Size-dependent survivorship in the web-building spiderAgelena limbata

Summary Stage-specific mortality rates and mortality factors for the web-building spiderAgelena limbata, which is suggested to be food-limited, were studied, and the relationship between body size of spiders and survivorship for instar 3 to adults was examined. The mortality rate of the egg sac stage including eggs, deutova (prenymphal stage), and overwintering instar 1 nymphs was low. The low mortality of this stage was partly due to maternal care that reduced the mortality caused by predation and/or abiotic factors. From emergence of instar 1 nymphs from egg sacs to reproduction, the stagespecific mortality rates were almost constant, 32–47%, and the time-specific mortality rates were also constant. These results suggest a Deevey (1947) type II survivorship curve inA. limbata, in contrast to other reports on the wandering or burrowing spiders which suggested type III curves. Important mortality factors for nymphs and adults were parasitism by an ichneumonid wasp and predation by spiders. There were great variations in body size (carapace width) ofA. limbata in the field. Smaller individuals survived at a lower rate to the next stage than larger individuals. This tendency was clearer for the population living under poorer prey availability.A. limbata was unlikely to starve to death in the field because every stage ofA. limbata could survive starvation for a long time in the laboratory, 22–65 days on average. I suggest that the size-dependent survivorship of this spider is associated with vulnerability of smaller individuals to parasitism and predation.     

Exploring the foraging environment of a natural enemy of Callosobruchus maculatus: Spatial egg distribution in stored cowpea

Knowledge of the spatial distribution of stored product insects may reduce the dependency on chemicals for control of these insects. Biological control, for instance, could be improved based on such knowledge. In this paper we describe the three-dimensional spatial oviposition pattern of Callosobruchus maculatus in stored cowpea. Individual C. maculatus females oviposited in clusters of 70±15 (SD) eggs. These clusters were variable in shape. In any cluster 90 to 95% of the eggs fitted into a volume of 19.1±3.5 cm³. The egg density was highest (0.6 eggs bean^–1) at the center of a cluster and decreased towards the periphery. A statistically significant relationship existed between the number of eggs n in a cluster and the cluster volume, V(cm³): V=11.5+0.11n. We also investigated the spatial egg distribution of beetles which emerged from egg clusters such as those produced by individual females. Their oviposition was not confined to one specific area but was scattered throughout the bean mass. A point pattern analysis showed that the density of the `parent' cluster had no effect on the spatial egg pattern. These results give insight into the foraging environment which the egg parasitoid Uscana lariophaga, a promising candidate for biological control of C. maculatus, is facing. We argue that the probability p of encountering at least one other bean with eggs after a parasitization is a function of the number n of beans that are visited: p=1–0.42 (0.37)^(n–1).     

SPATIAL DISTRIBUTION,SURVIVAL, AND FECUNDITY OF DIOON EDULE (ZAMIACEAE) IN A TROPICAL DECIDUOUS FOREST IN VERACRUZ,MEXICO, WITH NOTES ON ITS HABITAT

A natural habitat of the cycad Dioon edule Lindl. in central Veracruz, Mexico is described. Prolonged drought periods and fire are factors contributing to seedling death resulting in a population structure with a reverse ‘J’ or Deevey type III survivorship curve. The cycads exhibited a ‘clumped’ or contagious distribution on shallow soils of the habitat and on rocky outcrops. This distribution was attributed to exclusion by competition from other species for water, soil nutrients, and/or seed dispersal by rodents. Estimated coning frequency varied from 2.8 to 8.8 years in male plants and from 10.0 to 52.0 years in female plants, though shorter intervals are known to occur, especially following exceptionally wet years. Chamberlain's age estimate for D. edule has been confirmed and a life table is presented.     

Comparison of the physical properties of real and artificial salmonid eggs and of their performance when drifting in an experimental stream channel

