The history of development of our knowledge of animal groups follows a uniform pattern. First there is the problem of identification, usually beginning with description of new forms. This is followed by first attempts at a classification. Then regional systematic treatments appear. Finally a monograph becomes possible, based on the accumulated work of many people.Curiously, the mites have not followed this precise pattern. Because of their economic importance, mites have long attracted attention. Much has been written on these small but numerous and widely distributed arthropods. \et for years scientists did little to further knowledge of the systematics of mites. Only recently have new species been described in precise terms and old species redescribed adequately. Now the time is propitious for monographic treatment, and it is indeed fortunate that two such outstanding specialists as Professor A. Earl Pritchard and Dr. E. W. Baker have dedicated themselves to the job and have chosen the group that is most destructive to agricultural crops, the spider mites.In the following pages the authors have not only considered all previous work in the group, but they have gone to original sources of type speci¬ mens and living material in the museums of North America and Europe and the type localities of two continents. The revision of the Tetranychidae presented in this book not only provides the essential information re¬ quired by economic entomologists, but also provides illustrations of a caliber seldom equalled in taxonomic work and a sound classification for the use of taxonomists.

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We have constructed a plasmid-borne artificial operon that expresses the six subunits of the DnaX complex of Escherichia coli DNA polymerase III holoenzyme: tau, gamma, delta, delta', chi and psi. Induction of this operon followed by assembly in vivo produced two taugamma mixed DnaX complexes with stoichiometries of tau(1)gamma(2)deltadelta'chipsi and tau(2)gamma(1)deltadelta'chipsi rather than the expected gamma(2)tau(2)deltadelta'chipsi. We observed the same heterogeneity when taugamma mixed DnaX complexes were reconstituted in vitro. Re-examination of homomeric DnaX tau and gamma complexes assembled either in vitro or in vivo also revealed a stoichiometry of DnaX(3)deltadelta'chipsi. Equilibrium sedimentation analysis showed that free DnaX is a tetramer in equilibrium with a free monomer. An assembly mechanism, in which the association of heterologous subunits with a homomeric complex alters the stoichiometry of the homomeric assembly, is without precedent. The significance of our findings to the architecture of the holoenzyme and the clamp-assembly apparatus of all other organisms is discussed.

The nucleotide sequence for 40,469 bp of the linear Paramecium aurelia mitochondrial (mt) genome is presented with the locations of the known genes, presumed ORFs, and their transcripts. Many of the genes commonly encoded in mt DNA of other organisms have been identified in the Paramecium mt genome but several unusual genes have been found.Ribosomal protein genes rpsl4, rps12, and rpI2 are clustered in a region that also contains two other genes usually found in chloroplasts, but rpl14 is over 16 kbp away. The ATP synthase gene, atp9, is encoded in this mt genome, but the atp6, atp8, and COIII genes have not been identified. All of the identified genes are transcribed. Many mono-and poly-cistronic transcripts have been detected which cover most of the genome, including large regions where genes have yet to be identified. Based on sequence comparisons with known tRNAs, only those for phe, trp, and tyr are encoded in Paramecium mt DNA.

Petrobia latens feeds primarily on grasses and grains and is sometimes an important economic pest in the late winter or spring. It is known to occur in Europe, Africa, North America, and Australia. Sayed (1946) recorded this species from Upper Egypt, on onions, under the name Petrobia cephae Sayed. Specimens from Africa that we have studied are from Gutu, Southern Rhodesia, August, 1932 (A. Cuthbertson), on wheat. 2. Petrobia harti (Ewing)

