Parasitoid-induced mortality of Araneus omnicolor (Araneae, Araneidae) by Hymenoepimecis sp. (Hymenoptera, Ichneumonidae) in southeastern Brazil

All species included in the Polysphincta genus-group develop as ectophagous parasitoids of active spiders, killing their hosts prior to pupation. However, little information regarding natural history and ovipositing behavior of most species are available. In this study we inspected 85 webs of Araneus omnicolor to evaluate the frequency of parasitism and host size preferences of the wasp Hymenoepimecis sp. We also described the web characteristics of normal and parasitized spiders and the wasp ovipositing behavior. About 41% of the adult females of A. omnicolor inspected were parasitized. The highest incidence of parasitism was observed among relatively small females while no egg or larva was found in large individuals. Araneus omnicolor builds a strong web composed of an orb and barrier threads, where the spider rests within a curled leaf. The parasitoid larva builds its cocoon within this refuge, and modified cocoon webs were not observed. The ovipositing behavior of Hymenoepimecis sp. was very similar to that of Hymenoepimecis argyraphaga parasitizing Leucauge argyra, including the position of the sting, the killing of a previously attached larva, and the expelling of the egg from the base of the ovipositor.     

Mass predicts web asymmetry in Nephila spiders

The architecture of vertical aerial orb webs may be affected by spider size and gravity or by the available web space, in addition to phylogenetic and/or developmental factors. Vertical orb web asymmetry measured by hub displacement has been shown to increase in bigger and heavier spiders; however, previous studies have mostly focused on adult and subadult spiders or on several size classes with measured size parameters but no mass. Both estimations are suboptimal because (1) adult orb web spiders may not invest heavily in optimal web construction, whereas juveniles do; (2) size class/developmental stage is difficult to estimate in the field and is thus subjective, and (3) mass scales differently to size and is therefore more important in predicting aerial foraging success due to gravity. We studied vertical web asymmetry in a giant orb web spider, Nephila pilipes, across a wide range of size classes/developmental stages and tested the hypothesis that vertical web asymmetry (measured as hub displacement) is affected by gravity. On a sample of 100 webs, we found that hubs were more displaced in heavier and larger juveniles and that spider mass explained vertical web asymmetry better than other measures of spider size (carapace and leg lengths, developmental stage). Quantifying web shape via the ladder index suggested that, unlike in other nephilid taxa, growing Nephila orbs do not become vertically elongated. We conclude that the ontogenetic pattern of progressive vertical web asymmetry in Nephila can be explained by optimal foraging due to gravity, to which the opposing selective force may be high web-building costs in the lower orb. Recent literature finds little support for alternative explanations of ontogenetic orb web allometry such as the size limitation hypothesis and the biogenetic law.     

Does ontogenetic change in orb web asymmetry reflect biogenetic law?

Most orb web spiders face downward on the web hub, and their webs are vertically asymmetrical, that is, the lower part of the web is larger than the upper part and the ratio of the lower part to the whole web area increases as the spider grows. This phenomenon may reflect biogenetic law such that young animals exhibit a general ancestral trait whereas adults exhibit specific and derived traits. An alternative explanation is that vertical asymmetry may arise from the difference in time required by spiders to move up or down the web to capture prey. The present study tested these two hypotheses for Eriophora sagana. Subadults of this species build their webs with reverse asymmetry in that the upper part of the web area is larger than the lower part. In both subadults and adults, the upper proportion decreased with spider weight, and adult spiders built more symmetric webs. These results support the capture time difference hypothesis.     