The paper compares three batches of brown trout (Salmo trutta Linnaeus) eggs, two batches of Atlantic salmon (Salmo salar Linnaeus) eggs and the artificial trout eggs described by Ottaway (1981), in terms of their measurable physical characteristics. Attention was given to the effects of temperature and of the stage of development of the real eggs. Comparisons were also made between brown trout eggs and artificial eggs of the rate of fall in a water column, of the value and speed of attainment of terminal velocity when falling through water and of the pattern of settlement within an experimental stream channel.The main measurable characteristics of brown trout eggs (after water hardening) and of artificial eggs (values in parentheses) were: diameter 0.52–0.54 mm (0.54), fresh weight 0.078–0.086 g (0.09), density 1.071–1.075 g ml^–1 (1.066), and volume 0.073–0.081 ml (0.085). There was no evidence of any major change in these values as egg development proceeded. Salmon eggs had a similar density to trout and artificial eggs but had 30 to 70% greater weight and volume. The water-hardening of eggs caused their dry matter content to fall from 37–39% to 31–34% and there were corresponding increases in volume and fresh weight.The rate of fall of trout eggs and artificial eggs through water when timed from rest at the water surface to a depth of 125 cm was similar for both types of egg at 10 °C. This rate rose with increasing temperature at the same low rate of c. 0.02 cm s^–1 °C^–1 for both types of egg. Both types of egg had a terminal velocity of c. 8.8 cm s^–1 and both achieved terminal velocity in less than 1.5 s after release.The patterns of settlement of the two types of eggs in an experimental channel were similar.     

Analysis of contagiousness in the action of mortality factors on the western tent caterpillar population by using the\mathop m\limits^* - m relationshiprelationship

The contagiousness in the operation of mortality processes on the colonies of the western tent caterpillar, Malacosoma californicum pluviale, was analyzed from two different aspects: successive changes in the frequency distribution of the number of surviving individuals per colony in the course of development, and the distribution pattern of the individuals killed by some biotic mortality factors. Also, for a tachinid parasite, Tachinomyia similis, the analysis was made on the egg-laying pattern on colonies as well as on individual larvae. The methods of these analyses were all based on the relation of mean crowding (m) on mean . A braconid parasite, Rogas sp., tended to kill few individuals together once it attacked a colony, but its effect on host colonies was rather equivalent to the random removal of individuals from all the colonies. Diseases in the late-stage larvae before cocooning was contagious in their action. Nuclear polyhedrosis virus seemed to have no basic contagiousness in its action, but it caused highly contagious distribution of deaths among the colonies when its average incidence was high. A spore-forming Bacillus had a tendency to kill several individuals once it appeared in a colony, but the distribution of its incidence (no. of times it appeared per colony) was considered to be nearly at random. The female of Tachinomyia tended to lay more than one egg successively on the same colony. It also attacked individual larvae with a definite tendency for aggregation, which seemed to be resulted from the parasite's preference to large hosts. When the number of eggs laid on prefered hosts exceeded a certain threshold, however, the fly seemed to change its attention to less attractive, smaller individuals.     

Clumped distribution of oak leaf miners between and within plants

Leaf miners typically show non-random distributions both between and within plants. We tested the hypothesis that leaf miners on two oak species were clumped on individual host trees and individual branches and addressed whether clumping was influenced by aspects of plant quality and how clumping and/or interactions with other oak herbivores affected leaf-miner survivorship. Null models were used to test whether oak herbivores and different herbivore guilds co-occur at the plant scale. Twenty individual Quercus geminata plants and 20 Quercus laevis plants were followed over the season for the appearance of leaf miners and other herbivores, and foliar nitrogen, tannin concentration, leaf toughness and leaf water content were evaluated monthly for each individual tree. The survivorship of the most common leaf miners was evaluated by following the fate of marked mines in several combinations that involved intra- and inter-specific associations. We observed that all leaf miners studied were clumped at the plant and branch scale, and the abundance of most leaf-miner species was influenced by plant quality traits. Mines that occurred singly on leaves exhibited significantly higher survivorship than double and triple mines and leaves that contained a mine or a leaf gall and a mine and damage by chewers exhibited lowest survivorship. Although leaf miners were clumped at individual host trees, null model analyses indicated that oak herbivores do not co-occur significantly less than expected by chance and there was no evidence for biological mechanisms such as inter-specific competition determining community structure at the plant scale. Thus, despite co-occurrence resulting in reduced survivorship at the leaf scale, such competition was not strong enough to structure separation of these oak herbivore communities.     

Early survivorship of juvenile coral reef fishes

Data on early survivorship of newly settled reef fish were collected by monitoring individuals which recruited to 30 small lagoonal patch reefs over three summers. Preliminary survivorship curves spanning the first 45 days after settlement were derived for 17 species. Most species showed greatest rates of mortality in the first 1–2 weeks in the reef environment however there were substantial differences among species in the extent and the temporal pattern of this. In six species, 75% of individuals survived the 45 days, while in 5 others, 20% or fewer survived that long. In eight species, mortality was negligible after the first 14 days. In the other 9, significant mortality occurred in subsequent weeks. Patterns of survivorship did not appear to differ substantially among years in five of the six species for which data were adequate. In particular, survivorship did not appear to be different among years even when levels of recruitment varied greatly.     

