首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   11772篇
  国内免费   17篇
  完全免费   775篇
  2018年   297篇
  2017年   305篇
  2016年   281篇
  2015年   113篇
  2014年   368篇
  2013年   316篇
  2012年   566篇
  2011年   621篇
  2010年   341篇
  2009年   421篇
  2008年   558篇
  2007年   626篇
  2006年   611篇
  2005年   559篇
  2004年   483篇
  2003年   479篇
  2002年   419篇
  2001年   435篇
  2000年   432篇
  1999年   305篇
  1998年   225篇
  1997年   260篇
  1996年   258篇
  1995年   178篇
  1994年   196篇
  1993年   206篇
  1992年   285篇
  1991年   278篇
  1990年   283篇
  1989年   197篇
  1988年   191篇
  1987年   192篇
  1986年   142篇
  1985年   131篇
  1984年   109篇
  1983年   55篇
  1982年   72篇
  1981年   83篇
  1980年   77篇
  1979年   64篇
  1978年   59篇
  1977年   66篇
  1976年   75篇
  1975年   29篇
  1974年   33篇
  1973年   33篇
  1972年   35篇
  1971年   22篇
  1970年   19篇
  1969年   20篇
  1968年   11篇
  1967年   10篇
  1966年   8篇
  1965年   13篇
  1964年   21篇
  1963年   8篇
  1962年   11篇
  1961年   2篇
  1960年   7篇
  1959年   4篇
  1958年   4篇
  1957年   3篇
  1956年   2篇
  1955年   4篇
  1954年   1篇
  1953年   2篇
  1952年   2篇
  1951年   5篇
  1950年   4篇
  1948年   4篇
  1947年   1篇
  1944年   2篇
  1943年   1篇
  1942年   1篇
  1940年   3篇
  1938年   1篇
  1937年   2篇
  1936年   2篇
  1935年   5篇
  1934年   1篇
  1933年   1篇
  1931年   3篇
  1930年   1篇
  1929年   2篇
  1925年   2篇
  1922年   1篇
排序方式: 共有12564条查询结果,搜索用时 124 毫秒
1.
Insect hemocytes and their role in immunity   总被引:43,自引:0,他引:43  
The innate immune system of insects is divided into humoral and cellular defense responses. Humoral defenses include antimicrobial peptides, the cascades that regulate coagulation and melanization of hemolymph, and the production of reactive intermediates of oxygen and nitrogen. Cellular defenses refer to hemocyte-mediated responses like phagocytosis and encapsulation. In this review, we discuss the cellular immune responses of insects with emphasis on studies in Lepidoptera and Diptera. Insect hemocytes originate from mesodermally derived stem cells that differentiate into specific lineages identified by morphology, function, and molecular markers. In Lepidoptera, most cellular defense responses involve granular cells and plasmatocytes, whereas in Drosophila they involve primarily plasmatocytes and lamellocytes. Insect hemocytes recognize a variety of foreign targets as well as alterations to self. Both humoral and cell surface receptors are involved in these recognition events. Once a target is recognized as foreign, hemocyte-mediated defense responses are regulated by signaling factors and effector molecules that control cell adhesion and cytotoxicity. Several lines of evidence indicate that humoral and cellular defense responses are well-coordinated with one another. Cross-talk between the immune and nervous system may also play a role in regulating inflammation-like responses in insects during infection.  相似文献
2.
The Phylogeny of the Extant Hexapod Orders   总被引:33,自引:2,他引:31  
Morphological and molecular data are marshalled to address the question of hexapod ordinal relationships. The combination of 275 morphological variables, 1000 bases of the small subunit nuclear rDNA (18S), and 350 bases of the large subunit nuclear rDNA (28S) are subjected to a variety of analysis parameters (indel and transversion costs). Representatives of each hexapod order are included with most orders represented multiply. Those parameters that minimize character incongruence (ILD of Mickevich and Farris, 1981, Syst. Zool. 30, 351–370), among the morphological and molecular data sets are chosen to generate the best supported cladogram. A well-resolved and robust cladogram of ordinal relationships is produced with the topology (Crustacea ((Chilopoda Diplopoda) ((Collembola Protura) ((Japygina Campodeina) (Archaeognatha (Zygentoma (Ephemerida (Odonata ((((Mantodea Blattaria) Isoptera) Zoraptera) ((Plecoptera Embiidina) (((Orthoptera Phasmida) (Grylloblattaria Dermaptera)) ((((Psocoptera Phthiraptera) Thysanoptera) Hemiptera) ((Neuropteroidea Coleoptera) (((((Strepsiptera Diptera) Mecoptera) Siphonaptera) (Trichoptera Lepidoptera)) Hymenoptera)))))))))))))).  相似文献
3.
