Food limitation of population density in the orb-wed spider,Nephila clavata

Summary Field studies were conducted to clarify whether variation in food availability among habitats influences population density, and whether population density has a negative effect on foraging success in the orb-web spider,Nephila clavata. Lifetime food consumption per individual (i.e., foraging success) strongly correlated with mean body size of adult females and mean fecundity in populations. Also, there was a positive correlation between foraging success and population density. Since foraging success reflected potential prey availability in the habitat, food resource appeared to be a limiting factor for populations in this spider. Mean fecundity per individual correlated with population density of the following year, suggesting that decreased reproduction is a major component of food limitation on population density. Consistent defferences in mean body size between particular sites were observed over years, while such difference was less obvious in density. Thus, ranking of food abundance among habitats seems to be predictable between years. A field experiment revealed that an artificial increase in population density had no negative effect on the feeding rate of individuals, suggesting that intraspecific competition for food is not important in this species.     

Egg cluster size variation in relation to the larval food abundance inLuehdorfia puziloi (Lepidoptera: Papilionidae)

Summary Mean egg cluster size ofLuehdorfia puziloi yessoensis varied among habitats. The mean egg cluster size tended to be large when abundance of the larval food leaves expressed as the fresh weight of leaves per unit area at a given habitat was high. Since this variation was observed among closely located study plots (butterflies can easily move between study plots), the egg cluster size variation among habitats is likely to be a result of flexible response by females to varying food abundance for larvae.     

Variation in offspring size within a population of the web-building spiderAgelena limbata

Within a population of the web-building spiderAgelena limbata, the weight of the first instar nymphs ranged from 1.187 to 6.559 mg. Both intraclutch and interclutch variation were recorded. The mean weights were different among clutches and the coefficients of variation within a clutch ranged from 3.3 to 29.2%. Variation in the nymphal weight was certainly derived from variation in the egg weight because there was a high correlation between the two weights. Factors affecting interclutch variation in nymphal weight were examined by multiple regression analysis. Nymphal weight was positively correlated with the body size and food conditions of female parents, and negatively correlated with the clutch size. Among these three factors, the food conditions of female parents had the largest apparent effect on the interclutch variation. The results suggest that females with larger body size and more food produce larger offspring, and that there is a trade-off between offspring size and clutch size. Heavier nymphs had larger body size (carapace width) and may have larger energy reserves. Heavier nymphs survived experimental starvation for a significantly longer period.     

Prey capture tactics of spiders: An analysis based on a simulation model for spider's growth

Summary The prey capture tactics of spiders was analyzed, considering the energy gained by the capture of prey and that required for it. For the purpose of it, a growth model of spiders was constructed, expressing the flow rate of prey biomass to the spider's body by differential equations. Solving these equations under the differing values of three parameters, growth curves of spiders was obtained. These three parameters are the amount of prey biomass supplied daily to spiders,x ₀, the rate of prey capture of spiders, α, and a coefficient of the respiration rate required for the capture of prey,k. When the value ofk increased, spiders could grow only at high value ofx ₀. These results suggest that habitats with small prey biomass are preferred by spiders adopting a sit-and-wait tactics for prey capture, which requires small values ofk. Wolf spiders are one of these spiders showing that tactics. On the other hand, web-builders which require large amount of energy for spinning webs (namely, take large value ofk), are able to grow only in the habitats with large prey biomass. Each species of spiders are considered to locate in a certain point between both extremes of these tactics for the capture of prey.     

Decrease in respiratory rate in a wolf spider,Pardosa astrigera (L. Koch), under starvation