Reverse positional orientation in a neotropical orb-web spider, <Emphasis Type="Italic">Verrucosa arenata</Emphasis>

Most orb-web spiders face downwards in the web. A downward orientation has been proposed to be the optimal strategy because spiders run faster downwards and thus can catch prey quicker. Consequently, orb-web spiders also extend their web in the lower part, leading to top-down web asymmetry. Since the majority of orb-web spiders face downwards, it has been difficult to test the effect of orientation on prey capture and web asymmetry. In this study, we explored the influence of reverse orientation on foraging efficiency and web asymmetry in Verrucosa arenata, a neotropical orb-web spider that faces upwards in the web. We show that reverse orientation does not imply reverse web asymmetry in this species. V. arenata spiders captured more prey in the lower part of the web but more prey per area on the upper part. The average running speeds of spiders did not differ between upward and downward running, but heavier spiders took longer to capture prey while running upwards. We discuss these findings in the context of foraging efficiency and web asymmetry.     

Optimal foraging, not biogenetic law, predicts spider orb web allometry

The biogenetic law posits that the ontogeny of an organism recapitulates the pattern of evolutionary changes. Morphological evidence has offered some support for, but also considerable evidence against, the hypothesis. However, biogenetic law in behavior remains underexplored. As physical manifestation of behavior, spider webs offer an interesting model for the study of ontogenetic behavioral changes. In orb-weaving spiders, web symmetry often gets distorted through ontogeny, and these changes have been interpreted to reflect the biogenetic law. Here, we test the biogenetic law hypothesis against the alternative, the optimal foraging hypothesis, by studying the allometry in Leucauge venusta orb webs. These webs range in inclination from vertical through tilted to horizontal; biogenetic law predicts that allometry relates to ontogenetic stage, whereas optimal foraging predicts that allometry relates to gravity. Specifically, pronounced asymmetry should only be seen in vertical webs under optimal foraging theory. We show that, through ontogeny, vertical webs in L. venusta become more asymmetrical in contrast to tilted and horizontal webs. Biogenetic law thus cannot explain L. venusta web allometry, but our results instead support optimization of foraging area in response to spider size.     

Remote monitoring of vibrational information in spider webs

Spiders are fascinating model species to study information-acquisition strategies, with the web acting as an extension of the animal’s body. Here, we compare the strategies of two orb-weaving spiders that acquire information through vibrations transmitted and filtered in the web. Whereas Araneus diadematus monitors web vibration directly on the web, Zygiella x-notata uses a signal thread to remotely monitor web vibration from a retreat, which gives added protection. We assess the implications of these two information-acquisition strategies on the quality of vibration information transfer, using laser Doppler vibrometry to measure vibrations of real webs and finite element analysis in computer models of webs. We observed that the signal thread imposed no biologically relevant time penalty for vibration propagation. However, loss of energy (attenuation) was a cost associated with remote monitoring via a signal thread. The findings have implications for the biological use of vibrations by spiders, including the mechanisms to locate and discriminate between vibration sources. We show that orb-weaver spiders are fascinating examples of organisms that modify their physical environment to shape their information-acquisition strategy.     

Spider orientation and hub position in orb webs

Orb-web building spiders (Araneae: Araneoidea, Uloboridae) can be considered as territorial central place foragers. In territorial central place foragers, the optimal foraging arena is circular, with the forager sitting in its centre. In orb webs, the spider’s orientation (head up or head down) whilst waiting for prey on the hub of its web and the downwards–upwards asymmetry of its running speeds are the probable causes for the observed deviation of the hub from the web’s centre. Here, we present an analytical model and a more refined simulation model to analyse the relationships amongst the spider’s running speeds, its orientation whilst waiting for prey and the vertical asymmetry of orb webs. The results of our models suggest that (a) waiting for prey head down is generally favourable because it allows the spider to reach the prey in its web on average quicker than spiders waiting head up, (b) the downwards–upwards running speed asymmetry, together with the head-down orientation of most spiders, are likely causes for the observed vertical asymmetry of orb webs, (c) waiting head up can be advantageous for spiders whose downwards–upwards running speed asymmetry is small and who experience high prey tumbling rates and (d) spiders waiting head up should place their hub lower than similar spiders waiting head down.     

