life and when continuously mated with yeast this occurred during midlife. We show that poor larval food is not necessarily detrimental to key adult life‐history traits, but does exert an adult environment‐dependent effect, especially by affecting virgin life span and altering adult patterns of reproductive investment. Our findings are relevant because (1) they may explain differences between published studies on nutritional effects on life‐history traits; (2) they indicate that optimal nutritional conditions are likely to be different for larvae and adults, potentially reflecting evolutionary history; and (3) they urge for the incorporation of developmental nutritional conditions into the central life‐history concept of resource acquisition and allocation.     

Stress-tolerant P-solubilizing microorganisms

Drought, high/low temperature, and salinity are abiotic stress factors accepted as the main reason for crop yield losses in a world with growing population and food price increases. Additional problems create nutrient limitations and particularly low P soil status. The problem of phosphate fertilizers, P plant nutrition, and existing phosphate bearing resources can also be related to the scarcity of rock phosphate. The modern agricultural systems are highly dependent on the existing fertilizer industry based exclusively of this natural, finite, non-renewable resource. Biotechnology offers a number of sustainable solutions that can mitigate these problems by using plant beneficial, including P-solubilizing, microorganisms. This short review paper summarizes the current and future trends in isolation, development, and application of P-solubilizing microorganisms in stress environmental conditions bearing also in mind the imbalanced cycling and unsustainable management of P. Special attention is devoted to the efforts on development of biotechnological strategies for formulation of P-solubilizing microorganisms in order to increase their protection against adverse abiotic factors.     

Optimal moult strategies in migratory birds

Avian migration, which involves billions of birds flying vast distances, is known to influence all aspects of avian life. Here we investigate how birds fit moult into an annual cycle determined by the need to migrate. Large variation exists in moulting patterns in relation to migration: for instance, moult can occur after breeding in the summer or after arrival in the wintering quarters. Here we use an optimal annual routine model to investigate why this variation exists. The modelled bird's decisions depend on the time of year, its energy reserves, breeding status, experience, flight feather quality and location. Our results suggest that the temporal and spatial variations in food are an important influence on a migratory bird's annual cycle. Summer moult occurs when food has a high peak on the breeding site in the summer, but it is less seasonal elsewhere. Winter moult occurs if there is a short period of high food availability in summer and a strong winter peak at different locations (i.e. the food is very seasonal but in opposite phase on these areas). This finding might explain why only long-distance migrants have a winter moult.     

Indirect Costs of Parasitism are Shaped by Variation in the Type of Immune Challenge and Food Availability

Parasites can inflict indirect fitness costs to their hosts by eliciting costly immune responses. These costs depend on the type and amount of immunostimulants presented to the host immune system but also on the amount of resources available to fuel host immune responses. Here, we investigated how the relative costs of two different types of immune challenge are modulated by variation in food availability. We injected nestling tawny owls (Strix aluco) with either 10 μg of phytohaemagglutinin (PHA) or 20 μg of lipopolysaccharide (LPS), and subsequently raised them under two different food regimes (food-restricted vs. ad libitum). After controlling for food consumption, we found that LPS-injected nestlings lost more body mass than PHA-injected ones only when food-restricted. We also found that body mass gain of owlets fed ad libitum decreased with the intensity of the skin swelling response against LPS, but not PHA. These experimental and correlative results suggest that nestling tawny owls suffered greater immune costs when treated with LPS than PHA, and that variation in the costs of two different types of immune challenge can be exacerbated under conditions of low food availability. Our study highlights the importance of taking into consideration the interplay between host immunity and nutrition in the study of indirect costs of parasitism.     