Summary Effects of starvation on the suryival period and the respiratory rate in adults of a wolf spider,Pardosa astrigera (L. Koch), were investigated. The spiders used were divided into four groups: well-fed, starved and two limited food groups; in the latter two, each spider was supplied with one leafhopper every second or third day. Adult males and females ofP. astrigera could survive for a long time; 28.8±2.7 days and 54.4±18.9 days, respectively, without any food. The longevities shown here were 73.8% for males and 78.6% for females of those of well-fed spiders, indicating thatP. astrigera adults have a strong tolerance to starvation. The respiratory rate of well-fed adults showed no tendency to increase or decrease with their aging; the mean respiratory rates were 4.86×10⁻⁴ mg CO₂/mg f.w. (fresh body weight)/hr for males and 3.80×10⁻⁴ mg CO₂/mg f.w./hr for females. The respiratory rates of starved spiders increased during the first two days of starvation but decreased markedly from the third to the twelfth day, and thereafter retained an almost constant level for each sex. The mean respiratory rates after the twelfth day of starvation were 2.49×10⁻⁴ mg CO₂/mg f.w./hr for males and 2.76×10⁻⁴ mg CO₂/mg f.w./hr for females; these values were respectively 48.4% and 63.0% of those prior to starvation. The fresh body weight of starved spiders decreased linearly with time but the rate was small. The respiratory rates of the limited food groups tended to decline with time and thereby their weight losses were minimized. The decrease in the respiratory rate under starvation was considered not to be due to spider exhaustion or senescence but due to an intrinsic change in behaviour and/or metabolism, because when the spiders were supplied with ample food for five days after starvation, the respiratory rate and the body weight rapidly recovered to near the levels prior to starvation. It is suggested that starved spiders use a higher ratio of fat as catabolic substrate than normally fed or satiated ones. Feeding strategies of poikilo-therm predators are discussed. This work was partially supported by the Nippon Life Insurance Foundation Research Fund and Grant-in-Aid (No. 56480039) from the Ministry of Education, Science and Culture, Japan.     

Space partitioning among the wolf spiderPardosa amentata species group in Hokkaido, Japan

Summary There are 4 species belonging to the wolf spiderPardosa amentata species group in Hokkaido. The 4 species have separately occurred in Hokkaido in general, but a few species have sometimes occurred together at the same sites. Their biology at the coexisting sites, with special reference to partitioning of space was studied.P. agraria mainly inhabited fields and occurred widely in Hokkaido.P. hokkaido, P. brebivulva andP. lugubris inhabited the vicinity of forests and occurred in southern Hokkaido and on the side of the Sea of Japan in Hokkaido, all over Hokkaido excent the side of the Sea of Japan in central Hokkaido, and the lowlands and highlands in northern and eastern Hokkaido and mainly the highlands in other regions, respectively. In eastern Hokkaido whereP. brevivulva andP. lugubris have coexisted, as the number of spiders detected and the number of localities of the former were less than those of the latter, it was concluded that the former occurred mainly in southern Hokkaido and the Hidaka region. In southern Hokkaido whereP. hokkaido andP. brevivulva have coexisted, few individuals of both species were collected together at the same sites, and they were distributed allotopically. In short, the partitioning of space among theP. amentata species group occurring in Hokkaido was clear. From investigations on the coexisting sites and some other reports, it was considered that food, life cycle and diel activity periodicity amongPardosa occurring in Hokkaido were similar to one another. This may be one of the causes of the clear space partitioning amongPardosa. SincePardosa is a raptorial predator that does not weave but wanders on the ground and uses space horizontally, available prey resource forPardosa would be limited. It was therefore conjectured that the spider would be prone to space segregation, even if the spider could segregate the other niche dimensions. The partitioning of space betweenP. agraria andP. hokkaido inhabiting distinct habitats was loose when it was compared with that between species inhabiting similar habitats. As most of the spiders observed in the overlapping areas were females with egg cocoons, which required little food, the reduced feeding was considered to have allowed the loose partitioning.     

Preliminary studies on the respiratory energy loss of a spider,Lycosa pseudoannulata

Summary To elucidate the basic food requirement of spiders, the important polyphagous predators of rice-plant insect pests, an attempt was made to measure the respiratory energy loss of fasting spiders,Lycosa pseudoannulata. Relationship between fresh (y) and dry (x) weights of spiders inhabiting the bottom layer of the rice-plant community was represented by the following allometric equation:y=0.428x ^0.872. The carbon dioxide production by previously fed and unfed females under the dark at 29°C 100% R. H. was measured by a titration technique. The relationship between fresh body weight and CO₂ production by unfed animals could be represented by the equationM=aW ^b, M being the CO₂ output per individual per day andW the fresh body weight. The constantb, which determines the slope of curve, was 0.808. Respiration of the adult female with 100 mg fresh weight was 1.155±0.250 mg CO₂/100 g fresh weight/day or 48.69 mg CO₂/g dry weight/day. This value corresponds to 35.81 cal/g fresh weight/day or 150.94 cal/g dry weight/day. Supposing the calorific content of spiders to be 5820 cal/g dry weight, rate of the respiratory energy loss to total energy of the body was estimated to be 2.60%. This rate did not strongly contradict with the loss of fresh body weight before and after the measurement. The metabolic rate showed remarkable fluctuation with changing food supply. The CO₂ production of starved individuals decreased to 83.63±16.34% as compared with individuals which were fed before the measurement.     