Estimating cladoceran birth rate: use of the egg age distribution to estimate mortality of ovigerous females and eggs

The birth rate of natural cladoceran populations can change rapidly (during 2–3 days), reflecting rapid changes in their environment. If the egg ratio is calculated on the basis of egg age distribution, the birth rate can be estimated at short sampling intervals (shorter than egg stage duration) by modified Paloheimo's (1974) formula. When female size structure and age of eggs in clutches at the beginning and the end of a sampling interval are known, death rates of ovigerous females and eggs in separate size classes can be determined and incorporated in birth rate estimates. All these methods have been employed using the data on the population of Diaphanosoma brahyurum from the lake Obsterno (North-Western Belarus) in July–August, 1992. The birth rate values computed by the proposed methods and Poloheimo's formula differed significantly in many cases. The accuracy of birth rate estimations from various calculation methods was tested using a computer simulation. The model contains the essential features of cladoceran life history: distinct egg, juvenile and adult stages, development of eggs and reproduction. The population was divided into 25 age classes, each of 1 day duration. Durations of the egg, juvenile and adult stages were set at 3, 6 and 20 days, respectively. The embryogenesis was divided into three egg stages, each of 1 day duration. Survivorship was set from 0.2 up to 1.0 for each age class. The survivorship and brood size were changed through each of five time intervals (days) that allowed to simulate an increase or reduction of population density. Fecundity, survivorship and egg stage duration remained constant during each of 5 days that assumed stability of an environment (this does not occur in nature). Nevertheless, the egg ratio, proportion of juveniles and birth rates were variable even under these circumstances. Computer simulations showed that Poloheimo's formula evaluates birth rate with the relative error of 62% and usually overestimates its values. We propose methods to decrease errors of birth rate estimations by 3.5–5.5 times.     

Geographic Distribution of Mitochondrial DNA Variations among Grivet (Cercopithecus aethiops aethiops) Populations in Central Ethiopia

Since mitochondrial DNA (mtDNA) are maternally inherited without recombination, geographic distribution of mtDNA in semiterrestrial cercopithecines is considered to be influenced by female philopatry. I examined the effect of sex difference in migration patterns on geographic distribution in a habitat whose environment has changed frequently. I investigated ten groups (n = 77) of grivets (Cercopithecus aethiops aethiops) along a 600-km stretch of the Awash River, Ethiopia. I examined the mtDNA distribution among natural local populations whose nuclear variation was already shown to have a widely homogeneous distribution. RFLP analysis of whole mtDNA genome using 17 enzymes identified ten haplotypes in five clusters (haplogroups). Sequence divergence within haplogroups ranged from 0.17%–0.38%, while divergence between haplogroups ranged between 1.0%–2.5%. Haplogroups were distributed in blocks which ranged from 120–250 km along the Awash River. The haplotype distribution pattern of males indicated that they migrate between the boundaries of these blocks. Moreover, a clumped distribution pattern suggests the history of matrilineal distribution by group fission and geographic expansion.     

The combined use of acoustic tracking and echosounding to investigate the movement and distribution of common bream (Abramis brama) in the River Trent,England

Spatial behaviour and distribution of fishes along a 7.6-km lowland reach of the River Trent, England, were examined using two complementary telemetry techniques: acoustic tracking to assess the movement and activity of common bream Abramis brama (L.) and quantitative echosounding for measuring the density and distribution of fish shoals. Nine adult bream (39.3–53.2 cm) were tracked by means of intraperitoneally implanted acoustic transmitters from 19 July to 12 September 2000. Home range size varied between 0.35 and 5.40 km of river length over this period. Bream were relatively inactive during daylight hours, began moving near dusk, and tended to move throughout the night. A distinct daytime residence area was occupied by most tagged fish on most occasions, while river use at night was more variable between individuals. Mobile echosounding surveys, with the transducer beaming horizontally across the river, conducted at night between July and September 2000, showed a highly contagious fish distribution within the study reach. For 200-m sections of river, there was a negative correlation between the relative frequency of acoustic tracking fixes at night and mean fish densities, as measured by echosounding for targets larger than –50 dB (c. 5-cm long). However, there was a highly significant positive rank correlation between the relative frequency of acoustic tracking fixes and acoustic targets larger than –30 dB (c. 22-cm long), most of which in this river are bream. This suggests that telemetry and echosounding can, in this part of the River Trent, be combined to provide valuable spatial information at individual and population scales for bream.     