Drivers leading to higher food prices: biofuels are not the main factor

Since 2007, the overall rise in food prices in the USA was twice that of the overall inflation rate, led by inflation for poultry and dairy products in particular. Prominent studies have indicated that the main drivers associated with the rise in food prices in the past 3 yr are the increased energy costs (and the trickle-down impact on farm input costs) and the devaluation of the US dollar. However, currently, the impact of crude oil as one of the primary drivers in food prices has waned significantly, as crude oil prices have fallen dramatically since late 2008. The data reviewed here debunk the popular myth that food producers, particularly farmers growing corn or soybean for biofuel feedstocks, are making huge profits with high commodity prices. Producers continue to face extraordinary risks in their farming operations, and profit margins have narrowed considerably because of the high input prices driven by high energy costs. One of the primary solutions to the food price inflation is to increase the supply of commodity crops in a responsible way that is both respectful and sustainable regarding economic, social, and environmental aspects. At the center of this solution is implementing and developing new technologies to increase crop yields and nutritional values without increasing the amount of fossil-based inputs used in agriculture. Conventional breeding, molecular breeding, genomics, and biotechnology are pivotal technologies for increasing crop yields to meet these supply needs, combined with the impacts of other new technologies and best management practices in agricultural production.     

The Critical Role of Supervision in Retaining Staff in Obstetric Services: A Three Country Study

Millennium Development Goal (MDG) 5 commits us to reducing maternal mortality rates by three quarters and MDG 4 commits us to reducing child mortality by two-thirds between 1990 and 2015. In order to reach these goals, greater access to basic emergency obstetric care (EmOC) as well as comprehensive EmOC which includes safe Caesarean section, is needed.. The limited capacity of health systems to meet demand for obstetric services has led several countries to utilize mid-level cadres as a substitute to more extensively trained and more internationally mobile healthcare workers. Although this does provide greater capacity for service delivery, concern about the performance and motivation of these workers is emerging. We propose that poor leadership characterized by inadequate and unstructured supervision underlies much of the dissatisfaction and turnover that has been shown to exist amongst these mid-level healthcare workers and indeed health workers more generally. To investigate this, we conducted a large-scale survey of 1,561 mid-level cadre healthcare workers (health workers trained for shorter periods to perform specific tasks e.g. clinical officers) delivering obstetric care in Malawi, Tanzania, and Mozambique. Participants indicated the primary supervision method used in their facility and we assessed their job satisfaction and intentions to leave their current workplace. In all three countries we found robust evidence indicating that a formal supervision process predicted high levels of job satisfaction and low intentions to leave. We find no evidence that facility level factors modify the link between supervisory methods and key outcomes. We interpret this evidence as strongly supporting the need to strengthen leadership and implement a framework and mechanism for systematic supportive supervision. This will promote better job satisfaction and improve the retention and performance of obstetric care workers, something which has the potential to improve maternal and neonatal outcomes in the countdown to 2015.     

Working for food under conditions of varying motivation in broilers

Broiler chickens, Gallus gallus domesticus, have been selected for rapid weight gain and appear to be continously hungry. If this is so, then energy that broilers invest in obtaining food should be insensitive to their level of feed restriction. We tested this hypothesis by measuring the maximum price (i.e. maximum number of key pecks) broilers with different body weights would pay for a food reward under conditions of varying feed restriction. Two groups of 20 broilers were fed on 50 or 75% of the amount of feed a broiler would eat when fed ad libitum. Broilers were able to learn an operant task and were willing to work for food. Birds of the 50%-group paid a higher maximum price for a food reward and responded more quickly to food presentation than the birds of the 75%-group in the first test week and showed a nonsignificant tendency to pay a higher price and to respond more quickly in the second test week. Different levels of long-term feed restriction had an influence on their body weight and on their motivation to work for food. No short-term effect of changing feed restriction was found. Birds showed frustration behaviour after the last food reward had been obtained, indicating that they were still hungry. The results indicate that broilers are still sensitive to different levels of feed restriction and that they can balance their investments and profits to a certain extent.     