A 3.3-kilobase-pair fragment of Pseudomonas aeruginosa DNA containing the phospholipase C (heat-labile hemolysin) gene was sequenced, and the location of the gene was determined. The gene product contains at its NH2 terminus a 38-amino acid sequence which structurally resembles the signal peptides of other secreted proteins but is unusually long and positively charged (6+). The location of the translation start codon was determined by constructing a series of plasmids in which the promoter of a transcription vector was ligated to Pseudomonas DNA containing deletions at the 5' end of the gene. The plasmids were used to transform Escherichia coli, and the resulting clones were assayed for hemolysin activity. In addition, sizes of truncated proteins produced by mutants with translation terminators introduced at specific sites were analyzed in E. coli maxicells. The gene is transcribed, starting just upstream of the hemolysin gene, as an mRNA of approximately 2,800 bases. Analysis of the nucleotide sequence, analysis of mutants in maxicells, and transcriptional studies indicate that the hemolysin is part of an operon composed of two genes. Phosphate regulation of the operon is at the transcriptional level. The location of the 5' end of the transcript was determined by Si mapping.Phospholipase C (PLC) (phosphatidylcholine cholinephosphohydrolase) from Pseudomonas aeruginosa has been studied from different viewpoints: (i) as a hemolytic toxin that may contribute to the virulence of an opportunistic pathogen (22,23,44), (ii) as a secreted protein (10,12,25,48), and (iii) as a part of the phosphate regulon (13,14,19). Extracellular PLC hemolysins are also produced by organisms such as Clostridium perfringens (alpha toxin) and Staphylococcus aureus (beta toxin). In contrast, there is a different class of bacterial hemolysins produced by S. aureus (alpha toxin), Escherichia coli, and other organisms which are thought to have a nonenzymatic mechanism of action (18).The mechanism of pathogenicity for P. aeruginosa is complex, and the role of the hemolysin is not quite clear. PLC has been implicated as a virulence determinant in the pathogenesis of lung infection (22), and in another study, PLC production by urinary tract isolates was greater than that by lung, blood, or other isolates (4). Liu (23) observed that the PLC hemolysin and alkaline phosphatase are produced together during growth in low Pi and repressed during growth in high P, and proposed that these enzymes function cooperatively as a phosphate-scavenging mechanism. P. aeruginosa plcA mutants have been isolated which are deficient in the production of PLC and several other exported phosphate-repressible proteins (13, 17). Another class of mutants (plcB) hyperproduce most phosphaterepressible proteins and are also altered in Pi transport (14,17).PLC is also interesting for its properties as a secreted protein. The transport of proteins through the inner membrane of bacteria has been extensively studied in E. coli (for recent reviews, see references 33, 36, 41) bu...

DURING RECENT YEARS the taxonomy, bionomics, and control of phytophagous mites have become subjects of intensive study. Phytoseiids, the most important mite predators of plant-feeding mites, are now receiving attention from entomologists with regard to their taxonomy, bionomics, and the effects of agricultural chemicals on their populations. Gilliat (1935), Lord (1949), and Herbert (1952) regarded phytoseiid mites as among the most important predators of spider mites in Nova Scotia orchards. Mathys (1958) stated that mites of the genus Typhlodromus appear to be an effective factor limiting populations of spider mites in Swiss vineyards. Snetsinger (1959, 1960) found Typhlodromus to have a deterrent action on the development of spider mites on apple trees in Illinois. Chant (1958, 1960), however, regarded phytoseiids as of little importance in reducing spider-mite populations on deciduous fruit trees in England. In California, Smith and Summers (1949) found that a phytoseiid played an important part in controlling a heavy infestation of spider mites in a large acreage of strawberries, and Allen (1959) stated that another phytoseiid is the most important enemy of spider mites on strawberries. Huffaker and Kennett (1956) demonstrated the vital role of predation by still another phytoseiid in the control of the cyclamen mite on California strawberries. Fleschner (1958) indicated that phytoseiids may significantly control spider mites on citrus and avocado in southern California, and reasons for this effectiveness were advanced by Chant and Fleschner (1960). Chant (1959) and Chant and Fleschner (1960) have shown that certain phytoseiids may live and reproduce with it diet of pollen or aphid honeydew, which may be important in maintaining populations when mite hosts are not available. Of the few phytoseiid species described by earlier workers, practically none was accurately identified prior to the work of Garman (1948). Since that time about 250 species have been named or are being described. Never-1

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