Seedling-aided cache detection by heteromyid rodents   总被引:33,自引:0,他引:33  
S. Pyare  W.S. Longland 《Oecologia》2000,122(1):66-71
The possibility of communication between plants was proposed nearly 20 years ago, although previous demonstrations have suffered from methodological problems and have not been widely accepted. Here we report the first rigorous, experimental evidence demonstrating that undamaged plants respond to cues released by neighbors to induce higher levels of resistance against herbivores in nature. Sagebrush plants that were clipped in the field released a pulse of an epimer of methyl jasmonate that has been shown to be a volatile signal capable of inducing resistance in wild tobacco. Wild tobacco plants with clipped sagebrush neighbors had increased levels of the putative defensive oxidative enzyme, polyphenol oxidase, relative to control tobacco plants with unclipped sagebrush neighbors. Tobacco plants near clipped sagebrush experienced greatly reduced levels of leaf damage by grasshoppers and cutworms during three field seasons compared to unclipped controls. This result was not caused by an altered light regime experienced by tobacco near clipped neighbors. Barriers to soil contact between tobacco and sagebrush did not reduce the difference in leaf damage although barriers that blocked air contact negated the effect. Received: 15 February 2000 / Accepted: 1 April 2000  相似文献
4.
5.
应用底栖动物完整性指数B-IBI评价溪流健康   总被引:32,自引:5,他引:27       下载免费PDF全文
B-IBI指数是溪流生态系统健康评价常用指标之一。据安徽黄山地区溪流的33个底栖动物样点数据(11个参照点,22个受损点),对21个生物参数进行分布范围、Pearson相关性和判别能力分析,确定B-IBI指数由总分类单元数、EPT分类单元数、前3位优势分类单元%、粘附者%、敏感类群%和BI指数构成。分别用3分制、4分制和比值法统一各参数量纲,B-IBI指数值即为累加各构成指数的分值或比值。B-IBI箱线图分析表明,上述3种方法计算出的B-IBI值有较高判别能力(IQ=3),能很好地判别参照水体和受损水体,且3者之间具高相关性(r>0.90)。分别依据参照样点和所有样点B-IBI值建立健康评价标准,比较3分制、4分制和比值法对评价结果的准确性,表明用比值法统一各参数量纲并依据所有样点的B-IBI值建立的健康评价标准的准确性优于3分制和4分制法。建立了适合祁门县溪流生态系统健康评价B-IBI标准B-IBI>3.59健康,2.7~3.59亚健康,1.8~2.69一般,0.9~1.79差,B-IBI<0.9极差。祁门县21个样点的水体,14个健康,6个亚健康,1个一般。B-IBI与电导率(r=-0.62,p<0.01)和生境质量(r=0.65,p<0.01)显著相关。  相似文献
6.
我国东部底栖无脊椎动物主要分类单元耐污值   总被引:31,自引:5,他引:26       下载免费PDF全文
王备新  杨莲芳 《生态学报》2004,24(12):2768-2775
根据采自安徽南部山区溪流、江苏南京秦淮河流域和紫金山地区小水体的92个大型底栖无脊椎动物样本,以及文献记载的北京、天津、安徽、湖南和辽宁的135个大型底栖无脊椎动物样本数据,用调整后的多样指数分级标准H'>4极清洁,3~4清洁,2~3轻污,1~2中污,<1重污,确定了227个样本所在样点水质类型.采用Lenat(1993)耐污值计算法,共获得81个分类单元的耐污值,其中昆虫纲鞘翅目3科2属,广翅目1科2属,蜉蝣目7科14属,(礻责)翅目2科2属,毛翅目6科7属,蜻蜓目3科1属,双翅目3科3亚科12属,软体动物2属5种,环节动物1属4种,扁形动物1属,甲壳动物1属.根据采集经验和文献记载,核定了73个分类单元(科、属)的耐污值,并初步探讨了上述各分类单元耐污值所适用的地区.  相似文献
7.