Modelling interactions of food quality and quantity in homeostatic consumers

1. Boundaries between growth limitation by carbon (C) and phosphorus (P) in homeostatic heterotrophic consumers such as zooplankton will vary as demands on these two elements vary, as they should at different food quantities. At very low food quantity when production is close to zero, metabolic requirements (for carbon) become more important than growth requirements (for carbon and phosphorus in fixed proportion). Thus, the boundary separating C-and P-limited growth should be at a higher C : P ratio at low food quantity than at high food quantity.
2. A model including both metabolism and growth indicates that consumer growth should differ between foods of high vs. low phosphorus concentration only when food quantity is above a certain level. Thus, two foods might give identical consumer growth rates at low food quantity but give different consumer growth at higher quantity.
3. Solution of the model using parameters based on 2 mm Daphnia compared with a survey of C and P in seston of marine and freshwater sites supports earlier conclusions of the potential importance of food quality constraints on Daphnia growth.     

Evaluation of the ‘maximum price paid’ as an index of motivational strength for farmed silver foxes (Vulpes vulpes)

To measure farmed foxes’ motivations for full, naturalistic social contact, we constructed an apparatus where they could perform an operant to access stimuli, but then leave freely and thence determine their own bout lengths. Motivational measures based on demand curves can be invalid in such set-ups, and we therefore sought to validate the measure ‘maximum price paid’. This was achieved by measuring six silver fox males’ maximum operant responding for access or proximity to three resources differing in biological significance: food, vixens in oestrus and males. We predicted that if valid, maximum price paid would be highest for food and vixens. Maximum price were 970 ± 399 (S.E.) for food, 677 ± 173 (S.E.) for vixens and 389 ± 101 (S.E.) for other males (P < 0.05). In contrast, our complementary measures of motivation – price elasticity, expenditure and consumer surplus – did not differentiate between the resources, and ranked them in different orders (albeit not significantly). This was because the foxes rescheduled their behaviour with increasing costs, decreasing bout number while increasing bout length, to different extents with the three resources. Additional findings showed that all subjects ‘overpaid’, performing the operant response more than was required. This increased as the costs increased, perhaps due to increasing ‘time outs’ on the time-restricted schedule (DRH) as the task got harder. However, the overpayment was also highest when males were the resource, suggesting that operant responding was slowest and least efficient when working for less-valued resources. The resources present also affected how the foxes used the rest of the apparatus and influenced their behaviour; subjects staying more in the operant compartment when the resource was social (especially a female), but retreating to a distant compartment when it was food. While proximity to oestrous vixens elicited higher levels of tail wagging and only low levels of pacing, indicating a positive motivation, proximity to males elicited relatively high levels of pacing plus agonistic gaping, suggesting that the motives for seeking contact with males related to aggression. Thus, although our operant set-up reveals a drive to approach other males, the possible aggressive motives suggest that this sort of social contact would not necessarily improve their welfare in a traditional housing system. Overall, these results help improve the design and interpretation of preference tests, and confirm maximum price paid as a useful motivational measure for farmed foxes.     

Top-down and bottom-up factors in tidepool communities

Recent studies suggest that nutrient variation influences rocky intertidal community structure, however empirical evidence is rare. In the Gulf of Maine, tidepools that occur on seagull feeding roosts are potentially subjected to regular nutrient loading from seagull guano. The results of a survey conducted on Swan's Island, ME show that roost tidepools have very low macroinvertebrate and macroalgal diversity as well as very high phytoplankton biomass compared to non-roost tidepools. An experiment presented here tested basic food chain hypotheses in tidepool communities. These basic food chain models predict that in a tidepool with one trophic level (phytoplankton only), phytoplankton biomass will increase when nutrients are enriched. In contrast, these models predict that in two trophic level tidepools (phytoplankton and mussels) herbivory will prevent an increase in phytoplankton biomass when nutrients are enriched. A short term 2×2 factorially designed field experiment was used to test this basic conceptual model using herbivory by mussels and enrichment with nitrogen as the main effects. The results of this investigation are consistent with the predictions of basic food chain models, and indicate that over the short time interval of a few days, herbivory by mussels is sufficient to maintain low phytoplankton levels following enrichment with nitrogen. Experimental enrichment with phosphorus in this study had no effect on phytoplankton biomass. The results of this study suggest that periodic pulses of nitrogen into tidepools will have little effect on phytoplankton biomass when mussels are present and that longer-term chronic nitrogen influxes may be driving the patterns of community structure in tidepools occurring on roosts.     