On the failure of two butterfly species to respond to the presence of conspecific eggs prior to oviposition

Abstract. 1. The eggs of Euptychiine butterflies are laid singly, but their distributions tend to be contagious. However, these insects do not discriminate between egg-laden and egg-free plants.
2. Response to conspecific eggs is not part of the mechanism responsible for clumped egg distribution.     

The effect of donor sex and age on the number of sister chromatid exchanges in human lymphocytes growing in vitro

Summary After analysing the distribution of the numbers of sister chromatid exchanges (SCE) in 200 cells from one individual, we found no evidence to reject the hypothesis of a normal distribution (P>0.50). We then compared the mean numbers of SCE per cell in 18 individuals, three males and three females of each of three age groups (0–10, 30–40, and 60–70 years of age) by means of a one-way analysis of variance, and found that there was no significant difference among them at the level of 5%. When these data were analysed by means of a two-way analysis of variance to test separately the effect of sex and age, we found that the number of SCE per cell does not differ significantly between sexes, but differs with age (P>0.05). People in the age bracket 30–40 years have a higher number of SCE per cell. Age seems to affect both sexes equally.Supported by The Medical Research Council of Ireland     

Distributional responses to small-scale habitat variability by early juvenile silver hake, Merluccius bilinearis

Early juvenile (0-year) silver hake, Merluccius bilinearis, occurred at higher densities on bottoms with greater amphipod tube cover at a 55 m deep site in the Middle Atlantic Bight. The size range of individuals at this site was 1.5–5 cm total length. Additional observations at other sites showed that 0-year silver hake only occurred on silt-sand bottom with amphipod tubes at bottom water temperatures of 8.7–11.4 °C. This distribution pattern could be the result of either selective settlement into appropriate habitats or differential predation with higher survivorship in more complex habitats. We posit that 0-year silver hake occur in patches of dense amphipod tube cover to avoid visual predators and co-occur with preferred prey (i.e., amphipods and shrimps).     

Quantitative defense theory and patterns of feeding by oak insects

Summary Patterns of herbivory over a two year period on Quercus emoryi (Fagaceae) were correlated with seasonal and yearly changes in tannin and protein content. Quantitative defense theory predicts that tannin and protein content in apparent plants should be negatively and positively correlated, respectively, with degree of herbivory. Most herbivory occurred early in the growing season, but the pattern varied between the two years. Tannin and protein content sometimes varied negatively and sometimes positively with degree of herbivory; they did not consistently covary with herbivory. Protein content was positively correlated with herbivory in 1981–1982 but not in 1982–1983. Condensed tannin content was negatively correlated with herbivory in 1981–1982 but not in 1982–1983. Hydrolysable tannin content was positively correlated with herbivory activity. Multiple regression analyses indicated these phytochemical variables explained either no significant variation in herbivory (1982–1983) or did so in a fashion opposite (1981–1982) to the predictions of the theory of quantitative defense.Feeding by oak insects was not solely a function of seasonal changes in quantitative defenses and nutrients. Obviously, population dynamics of the insects are sensitive to factors other than phytochemistry of the trees and I discuss other factors that can influence patterns of herbivory.     

Fine-scale temporal variation in recruitment of a temperate demersal fish: the importance of settlement versus post-settlement loss