Optimal web investment in sub-optimal foraging conditions

Orb web spiders sit at the centre of their approximately circular webs when waiting for prey and so face many of the same challenges as central-place foragers. Prey value decreases with distance from the hub as a function of prey escape time. The further from the hub that prey are intercepted, the longer it takes a spider to reach them and the greater chance they have of escaping. Several species of orb web spiders build vertically elongated ladder-like orb webs against tree trunks, rather than circular orb webs in the open. As ladder web spiders invest disproportionately more web area further from the hub, it is expected they will experience reduced prey gain per unit area of web investment compared to spiders that build circular webs. We developed a model to investigate how building webs in the space-limited microhabitat on tree trunks influences the optimal size, shape and net prey gain of arboricolous ladder webs. The model suggests that as horizontal space becomes more limited, optimal web shape becomes more elongated, and optimal web area decreases. This change in web geometry results in decreased net prey gain compared to webs built without space constraints. However, when space is limited, spiders can achieve higher net prey gain compared to building typical circular webs in the same limited space. Our model shows how spiders optimise web investment in sub-optimal conditions and can be used to understand foraging investment trade-offs in other central-place foragers faced with constrained foraging arenas.     

Prey detection without successful capture affects spider’s orb-web building behaviour

Animals obtain information from past foraging experience to adjust their foraging activity according to their environment. The ability of spiders to obtain information from unsuccessful predation experiences was investigated by examining the effects on web building, a significant foraging investment, of prey detection without successful capture in the orb-web spider Cyclosa octotuberculata. Four treatments were employed: (1) successful capture and feeding: one syrphid fly was allowed to be captured and consumed by the spider on the web; (2) single prey-item detection: a syrphid fly was placed on the web to lure the spider, but was removed before capture; (3) five prey-item detection: above prey-item detection stimulus was given five times; and, (4) control: neither prey nor feeding on the web. While control spiders decreased the total thread length and capture area of their webs, prey-item detection spiders in both conditions increased them, indicating that the spider obtained information from unsuccessful predation experience to adjust their foraging investment. The fed spiders exhibited a significantly greater increase than the prey-detection-only spiders, suggesting that prey detection alone and prey detection with consumption had different informational effects. Total thread length did not differ between single and five prey-item detection spiders, but distance between two adjacent sticky spirals increased only in the former spiders, possibly because five times unsuccessful predations prevented spiders to reduce web stickiness. It suggests that the spider changed web morphology according to the number of prey detection.     

Host selection by a kleptobiotic spider

Why do kleptobiotic spiders of the genus Argyrodes seem to be associated with spiders of the genus Nephila worldwide? Observations following introduction of experimental insect prey of different sizes and weights on to host webs revealed that: (1) small prey are more effectively retained on the web of Nephila clavipes than on the web of another common host, Leucauge venusta. (2) N. clavipes did not consume small prey that accumulated on the web whereas larger, heavier prey were enveloped and stored. (3) We observed clear partitioning of prey items between N. clavipes and Argyrodes spp.; diet selection by Argyrodes did not overlap with that of N. clavipes but closely overlapped with that of L. venusta. (4) L. venusta responds very quickly to prey impact whereas N. clavipes is slower, offering a temporal window of opportunity for Argyrodes foraging. (5) The ability of L. venusta to detect and respond to small items also means that it acts aggressively to Argyrodes spp., whereas N. clavipes does not. Consequently, food-acquisition behaviours of Argyrodes were clearly less risky with N. clavipes compared with L. venusta. We conclude that when a kleptobiotic organism has a choice of various host species, it will opt for the least risky host that presents the highest rate of availability of food items. The fact that Nephila species present such characteristics explains the worldwide association with Argyrodes kleptobiotic spiders.     

