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The Physiology of Hibernation and Its Role in the Geographical Distribution of Populations of the Culex Pipiens Complex

Culex pipiens pipiens and C. p. fatigans were subjected to a comparative study on the role of temperature in growth, longevity, respiration, fat accumulation and utilization, and egg production. In both subspecies, the lower the temperature the longer was the developmental time, the heavier was the body weight, the greater was the amount of fat, and the longer was the wing length. However, the proportion of fat in the mosquitoes reared at all temperatures remained constant. In both subspecies sugar-fed mosquitoes stored a large amount of fat, although this was not as great as that observed in females caught in nature during winter. The lower the temperature the lower was the rate of oxygen consumption. The sugar-fed mosquitoes consumed oxygen at a higher rate than did young and unfed mosquitoes. Young and unfed mosquitoes of both subspecies had the same rate of oxygen consumption; but following sugar feeding, this rate was significantly higher in pipiens than in fatigans. The rearing temperature had no effect on the rate of oxygen consumption of the adult mosquitoes. The dry body weight of previously sugar-fed pipiens and fatigans decreased during simulated hibernation; however, the rate of fat decrease in pipiens was greater than in fatigans. At the time when fatigans females were dying, their fat content was almost twice that of pipiens; at this time the surviving pipiens females outnumbered fatigans females by 3 to 1. As temperature decreased, the time required for blood digestion and for oviposition increased, and fewer females oviposited. At 5°C pipiens females digested their blood meals in about 60 days but no oviposition took place. On the other hand, fatigans did not survive long enough to digest their blood meals.

From the above experimental results it was concluded that (1) the larval rearing temperature has no effect on the ability of the emerging mosquitoes to go into hibernation directly. (2) Prior to overwintering, adult female mosquitoes require carbohydrates, or a combination of carbohydrates and a blood meal to carry them through the winter. (3) Females of southern fatigans, like those of northern pipiens, are capable of accumulating fat reserves if given a perhibernation carbohydrate diet. (4) Hibernating pipiens females outlive fatigans females because they are able to utilize their fat reserves, whereas fatigans can not do so. (5) The inability of fatigans females to survive long hibernation periods at low temperatures may be an important reason for the absence of this subspecies from colder regions.