A laboratory study on the predatory miteTyphlodromus pyri (Acarina: Phytoseiidae): I. The effect of temperature and food consumption on the rate of development of the eggs and immature stages

Summary Development periods were determined for eggs and juvenile stages of the predatory phytoseiid miteTyphlodromus pyri at four constant temperatures and three feeding levels. For the non-feeding stages (eggs and larvae) power curve functions were fitted to development rate in terms of temperature. The results are compared to the general arthropod response to temperature. A non-linear model was fitted to nymphal development rate in terns of temperature and food, in which the weight gain over an instar is assumed to be a linear function of daily food consumption. Examples of the same shape development rate—food relationship are given for two more phytoseiid mites and for other arthropods. The results are discussed with respect toT. pyri as an integrated control agent.     

The ingestion in wolf spiders I. Capacity of gut ofLycosa pseudoannulata

Summary An wolf spider,Lycosa pseudoannulata, collected from the field were reared in test tubes supplied with fruit flies as the food, and the number and weight of fruit flies killed per unit time and the weight of residuum of killed flies were recorded. When the cumulative number of flies killed by a spider was plotted against time after the beginning of food supply, the angle of the curve obtained decreased gradually until a constant value. In this time, it seems that the gut of spider had saturated with food and the increase rate of number of flies killed reflects the rate of food disappeared from gut by assimilation and egestion. The amount of ingestion was obtained by subtracting the weight of residuum and the gain of live weight of the spider from the weight of killed flies. After the gut has saturated, the amount of food remained in the gut is considered to be equal to the capacity of gut, which is an essential factor in the study of predation. The amount of food disappearance from gut was also estimated. There was a linear relationship between the log body weight of spiders and the log capacity of gut. The linear relationship was also seen between the log total amount of ingestion and the log total amount of food disappeared from gut.     

Factors affecting body size variation within a population of an herbivorous lady beetle,Henosepilachna niponica (Lewis)

Summary Size variation in newly-emerged adults was examined in two different local populations of an herbivorous lady beetle,Henosepilachna niponica, for 1976–80. Mean adult size of both sexes changed rather synchronously in the two populations over 5 years. Body size of adult beetles apparently decreased with increasing leaf damage of the plants on which they developed. Adult beetles which emerged late in the season, associated with increasing food deterioration, were smaller than those which emerged early. Ecological consequences of adult size variation is discussed in terms of oviposition site selection.     

The relationship between size, budding rate, and growth efficiency in three species of hydra

Summary Three species of the fresh water carnivore hydra,H. littoralis, H. pseudoligactis, andC. viridissima present a graduation in size with the first species the largest and albinoChlorohydra the smallest. When presented with a daily overabundance of food (artemia), considerable variation in food intake and gross efficiency of growth (proportion of food energy consumed that is turned into new protoplasm or buds) existed among the species. The degree of association between size of species and food intake was highly significant. However, budding efficiency among the species was found to be independent of food intake (when the effects of species size were eliminated) and of species size (when the effects of food intake were removed). However, species with high (low) efficiencies have significantly higher (lower) reproductive rates. A lowering of the temperature from 25° to 15° C. increased the size of the species, increased food intake, but decreased reproductive rate. In all species exceptH. pseudoligactis a corresponding increase in the production of bud energy with no change in efficiency also occurred. On the other hand, lowering of the temperature forH. pseudoligactis significantly lowered reproductive efficiency but had no effect on the total calorific output of buds. This species, in constrast to the others, appears to have a compensatory ability to adjust its efficiency to maintain a high calorific output when temperature increases. It was also found that albinoChlorohydra have budding efficiencies of around 35 percent which are not influenced by changes in food intake or light. Normal green hydras, however, have efficiencies which range from 40 to 62 percent above their albino counterparts when fed once a day and once every two days in light respectively. It it concluded first, that the symbiotic algae in the gastrodermals cells of green hydra contribute quantitatively in the order of the above amounts to the growth process in this species, and second, that green hydras have the ablity to increase their growth efficiency when food intake is reduced thus reducing the drop in calorific but output that normally occurs in the albino (control) form. This study was supported by a National Science Foundation Grant (GB-912)     

Resource allocation and time budgeting in adults of the cockroach,Periplaneta americana: The interaction of behaviour and metabolic reserves