Habitat patch size and isolation as predictors of occupancy and number of argyrodine spider kleptoparasites in Nephila webs

How fully a suitable habitat patch is utilized by organisms depends crucially on patch size and isolation. Testing this interplay is made difficult in many systems by the arbitrariness of defining a "habitat patch", measuring its boarders, and relatively low detection probability of the inhabitants. Spider webs as habitat patches for obligate web kleptoparasites are free from these problems. Each individual web is a highly discrete and readily measured habitat patch, and the detection probability of argyrodine spider kleptoparasites is very nearly 1. Hence, spider webs emerge as simple systems for ecological models such as patch occupancy and metapopulation biology. Recently, I showed that the distribution of kleptoparasites among host webs relates both to web (patch) size as well as patch connectivity. Here, I test the relative importance of patch size versus isolation in explaining patch occupancy and abundance of inhabitants. I find that (1) web size is the better predictor of patch occupancy and abundance. (2) Web size is overall positively correlated with abundance, but predicts it most precisely among interconnected webs and not at all among the most isolated webs. Hence, patch occupancy and inhabitant abundance is explained by a rather complex interplay between patch size and isolation.     

富营养化水体中光学活性物质的垂向分布及其对遥感反射光谱的影响

富营养化水体中光学活性物质的垂向分布对水下光场分布具有重要影响,决定了水色参数定量遥感反演的精度.基于2011年8月和11月在太湖进行的垂直分层采样,分析了水体中光学活性物质含量的垂向分布特征,并探讨了其影响因素及其对水体遥感反射光谱的影响.结果表明,风速是影响富营养化水体中光学活性物质垂向分布的重要因子之一.风速较小(小于3.0 m·s~(-1))或无风时,表层叶绿素a浓度急剧增加,易形成水华;而风速大于5.0 m·s~(-1)时,水体发生垂直混合,底层叶绿素a浓度最大;其它风速条件下,叶绿素a浓度在某一水深处出现极大值.在风速小于5.0 m·s~(-1)的情况下,悬浮物浓度和有色溶解有机物(CDOM)的垂向分布相对均一;当风速超过5.0 m·s~(-1)时,底泥再悬浮过程对两者的贡献较大.在未发生藻华情况下,0~0.5 m水深范围内的叶绿素a浓度均值与波段比值的相关系数最大(r=0.86),对水体遥感反射比的影响较大;而表层悬浮物浓度对水体遥感反射比的影响最大,研究结论可为水色参数含量的精确估算提供方法论支持.     

1957-2009年黄土高原地区风速变化趋势

用88个气象站1957-2009年的月平均风速和最大风速日值资料,采用距平累积法、 5 a趋势滑动法、 Mann-Kendall趋势检验法等分析了黄土高原地区风速的变化趋势及其空间分异等特征。结果表明:①黄土高原地区1957-2009年多年平均风速为2.36 m/s,水蚀区、 水蚀风蚀交错区和风蚀区年均风速分别为2.36、 2.17、 2.60 m/s,年际变化倾向率分别为-0.008 4、 -0.009 4和-0.018 8 m·s^-1·a^-1,并均通过了0.001的极显著性检验。3个区域均是冬、 春季的平均风速对全年趋势演变贡献率较大,年均风速也均在1981年发生偏强转为偏弱的跃变,20世纪70年代以后平均风速逐渐减小。②黄土高原平均风速减少的主要原因是最大风速为5级或5级以上的发生日数减少。大风频率从1970年代至2000年代呈显著减少趋势,风蚀区减少幅度最大,减少了10%以上,水蚀风蚀交错区减少1%~5%,到2000年代,大多数站点的大风频率均降低为<2%。③水蚀风蚀交错区和风蚀区年均大风日数较多,而水蚀区和黄土高原西部地区年均大风日数较少。根据大风年均发生日数,将大风天气划分为大风天气较少区(年均大风日数<10 d)、 较多区(10~50 d)、 多发区(50~100 d)和频发区(>100 d)。大风较多区在黄土高原地区分布最广,其次是较少区,无频发区。70年代至90年代,风蚀区和水蚀风蚀交错区的站点大多为大风较多区,其中70年代中宁和包头为大风多发区;2000年代以后,大部分地区转为大风较少区。     