Birds and biogeography of Mount Mecula in Mozambique’s Niassa National Reserve

The montane forests of northern Mozambique’s isolated massifs are inhabited by numerous range-restricted and threatened bird species, but until recently were extremely little-known. We report on a first avifaunal survey of the isolated montane habitats of Mt Mecula (1 442 m), Niassa National Reserve, notable as the only currently protected montane area in northern Mozambique. Mount Mecula’s moist forest is small (approximately 136 ha in total) and patchy, and although known botanically to have some montane affinities, was found to support an avifauna more typical of riparian forests of medium to low altitude. The only montane forest species recorded was Lemon Dove Aplopelia larvata. Other montane elements included Vincent’s Bunting Emberiza (capensis) vincenti, one of six species recorded new to the Niassa National Reserve list. Overall, it appears that despite its intermediate location, Mt Mecula does not represent a biogeographical ‘stepping stone’ for montane forest bird species. This probably owes to its remoteness from the Eastern Arc Mountains of Tanzania to the north and the massifs of other parts of northern Mozambique, to the south and west.     

Evaluation of quality changes in udder quarter milk from cows with low-to-moderate somatic cell counts

Much emphasis has been put on evaluating alterations in milk composition caused by clinical and subclinical mastitis. However, little is known about changes in milk composition during subclinical mastitis in individual udder quarters with a low-to-moderate increase in milk somatic cell count (SCC). This information is needed to decide whether milk from individual udder quarters with a moderate-to-high increase in milk SCC should be separated or not. The aim of this study was to determine how milk composition in separate udder quarters is affected when cow composite milk has low or moderately increased SCC levels. Udder quarter and cow composite milk samples were collected from 17 cows on one occasion. Milk yield was registered and samples were analyzed for SCC, fat, total protein, whey proteins, lactose, citric acid, non-protein nitrogen (NPN), lactoferrin, protein profile, free fatty acids (FFAs), lactate dehydrogenase (LDH), proteolysis, sodium and potassium. Bacteriological samples were collected twice from all four quarters of all cows. The cows were divided into three groups depending on their SCC at udder quarter level. The first group comprised healthy cows with four udder quarters with low SCC, <50 000 cells/ml; composition was equal when opposite rear and front quarters were compared. In the second and the third groups, cows had one udder quarter with 101 000 cells/ml < SCC < 600 000 cells/ml and SCC > 700 000 cells/ml, respectively. The remaining udder quarters of these cows had low SCC (<100 000 cells/ml). Despite the relatively low average cow composite SCC = 100 000 cells/ml of Group 2, milk from affected udder quarters exhibited lower casein number, content of lactose and β-casein (β-CN), while the content of whey protein, sodium, LDH and α-lactoalbumin (α-la) were higher compared to healthy opposite quarters. In addition to these changes, milk from affected udder quarters in Group 3 also exhibited lower values of potassium and αs1-casein (αs1-CN) and higher values of lactoferrin when compared to milk from opposite healthy quarters. This indicates that even when the SCC in cow composite milk is low, there might exist individual quarters for which milk composition is changed and milk quality impaired.     