In order to understand variability in recruitment to populations of benthic and demersal marine species, it is critical to distinguish between the contributions due to variations in larval settlement versus those caused by post-settlement mortality. In this study, fine-scale (1–2 days) temporal changes in recruit abundance were followed through an entire settlement season in a temperate demersal fish in order to determine 1) how dynamic the process of recruitment is on a daily scale, 2) whether settlement and post-settlement mortality are influenced by habitat structure and conspecific density, and 3) how the relationship between settlement and recruitment changes over time. Settlement is considered to be the arrival of new individuals from the pelagic habitat, and recruitment is defined as the number of individuals surviving arbitrary periods of time after settlement. Replicate standardized habitat units were placed in 2 spatial configurations (clumped and randomly dispersed) and monitored visually for cunner (Tautogolabrus adspersus) settlement and recruitment every 1–2 days throughout the settlement season. The process of recruitment in T. adspersus was highly variable at a fine temporal scale. Changes in the numbers of recruits present on habitat units were due to both settlement of new individuals and mortality of animals previously recruited. The relative importance of these two processes appeared to change from day to day. The magnitude of the change in recruit number did not differ between the clumped and random habitats. However, post-settlement loss was significantly greater on randomly dispersed than clumped habitats. During several sampling dates, the extent of the change in recruit abundance was correlated with the density of resident conspecifics; however, on other dates no such relationship appeared to exist. Despite the presence of significant relationships between the change in recruit number and density, there was no evidence of either density-dependent mortality or settlement. Initially, there was a strong relationship between settlement and recruitment; however, this relationship weakened over time. Within 2 months after the cessation of settlement, post-settlement loss was greater than 99%, and no correlation remained between recruitment and the initial pattern of settlement. The results of this study demonstrate that the spatial arrangement of the habitat affects the rate and intensity of post-settlement loss. Counter to much current thinking, this study suggests that in order to understand the population ecology of reef fishes, knowledge of what habitats new recruits use and how mortality varies with structural aspects of the habitats is essential.     

Spatial distribution of Eurya japonica in an old-growth evergreen broad-leaved forest,SW Japan

Abstract. The spatial distribution of Eurya japonica, an evergreen understorey species, was studied on a 4-ha permanent plot in an old-growth, temperate, evergreen broad-leaved forest in southwestern Japan. The pattern of spatial dispersion varied with size class, the environmental heterogeneity produced by tree-fall gaps and microtopography and the scale of observation. Seedlings (0.05 to < 0.5 m high) had the most clumped distributions, followed by saplings (0.5 to 1.3 m high) and adults (> 1.3 m). Individuals were more clumped at the smallest grid size, 1.56m × 1.56 m, and became less clumped as grid size increased for all size classes. On grids of less than 6.25 m × 6.25 m, no spatial correlations were found between seedlings and larger size classes; positive spatial correlations were found between saplings and adults at this scale. Abundant seedlings appeared in tree-fall gaps formed by uprooted trees, but seedlings were frequent on ridge tops and upper slopes. Saplings and adults also occurred on upper slopes, but their spatial distribution had little relation to current tree-fall gaps. The spatial distribution of E. japonica individuals was largely attributed to the distribution of tree-fall gaps and the differential responses to microtopographic variation by individuals at different size classes.     

Woody vegetation spatial patterns in a semi-arid savanna of Burkina Faso,West Africa

Spatial patterns of woody individuals were studied in a semi-arid savanna of West Africa located in Burkina Faso at and around 14° 12 N and 2° 27 W. The study was based upon a 10.24 ha plot within which individuals were mapped. Spatial pattern analysis was carried out using second order characteristics of point processes as K functions and pair correlations. The overall density amounted to 298 individuals ha^-1. The most abundant species were Combretum micranthum G. Don., Grewia bicolor Juss. and Pterocarpus lucens Lepr. Anogeissus leiocarpus (D.C.) G. et Perr. was also an important constituant of this vegetation type, owing to its taller stature. Clumped spatial distributions were identified for all species except for two, for which complete spatial randomness (CSR) was found (including P. lucens, a dominant woody plant). No regular pattern was found even when tall individuals were considered alone. Aggregation dominates interspecific relationships, resulting in multispecific clumps and patches. The overall aggregation pattern was constituted by two different structures. A coarse-grain pattern of ca. 30–40 m was based on edaphic features, and expresses the contrast between sparse stands on petroferric outcrops and denser patches on less shallow soils. A finer-grain pattern made of clumps ca. 5–10 m wide, with no obvious relation to pre-existing soil heterogeneity. There was no overall pattern for saplings (between 0.5 m and 1.5 m in height) irrespective of species, and thus no obvious common facilitation factor. For species with a high recruitment level there was no significant relationship between mature adult and saplings. The only case of clumped saplings with randomly distributed adults was found in P. lucens. However, this cannot be unequivocally interpreted as density dependent regulation since the existence of such a process was not consistent with the spatial distribution of dead P. lucens individuals (victims of the last drought). The mean density around dead P. lucens was lower than around surviving ones, indicating that the last drought tended to reinforce clumping rather than promote a regular pattern of trees. Spatial pattern analysis yielded no evidence supporting a hypothesis of stand density regulation through competition between individuals. Other processes, as surface sealing of bare soils or insufficient recruitment, may play a more important role in preventing a savanna-like vegetation from turning into denser woodlands or thickets.     