Wind tunnel experiments on the resuspension of particulate material

A programme of wind tunnel experiments has been carried out in which the resuspension of particulate material of four sizes (4, 10, 18 and 22 μm diameter) from concrete and grass surfaces has been studied. The importance of wind speed has been investigated and the time dependence of the resuspension rate has been evaluated for periods shortly after the onset of resuspension. The results indicate rapidly falling values of the resuspension rate which are approximately related to 1/time. The increase in the resuspension rate with increasing wind speed was most pronounced for large particles, leading to the conclusion that resuspension can be dominated by short-duration episodes which coincide with conditions that favour resuspension. Resuspension rates, averaged over approximately 1 h after the onset of conditions leading to resuspension, were in the range 1 × 10⁻⁶ to 5 × 10⁻⁶ s⁻¹, at moderate wind speeds (less than 5 m s⁻¹), with little difference between the grass and concrete surfaces. For high wind speeds (greater than 6.5 m s⁻¹), several per cent of the total surface deposit of 22 μ diameter particles was removed from both the concrete and grass surfaces within 10 s of the onset of wind.     

风浪扰动对太湖水体中砷在水相-悬浮物相之间分配的影响

在太湖冬季不同风速下(0.8、1.8、2.7、3.2、4.0 m·s-1)采集了不同深度(水深0.1、1.0和1.8 m,分别记为上、中和下层)的水样,测定了水体和悬浮物中的砷含量及水质参数,计算了砷在两相之间的分配系数,以期了解在太湖生物活动最低的阶段,风浪扰动作用对砷在水相和悬浮物相之间分配的影响.结果显示,随着风浪作用的加强,水体总砷含量有所增加,相应的悬浮物总砷含量及砷的分配系数降低,说明风浪作用可以降低悬浮物吸附砷的能力,以至于促进砷从悬浮物中的释放,进而使水体中的溶解态砷增加.相关性分析及变化趋势比较分析发现,砷分配系数的降低与在风浪扰动的影响下悬浮物浓度和溶解氧的增加,以及叶绿素a浓度的下降有关.不同风速下各参数的垂向分布变化显示,风浪扰动对中下层的砷分配系数以及中层的悬浮物砷含量影响较大;风速变大时,中层溶解态砷含量升高,上层与下层溶解态砷含量降低;而风浪扰动作用对水体总砷的分层变化影响不大.悬浮物浓度、砷浓度及分配系数均在3~4 m·s-1风速下发生变化,且呈现与之前不同的分层特征.4 m·s-1可能是一个临界风速,在4 m·s-1以上的风速下风浪占主导作用,而4 m·s-1以下是湖流与风浪共同作用的结果.     

Web-building spiders attract prey by storing decaying matter

The orb-weaving spider Nephila edulis incorporates into its web a band of decaying animal and plant matter. While earlier studies demonstrate that larger spiders utilise these debris bands as caches of food, the presence of plant matter suggests additional functions. When organic and plastic items were placed in the webs of N. edulis, some of the former but none of the latter were incorporated into the debris band. Using an Y-maze olfactometer, we show that sheep blowflies Lucilia cuprina are attracted to recently collected debris bands, but that this attraction does not persist over time. These data reveal an entirely novel foraging strategy, in which a sit-and-wait predator attracts insect prey by utilising the odours of decaying organic material. The spiders habit of replenishing the debris band may be necessary to maintain its efficacy for attracting prey.     