Summary Activity (to food, water and shelter) and resource allocation (mass budget, and size of various body components) were examined in populations of adultPeriplaneta americana subjected to periods of starvation or sugar feeding. Following 13 days of starvation, roaches ate 5 times their normal daily ration and feeding required about 20 days to return to pre-starvation levels. When sucrose was substituted for the usual dog-food diet for 13 days, there was also a large increase in feeding initially. When the dog food was re-established, however, there was very little feeding for about one week. Although reproduction was markedly curtailed by starvation, females fed sucrose continued reproducing for at least 59 days. The results suggested that the roaches were mainly energy limited. Although the consumption of roaches was strongly affected by reserve depletion, eating was a small component of the time budget, and overall activity was relatively unchanged by starvation or sugar-feeding. There was a slight decrease in activity during starvation, but the circadian pattern remained unchanged. Females carrying o?thecae were highly active and were apparently attracted to food (even though they ate little). Thus the activity of the population was dominated by circadian rhythmicity and the reproductive cycle.     

Why doLuehdorfia butterflies lay eggs in clusters?

Summary Luehdorfia butterflies lay eggs in clusters. Clones of their host plants (Asiasarum andHeterotropa) are distributed pacthily among the understory of deciduous forests. Groups ofLuehdorfia larvae often exhaust the clones and may wander over the forest floor seeking new clones. The highest mortality observed is during this wandering period. To elucidate whyLuehdorfia butterflies lay eggs in clusters, a simulation experiment was made for hypothetical populations which lay eggs in clusters or singly. Field data on larval mortality, consumption, density of host clones and leaf weight forLuehdorfia japonica were incorporated into the model. The predictions of the simulation were: (1) When the egg density is low, the single egg type could leave many more pupae than the egg clustering type, but when the egg density is high, the former might leave smaller number of pupae than the latter; and (2) There are optimal sizes of egg clusters for different egg densities and the optimal size becomes larger as the egg density increased. This work was supported in part by Grant-in-Aid No. 439017 and No. 56480039 from the Ministry of Education, Science and Culture, Japan.     

The effects and limits of territoriality on population regulation in grey red-backed voles,Clethrionomys rufocanus bedfordiae

Summary The effects of breeding territoriality on the stability of grey red-backed vole (Clethrionomys rufocanus bedfordiae) populations were investigated on a control grid and a grid on which the voles were fed, in an outdoor enclosure in Hokkaido, Japan. Vole populations were monitored by live trapping from 1984 to 1986: (1) Population density was 2–7 times greater on the experimental grid to which food was added than on the control grid. Reproductive output was more closely associated with the difference in density between grids than survival or dispersal (immigration and emigration) rates. (2) The number of adult females and pregnancy rate of the experimental population were significantly greater than those of the control one. The difference in the number of adult females between the populations was greater than that in pregnancy rate. (3) The proportion of successful litters and the number of weanlings per litter were not significantly different between the control and experimental population. (4) Adult females held territories on both the control and experimental grid; they were spaced out more than would be expected from random occupation. The territories overlapped more on the experimental grid than on the control grid. (5) Mean territory size of adult females on the experimental grid was about half of that on the control grid. The territory size was correlated negatively with population density. (6) The proportion of trap sites that were used by adult females was significantly greater on the experimental grid than on the control grid. This suggests that adult females on the experimental grid used the area more extensively. This factor, in association with territory size and overlapping of territory, was also important in causing the difference in the number of adult females between the grids. (7) These results call into question the hypothesis that territoriality stabilizes the density in populations ofClethrionomys.     

An individual-based model for competingDrosophila populations

An individual-based model forDrosophila is formulated, based on competition amongst larvae consuming the same batch of food. The predictions of the model are supported by data for single speciesDrosophila populations reared in the laboratory. The model is used to build a simple discrete model for the dynamics ofDrosophila populations that are kept over a number of generations. The dynamics of a single species is shown to give either a stable equilibrium or fluctuations which can be periodic or chaotic. When the dynamics of a species in the absence of the other is periodic or chaotic, we found coexistence or two alternative states, on neither of which the species can coexist.     

