JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 128.235.251.160 on TueHOST-PARASITE LIFE TABLES 119 to environmental extremes of drought and flash floods. 2. The natural life span for most Rhinichthys oscults in the Chiricahua Mountains is less than 3 years. None was found to reach 4 years.3. High mortality rates during summer drought must be interpreted, in part, as an acceleration of death among older fish since most fish are expected to die before they are 3 years old. Among younger fish, drought directly elevates the mortality rate.4. Mortalities during a drought are caused directly by the disappearance of water and indirectly by starvation of the fish which are crowded into reduced habitat with inadequate food. 5. Flash floods are an important cause of mortality among fish of the year if they occur while the fish are very small. The greatest potential loss of fish of the year occurs in late summer when the initial flash flood induces major reproduction and is then followed by another flash flood.6. Flash floods are not a significant cause of mortality among older fish. 7. Temporary waters may persist in some sections through a period of wet years. The populations of fish inhabiting them are derived from upstream sections during a flash flood. All fish that were located in downstream temporary sections in the spring of 1960, were one-year-olds. 8. Temperatures do not rise to lethal levels in any of the flowing streams. In exposed shallow isolated pools, the temperatures may become lethal for oldcr fish, but not for fish of the year. 9. Predators play a minor role. Only one, the garter snake, Thamnophtis cyrtopsis, is common and apparently important.10. Although fish populations are greatly reduced by a year of drought, there is no immediate threat of their local extinction. ACKNOWVLEDGMENTSThese studies were made possible . 1946. Lethal temperature relations for a sample of young speckled trout (Salvelinus fontinalis). Univ. Toronto Stud. Biol. Ser. 54; Pub. Ont. Fish. Res. Lab. 66. Hart, J. S. 1952. Geographic variations of some physiological and morphological characters in certain freshwater fish. Univ. Toronto Biol. Ser. 60; Pub. Ont. Fish. Res. Lab. 72. John, K. R. 1963. The effect of torrential rains on the reproductive cycle of Rhinichthys osculus (Girard) in the Chiricahua Mts., Arizona. Copeia, 1963 (2): in press.

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Three species of hymenopterous parasites of the spotted alfalfa aphid were reared over a range of constant temperatures. At each thermal level, life-table data were obtained for each species. These data were used in computing certain statistics concerning reproduction and rates of potential population increase which are valuable in assaying the effectiveness of each species as an aphid parasite. The temperatures studied ranged from 10 to 35°C, and the relative humidity was held as constant as possible between 40 and 60o/o. Artificial illumination and photoperiods were identical in all studies.The studies showed that the braconid, Trioxys uti/is Muesebeck, generally has the highest mean total fecundity of the three species, although the aphelinid, Aphelinus semijlavus Howard, produced more eggs in the temperature range of 18 to zzoc. The gross reproductive rate and net reproductive rate of T. uti/is were also higher than the other parasites throughout most of the temperature range studied, although again A. semijlavus showed higher reproductive rates between 24 and 27°C. The innate capacity for increase, r m' of T. uti/is was decidedly the highest of the three parasites at all temperatures.The braconid, Praon palitans Muesebeck, was inferior to the other two parasites in nearly all phases of these life-table statistics. Its inclination to enter a facultative hibernal diapause at relatively mild temperatures, plus its intolerance of higher temperatures, which the other two species endured, limited its thermal range of effectiveness.Statistics such as total fecundity, gross reproduction rate, net reproduction rate, and innate capacity for increase are discussed relative to their value in indicating the potential effectiveness parasites can be expected to possess in the field. ment of Agriculture,