Large pollen loads of a South African asclepiad do not interfere with the foraging behaviour or efficiency of pollinating honey bees

The pollen of asclepiads (Asclepiadoideae, Apocynaceae) and most orchids (Orchidaceae) are packaged as large aggregations known as pollinaria that are removed as entire units by pollinators. In some instances, individual pollinators may accumulate large loads of these pollinaria. We found that the primary pollinator of Cynanchum ellipticum (Apocynaceae—Asclepiadoideae), the honey bee Apis mellifera, accumulate very large agglomerations of pollinaria on their mouthparts when foraging on this species. We tested whether large pollinarium loads negatively affected the foraging behaviour and foraging efficiency of honey bees by slowing foraging speeds or causing honey bees to visit fewer flowers, and found no evidence to suggest that large pollinarium loads altered foraging behaviour. C. ellipticum displayed consistently high levels of pollination success and pollen transfer efficiency (PTE). This may be a consequence of efficiently loading large numbers of pollinaria onto pollinators even when primary points of attachment on pollinators are already occupied and doing so in a manner that does not impact the foraging behaviour of pollinating insects.     

Direct evidence of hybodont shark predation on Late Jurassic ammonites

Sharks are known to have been ammonoid predators, as indicated by analysis of bite marks or coprolite contents. However, body fossil associations attesting to this predator–prey relationship have never been described so far. Here, I report a unique finding from the Late Jurassic of western France: a complete specimen of the Kimmeridgian ammonite Orthaspidoceras bearing one tooth of the hybodont shark Planohybodus. Some possible tooth puncture marks are also observed. This is the first direct evidence of such a trophic link between these two major Mesozoic groups, allowing an accurate identification of both organisms. Although Planohybodus displays a tearing-type dentition generally assumed to have been especially adapted for large unshelled prey, our discovery clearly shows that this shark was also able to attack robust ammonites such as aspidoceratids. The direct evidence presented here provides new insights into the Mesozoic marine ecosystem food webs.     

Using Hidden Markov Models to characterise intermittent social behaviour in fish shoals

The movement of animals in groups is widespread in nature. Understanding this phenomenon presents an important problem in ecology with many applications that range from conservation to robotics. Underlying all group movements are interactions between individual animals and it is therefore crucial to understand the mechanisms of this social behaviour. To date, despite promising methodological developments, there are few applications to data of practical statistical techniques that inferentially investigate the extent and nature of social interactions in group movement. We address this gap by demonstrating the usefulness of a Hidden Markov Model approach to characterise individual-level social movement in published trajectory data on three-spined stickleback shoals (Gasterosteus aculeatus) and novel data on guppy shoals (Poecilia reticulata). With these models, we formally test for speed-mediated social interactions and verify that they are present. We further characterise this inferred social behaviour and find that despite the substantial shoal-level differences in movement dynamics between species, it is qualitatively similar in guppies and sticklebacks. It is intermittent, occurring in varying numbers of individuals at different time points. The speeds of interacting fish follow a bimodal distribution, indicating that they are either stationary or move at a preferred mean speed, and social fish with more social neighbours move at higher speeds, on average. Our findings and methodology present steps towards characterising social behaviour in animal groups.     

基于风廓线仪等资料的珠江三角洲污染气象条件研究

利用珠江三角洲区域空气质量资料和珠海、南沙、增城的逐30 min风廓仪观测资料,以及清远探空站每天08时和20时的温度探空资料等,研究了2012年10月珠江三角洲出现的大范围持续性污染天气期间的大气边界层特征.结果表明:2012年10月珠江三角洲出现的大范围持续性污染天气主要是受到弱冷空气南下和台风外围下沉气流天气形势的影响;在污染日,增城、南沙和珠海低层风速普遍小于3 m·s-1,300m高度以下风速甚至小于2 m·s-1,增城和珠海观测站的通风量普遍小于5000~6000 m2·s-1;非污染日的通风量则远远大于污染日;在近海的南沙站和珠海站,海陆风等局地环流的的作用不利于污染物的扩散,海风约出现于下午6时,最大影响高度约600~800 m;逆温层是导致重污染的天气的重要原因,清远探空站频繁出现逆温层,甚至出现多层低空逆温,污染日的逆温层平均厚度和平均强度都较大.