Cytochromes P450 and insecticide resistance.   总被引:30,自引:0,他引:30  
The cytochrome P450-dependent monooxygenases (monooxygenases) are an extremely important metabolic system involved in the catabolism and anabolism of xenobiotics and endogenous compounds. Monooxygenase-mediated metabolism is a common mechanism by which insects become resistant to insecticides as evidenced by the numerous insect species and insecticides affected. This review begins by presenting background information about P450s, the role of monooxygenases in insects, and the different techniques that have been used to isolate individual insect P450s. Next, insecticide resistance is briefly described, and then historical information about monooxygenase-mediated insecticide resistance is reviewed. For any case of monooxygenase-mediated resistance, identification of the P450(s) involved, out of the dozens that are present in an insect, has proven very challenging. Therefore, the next section of the review focuses on the minimal criteria for establishing that a P450 is involved in resistance. This is followed by a comprehensive examination of the literature concerning the individual P450s that have been isolated from insecticide resistant strains. In each case, the history of the strain and the evidence for monooxygenase-mediated resistance are reviewed. The isolation and characterization of the P450(s) from the strain are then described, and the evidence of whether or not the isolated P450(s) is involved in resistance is summarized. The remainder of the review summarizes our current knowledge of the molecular basis of monooxygenase-mediated resistance and the implications for the future. The importance of these studies for development of effective insecticide resistance management strategies is discussed.  相似文献
8.
The five regions of homologous DNA which are interspersed in the genome of the baculovirus Autographa californica nuclear polyhedrosis virus increased the expression of a delayed-early gene of this virus. Although this activity was first observed as a 10-fold trans effect, the homologous region 5 (hr5) enhanced the expression of linked genes 1,000-fold. The hr5 enhancer also exhibited the other characteristics associated with viral enhancer elements, including orientation independence and the abilities to function at a distance from the linked promoter, to regulate heterologous promoters, and to increase the number of RNA polymerase molecules transcribing the linked genes. The expression of the immediate-early regulatory gene was not enhanced by cis-linked hr5, although the enhancer function may require the immediate-early regulatory gene product. The hr5 enhancer was relatively insensitive to competition by an excess of enhancer molecules. The nucleotide sequence of hr5 revealed two different conserved repeats separated by nonhomologous DNA. Deletion analysis of the hr5 enhancer indicated that a 30-base-pair inverted repeat was essential for enhancer function.  相似文献
9.
Insect resistance management in GM crops: past, present and future   总被引:30,自引:0,他引:30  
Transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) were first commercialized in 1996 amid concern from some scientists, regulators and environmentalists that the widespread use of Bt crops would inevitably lead to resistance and the loss of a 'public good,' specifically, the susceptibility of insect pests to Bt proteins. Eight years later, Bt corn and cotton have been grown on a cumulative area >80 million ha worldwide. Despite dire predictions to the contrary, resistance to a Bt crop has yet to be documented, suggesting that resistance management strategies have been effective thus far. However, current strategies to delay resistance remain far from ideal. Eight years without resistance provides a timely opportunity for researchers, regulators and industry to reassess the risk of resistance and the most effective strategies to preserve Bt and other novel insect-resistant crops in development.  相似文献
10.
The molecular biology of knockdown resistance to pyrethroid insecticides   总被引:29,自引:0,他引:29  
The term "knockdown resistance" is used to describe cases of resistance to diphenylethane (e.g. DDT) and pyrethroid insecticides in insects and other arthropods that result from reduced sensitivity of the nervous system. Knockdown resistance, first identified and characterized in the house fly (Musca domestica) in the 1950's, remains a threat to the continued usefulness of pyrethroids in the control of many pest species. Research since 1990 has provided a wealth of new information on the molecular basis of knockdown resistance. This paper reviews these recent developments with emphasis on the results of genetic linkage analyses, the identification of gene mutations associated with knockdown resistance, and the functional characterization of resistance-associated mutations. Results of these studies identify voltage-sensitive sodium channel genes orthologous to the para gene of Drosophila melanogaster as the site of multiple knockdown resistance mutations and define the molecular mechanisms by which these mutations cause pyrethroid resistance. These results also provide new insight into the mechanisms by which pyrethroids modify the function of voltage-sensitive sodium channels.  相似文献
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号