The role of parasitoids in evolution of habitat and larval food plant preference by three Pieris butterflies

This article attempts to explain that parasitoids provide the evolutionary pressure responsible for relationships between habitat use and larval food plant use in herbivorous insects. Three species of butterflies of the genus Pieris, P. rapae, P. melete, and P. napi use different sets of cruciferous plants. They prefer different habitats composed of similar sets of cruciferous plants. In our study, P. rapae used temporary habitats with ephemeral plants, P. melete used permanent habitat with persistent plants, although they also used temporary habitats, and P. napi used only permanent habitat. The choice experiment in the field cages indicated that each of the three butterfly species avoided oviposition on plants usually unused in its own habitat, but accepted the unused plants which grew outside its own habitat. Their habitat use and plant use were not explained by intrinsic plant quality examined in terms of larval performance. Pieris larvae collected from persistent plants or more long lasting habitats were more heavily parasitized by two specialist parasitoids, the braconid wasp Cotesia glomerata and the tachinid fly Epicampocera succincta. The results suggest that Pieris habitat and larval food plant use patterns can be explained by two principles. The evolution of habitat preference may have been driven by various factors including escape from parasitism. Once habitat preference has evolved, selection favors the evolution of larval food plant preferences by discriminating against unsuitable plants, including those which are associated with high parasitism pressures. Received: December 3, 1998 / Accepted: January 20, 1999     

Phenological studies on the wing dimorphism of a semi-aquatic bug,Microvelia douglasi (Heteroptera: Veliidae)

Summary Seasonal changes in the percentage of macropterous individuals in natural population ofMicrovelia douglasi in permanent habitats were investigated in Shimane, western part of Japan. Macropterous forms of 5th instar nymphs occurred most frequently from late June to early July. However, the seasonal appearance of macropterous adults differed between habitats and years. There were significant effects of density, temperature, photoperiod and food availability on the percentage of macropters under laboratory conditions. The percentage of macropterous forms generally increased with density, but the mode of the response was modified by temperature, photoperiod and food availability. When insects were reared under long day photperiod (16L8D) with abundant food (0.5–1.0 g/container/day), the percentage of macroptery varied most among the different density treatments at the intermediate temperature (24°C) and the difference was reduced at higher temperature (28° and 32°C). Effects of temperature were small under the short day photoperiod (12L12D). Under 12L12D photoperiod sensitivity of density responses were higher at 32°C and lower at 24°C than those under 15L8D photoperiod. When the insects were supplied with food every other day, the density response became obscure. Ecological significance of environmental responses in the wing morph determination was discussed in relation to the heterogeneous environment of water surface.     

The ingestion in wolf spiders II. The expression of degree of hunger and amount of ingestion in relation to spider's hunger

Summary To express the degree of hunger during both of feeding and unfeeding periods of spiders, equations for the amount of ingestion and food disappearance from the gut were presented using three components: capacity of gut, rate of ingestion and rate of food disappearance. The degree of hunger was expressed by the rate of unfilled capacity of gut to the capacity of gut based on these equations. The rates of ingestion and food disappearance were estimated from the results of experiment with a wolf spider,Pardosa laura. The equations obtained well applied to the experimental result. By changing values of these rates, it was revealed that the ratio of rates of ingestion to food disappearance determine the amount of ingestion.     

Continuous population estimates forDendroctonus frontalis Zimm. (Coleoptera: Scolytidae) occurring in infestations

Summary Infestations ofDendroctonus frontalis Zimm. are often observed to enlarge continuously by the colonization of new hosts in a pattern similar to a forest fire. This pattern of infestation growth presents unique problems in quantitatively estimating populations ofD. frontalis. Beetle populations on each infested tree in an infestation go through five processes: attack, oviposition, reemergence, survivorship, and emergence. These processes, which have been described mathematically in the literature, each take several days for completion. In order to follow the distribution and abundance ofD. frontalis throughout the course of development of a spot, we need a daily estimate of the number of beetles involved in each process on every tree. Since it is not practical to sample each tree daily, we developed a procedure whereby quantitative estimation procedures for within-tree populations were used in combination with the mathematical models for the life processes to produce a daily record of the number of adults successfully attacking trees, the number of eggs oviposited, the number of beetles reemerging, number of beetles surviving within the trees, and the number of beetles emerging. These daily estimates were then summarized for all trees in the spot for the duration of the infestation. The daily record of populations ofD. frontalis, used with information on infestation geometry, were suggested to be of value in describing and elucidating several important facets of population dynamics including dispersal patterns within infestations, between tree beetle loss (mortality), and time lags among the various population processes. The information reported can be used to develop simulation models of population dynamics or to validate existing models. Texas Agric. Experiment Stn. TA No. 14689.