The neonate nutrition hypothesis: early feeding affects the body stoichiometry of Daphnia offspring

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Regulation of gut function varies with life-history traits in chuckwallas (Sauromalus obesus: Iguanidae)

We examined the effects of hibernation and fasting on intestinal glucose and proline uptake rates of chuckwallas (Sauromalus obesus) and on the size of organs directly or indirectly related to digestion. These lizards show geographic variation in body size and growth rate that parallels an elevational gradient in our study area. At low elevation, food is available only for a short time during the spring; at high elevation, food may also be available during summer and autumn, depending on rainfall conditions in a given year. We hypothesized that low-elevation lizards with a short season of food availability would show more pronounced regulation of gut size and function than high-elevation lizards with prolonged or bimodal food availability. Hibernating lizards from both elevations had significantly lower uptake rates per milligram intestine for both nutrients, and lower small intestine mass, than active lizards. The combination of these two effects resulted in significantly lower total nutrient uptake in hibernating animals compared to active ones. The stomach, large intestine, and cecum showed lower masses in hibernators, but these results were not statistically significant. The heart, kidney, and liver showed no difference in mass between hibernating and nonhibernating animals. Lizards from low elevations with a short growing season also showed a greater increase in both uptake rates and small intestine mass from the hibernating to the active state, compared to those from high elevations with longer growing seasons. Thus, compared to those from long growing season areas, lizards from short growing season areas have equal uptake capacity during hibernation but much higher uptake capacity while active and feeding. This pattern of regulation of gut function may or may not be an adaptive response, but it is consistent with variation in life-history characteristics among populations. In areas with a short season, those lizards that can extract nutrients quickly and then reduce the gut will be favored; in areas where food may be available later in the year, those lizards that maintain an active gut would be favored. While other researchers have found much greater magnitudes of gut regulation when making comparisons among species, we find the different patterns of change in gut function between different populations of chuckwallas particularly intriguing because they occur within a single species.     

To See or Not to See: Do Front of Pack Nutrition Labels Affect Attention to Overall Nutrition Information?

Background

Front of pack (FOP) nutrition labels are concise labels located on the front of food packages that provide truncated nutrition information. These labels are rapidly gaining prominence worldwide, presumably because they attract attention and their simplified formats enable rapid comparisons of nutritional value.

Methods

Eye tracking was conducted as US consumers interacted with actual packages with and without FOP labels to (1) assess if the presence of an FOP label increases attention to nutrition information when viewers are not specifically tasked with nutrition-related goals; and (2) study the effect of FOP presence on consumer use of more comprehensive, traditional nutrition information presented in the Nutritional Facts Panel (NFP), a mandatory label for most packaged foods in the US.

Results

Our results indicate that colored FOP labels enhanced the probability that any nutrition information was attended, and resulted in faster detection and longer viewing of nutrition information. However, for cereal packages, these benefits were at the expense of attention to the more comprehensive NFP. Our results are consistent with a potential short cut effect of FOP labels, such that if an FOP was present, participants spent less time attending the more comprehensive NFP. For crackers, FOP labels increased time spent attending to nutrition information, but we found no evidence that their presence reduced the time spent on the nutrition information in the NFP.

Conclusions

The finding that FOP labels increased attention to overall nutrition information by people who did not have an explicit nutritional goal suggests that these labels may have an advantage in conveying nutrition information to a wide segment of the population. However, for some food types this benefit may come with a short-cut effect; that is, decreased attention to more comprehensive nutrition information. These results have implications for policy and warrant further research into the mechanisms by which FOP labels impact use of nutrition information by consumers for different foods.     

Effect of Food Quantity and Quality on Population Growth Rate and Digestive Activity in the Euryhaline Rotifer Brachionus plicatilis Müller