白马雪山阳坡林线方枝柏种群的生态特征

方枝柏(Sabina saltuaria)是青藏高原特有林线树种。对白马雪山阳坡海拔4 390 m以上林线0.42 hm²样地方枝柏种群进行每木调查,分析了种群结构、数量动态及空间分布格局。结果表明:1)在种群生物学特征、环境因子及人为干扰等综合影响下,林线区方枝柏幼苗数量非常少。幼树在种群中占了很大比重,种群个体数随径级的增加而逐渐减少,密度为成年树>幼树>幼苗;2)种群存活曲线介于Deevey-Ⅱ型和Deevey-Ⅲ型之间,种群结构更接近稳定型。死亡率曲线和消失率曲线都在龄级Ⅲ出现一个高峰,可能是由于方枝柏处于青壮年期,个体对营养空间的需求不断增大,对空间、光照和养分等生存因子的激烈竞争引起的自疏过程,导致死亡率有所上升;3)方枝柏种群各龄级的空间格局基本上是聚集型,随龄级增加,聚集尺度和强度都增加;各龄级关系密切,都呈显著正相关,其中幼苗和成年树的相关性最强,幼苗和幼树相关性最弱。林线方枝柏种群空间分布格局是种群生物学特性、环境条件及人为干扰等因素共同作用的结果,这种格局有利于整个种群的生存和发展,也是高山生态系统恶劣生境中种群的一种适应对策。该研究采用森林罗盘仪与测距仪相结合进行每木定位的方法,准确度高且大大降低工作强度,是高海拔区恶劣条件下开展类似工作的推荐之法。研究结果也再次体现了空间点格局分析方法在格局研究中的有效性和优越性。     

Egg production rates of the neritic marine copepod Acartia tumida Willey in the Aleutian Archipelago

Acartia tumida, a neritic copepod of the northern North Pacific Ocean and Bering Sea, is an unusually large member of its genus, adult females measuring 2.0–2.4 mm in total length. In the summers of 1986 and 1987 we investigated egg production of A. tumida in nearshore habitats of several islands in the Aleutian Island chain. A. tumida was found within protected embayments, where it could reach adult densities as high as 1000m^–3. Highest egg production rates were measured at Amchitka Island (up to 86 eggs copepod^–1 d^–1 at 6°C), where the phytoplankton was dominated by chain-forming Thalassiosira spp. In situ egg production rates at Amchitka were more than twice as high as maximum rates measured with cultured T. weissflogii, a single cell diatom, or during blooms of chain-forming Chaetoceros spp. at Adak and Kiska Islands. Approximately 12–24 h was necessary for recent feeding to be reflected in egg production. At high food concentrations 75% of spawning occurred at night and in discrete clutches, a pattern not observed at a lower food concentration.     

Population structure of the rare damselfly,Ischnura gemina (Kennedy) (Odonata: Coenagrionidae)

Summary The population structure of the endemic San Francisco Bay Area damselfly, Ischnura gemina, is examined using mark-recapture methods. Average daily movements, sex ratios, population size, maturation times, survivorship, and dispersion patterns, were recorded and calculated from two small (each less than one hectare) sites 150 m apart in Glen Canyon, San Francisco. Of 563 adults marked over 36 days, 412 (73%) were recaptured at least once. Average daily movements for males and females were less than 6 m, suggesting local movements. However, directional movements of 150 m were observed from one site to the other, indicating dispersal potential. One of the populations was a satellite composed entirely, of emigrating individuals from the other site; no larvae or teneral adults were found at the satellite area. Males were more aggregated than females at both sites. Both sexes were highly clumped at one site but were nearly randomly dispersed at the other site. Total population size for both sexes tended to be constant throughout the sampling period, at about 250. Adult population estimates showed more males were present than, females, but larval counts at one site indicated only a slight excess of males. Average life span estimates ranged from 6.5 days (females) to 23.3 days (males). One male lived at least 36 days. Maturation time for males was about 5–7 days, 7–10 days for females. A long life span and long flight season (March to November) are probably adaptations to the foggy San Francisco climate. All populations of I. gemina located to date are small, possibly originating from founders from nearby demes, and may be subject to different selection pressures. The dispersal potential of I. gemina may increase its chance of survival should small urban demes be threatened with destruction.