We investigated the nutritional effects of both food quantity and quality on Brachionus plicatilis. Decomposition of particulate and dissolved organic matter by rotifer digestive enzymes play a crucial role in rotifer nutrition. Among other enzymes, rotifers produce phosphatases, non‐specific enzymes that allow for the release of orthophosphate from a variety of organic phosphorus compounds. Phosphatase saturation was measured in B. plicatilis homogenates using the spectrofluorimetric method. We examined population growth rate, reproduction and phosphatase activity in the homogenate of rotifers (PA_RH) fed by nutrient‐replete algal food supplied at different quantities. Population growth rate, number of eggs per individual and PA_RH were affected by food quantity. Growth rate and number of eggs per individual significantly increased in rotifers fed by food supplied at the highest quantity. The highest population growth rate was reached by rotifers fed by nutrient‐replete food, while it did not significantly differ between rotifers fed on nitrogen (N)‐depleted and phosphorus (P)‐depleted food. The number of eggs per individual was more affected by N than P supply. PA_RH and rotifer RNA content were not influenced by different food quality. The results indicate that B. plicatilis is not able to regulate its digestive apparatus in terms of efficiently getting access to essential nutrients when scarce, but do this when nutrient‐replete food is available in different quantity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The response of luteinizing hormone secretion to photoperiod is modified by the level of nutrition in female Mediterranean goats

This paper reports the influence of nutrition on the photoperiodic control of luteinizing hormone (LH) secretion in female Mediterranean goats (i.e., goats from the Mediterranean area in general). Ovariectomized, oestradiol-treated goats were subjected to two consecutive intervals of 3 months of long days followed by 3 months of short days (group LDSD, N=20), or vice versa (group SDLD, N=20). The LDSD and SDLD does were also randomly assigned to one of two nutrition groups that received either 1.1 (H group, N=10) or 0.7 (L group, N=10) times their maintenance requirements. Live weight and body condition score were determined weekly and LH concentrations twice per week. To establish the pulsatility of secretion of LH, three periods of intensive sampling were undertaken. Melatonin was determined after a period of 45 short or long days. All photoperiod/nutrition groups showed large variations in LH concentrations according to photoperiod, with nutrition having a significant effect (P<0.001). The mean time between the shift from long to short days and the stimulation of LH secretion, and between the shift from short to long days and the inhibition of LH secretion, was different in each nutrition group (at least P<0.05). No differences were seen in the frequency of LH pulses between the nutrition groups, but differences between sampling periods were observed (P<0.001). Melatonin secretion was not affected by food supply. These results confirm: (1) that Mediterranean female goats are sensitive to photoperiod, (2) that this environmental cue may control the timing of pituitary activity under natural conditions, and (3) suggest that nutrition plays an important role in the effect of photoperiod on LH secretion.     

Plasticity of insect reproduction: testing models of flexible and fixed development in response to different growth rates

We tested alternative developmental hypotheses describing when during an insect oviposition cycle reproductive tactics are determined. Newly eclosed adult females of the grasshopper Romalea guttata were raised on eight different feeding treatments consisting of a low food diet, a high food diet, and changes from high to low food, or low to high food, at different times during the first oviposition cycle. When initial food availability was high, a decline in food availability >7 days after adult eclosion produced no significant increase in time to oviposition compared to constant high food. In contrast, when initial food availability was low, an increase in food availability as late as day 14 produced a significant decrease in time to oviposition compared to constant low food. Thus, time to oviposition is determined by feeding rate early in the oviposition cycle, but the time of this determination is later when food availability is lower. Masses of individual eggs were unaffected by these treatments. When initial food availability was high, a decrease in food availability on day 21 produced no significant change in numbers of eggs in a clutch compared to constant high food. In contrast, when initial food availability was low, an increase in food availability after day 7 produced no significant change in number of eggs in a clutch compared to constant low food. Changes in egg production resulted from oocyte resorption, which appeared to become unresponsive to food availability between day 14 and day 21. Our results refute the hypothesis that reproductive tactics are continuously flexible. Development toward oviposition seems to be structured so that reproductive tactics become independent of feeding late during the first oviposition cycle. Reproductive tactics become unresponsive to food at different times for groups initially receiving low or high food, suggesting that a particular developmental state, rather than some absolute time, marks the shift to development that is unresponsive to␣food. Plasticity in reproductive tactics appears to be␣controlled by hormones in a manner similar to the hormonal control of plasticity of metamorphosis in other insects. Received: 21 September 1997 / Accepted: 10 March 1998