As was observed in the field experiments, voluntary flights were very seldom made above 10,000 feet. Flights of very short duration were made in response to the touch stimulus at altitudes ranging from 10,000 feet to about 30,000 feet. At altitudes of 10,000 feet and above the mosquitoes were noticeably sluggish in their movements. This sluggishness became, increasingly apparent with decrease in air pressure. Voluntary movements other than a slow paddling of the hind legs and an occasional rubbing of the thorax and wings with the middle pair of legs, virtually ceased above 30,000 feet. Mosquitoes maintained at altitudes ranging from 30,000 feet to 40,000 feet for periods of up to 48 hours did not change their positions on the container wall. In response to contact stimuli movements of a few steps were made, sometimes in a sideways direction. When knocked off their feet by means of the thread attached to the top of the container the mosquitoes showed a sluggish righting response. The abdomens of gorged females of Aëdes notoscriptus became grossly distended as the air pressure in the chamber was reduced. At altitudes of from 30,000 feet to 40,000 feet the posterior extremity of the abdomen of ungorged mosquitoes was sometimes curved downwards and forwards beneath the body.

Five male and five female Aëdes notoscriptus were transferred to a rubber-stoppered tube which was placed inside the pressure chamber. Air was now pumped from the chamber. When the air pressure outside the stoppered tube was equivalent to that prevailing at just over 30,000 feet, the rubber stopper was blown out. Thus the atmosphere about the tubed mosquitoes was explosively decompressed. The insects had till this moment been making frequent voluntary flights, and otherwise behaving in a normal manner. They now dropped immediately to the bottom of the tube, moved a few steps, and became still. When restored to normal atmospheric pressure eight hours later they made an immediate recovery, and resumed normal activity.

Mosquitoes which had been held at a reduced air pressure equivalent to that at an altitude of 40,000 feet, for as long as 48 hours, resumed normal activity as soon as they were restored to altitudes below 10,000 feet. Immediate restoration to ground level from an altitude of 40,000 feet was also followed by an immediate resumption of normal behaviour. It will be remembered that Whitfield and Lefroy (Whitfield, 1939) carried out experiments upon the ability of certain insects to withstand various degrees of vacuum, and concluded that none of these insects showed any signs of distress. Their experiments were carried out in a specially built iron cylinder. Whitfield further states that the results of this investigation are substantiated by the observations of various Imperial Airways captains. These officers informed him that on occasion the descent from conditions of high altitude (12,000 feet or more) and low temperatures was most rapid, but that this had no apparent effect upon the insects subsequently found in their aircraft. It is evident that Whitfield had no means of observing insects while they were actually exposed to varying degrees of vacuum in his iron cylinder. Thus, while drawing the correct conclusion that insects show no signs of

distress after exposure to reduced air pressures even when their restoration to normal atmospheric pressure is extremely rapid, he overlooks the important fact that the behaviour of insects is abnormal during such exposure. As has already been indicated, this fact was first realist by Bert in 1877 (Hitchcock and Hitchcock, 1943). Hicks and Chand (1936) noted an apparent sluggishness in the movements of two female Aëdes aegypti held for an hour in an altimeter-testing apparatus at a reduced air pressure equivalent to that at 10,000 feet. They did not investigate the effects of still more reduced pressures and were forced to abandon their experiments at an early stage as their apparatus was required by the airways company from which they had borrowed it.

Although the experimental mosquitoes never made a flight response to the touch stimulus at greater altitudes than about 30,000 feet, they sometimes fluttered their wings without flying. A stroboscopic estimation of the wing-rate at progressively reduced air pressures was made, using a modified form of the apparatus devised by Williams and Chadwick (1943). The technique involves attaching the insects concerned to mounts without causing them injury or interfering with their flight movements. Once mounted, the insects are positioned inside a glass tube which can be evacuated by means of a pump. This tube is placed in front of an Edgerton stroboscope, having a neon-filled bulb of which the flash-frequency is controllable by an adjustable electric oscillator. The flash-frequency of this bulb is tuned to synchronise with the wing-rate of the insects, in which flight is initiated or inhibited by means of a tarsal stimulus.

Aëdes notoscriptus could not be induced to make flight movements below a temperature of 8° C. Sporadic movements began between 8° C. and 8° C., the wing-rate becoming progressively more rapid with rise in temperature above this level. Because of an electrical fault in the only stroboscope available at the time, it was not possible to make an accurate series of wing-rate readings. However, no change in the wing-rate could be detected when the experimental tube was evacuated to 40,000 feet at 15° C., although the actual form of the beat appeared to alter between about 25,000 feet and 30,000 feet. It is likely that above this level insufficient “lift” is developed to render flight possible in nature. This conclusion is substantiated by the fact that unmounted Aëdes notoscriptus could not be induced to fly at greater altitudes than 30,000 feet. It is of interest to note that Chadwick (1939), working on Drosophila repleta, was also unable to induce flight movements at sower temperatures than 8° C.′ to 10° C.

Unmounted mosquitoes of both sexes were placed in a glass cylinder at normal atmospheric pressure, and the temperature was progressively reduced from the prevailing room temperature of 15° C. Behaviour was normal above about 12° C., and voluntary flights were frequently made. Between about 10° C. and 12° C. some degree of sluggishness in the movements of the mosquitoes was apparent and fewer voluntary flights were made. From 8° C. to 10° C. the insects were sluggish in their movements. Although voluntary flights were not observed within this range of temperatures, brief flights were made in response to the stimulus of sharply

tapping the side of the cylinder. Between about 3° C. and 8° C. there was no flight response to the tapping stimulus, although sluggish movements and a righting response were still made. The mosquitoes collapsed on the bottom of the cylinder as soon as they were exposed to temperatures below 3° C. They also became inert if held at temperatures below 8° C. for long periods. Some insects of both sexes still made occasional feeble movements, both voluntarily and in response to the tapping stimulus, after they had collapsed On to their backs.

Batches of male and female Aëdes notoscriptus were exposed to a temperature of −2° C. for 24 hours, some at normal atmospheric pressure and others at reduced air pressures equivalent to those at 10,000 feet and 30,000 feet. Throughout their exposure to −2° C. these insects were quite inert, and lay on the bottom of the cylinder. Some of them lay with the wings partly spread and the legs in unnatural positions, and seemed as if dead. All of these mosquitoes were stirring within five minutes of their return to room temperature (15·5° C.). Within a further five minutes their behaviour appeared quite normal, and voluntary flights were made.

From these observations and those made during the field experiments already discussed, it appears that exposure to temperatures below about 10° C. interferes not only with the flight movements of Aëdes notoscriptus but with the general bodily activity of this mosquito as well.

Survival of Aëdes notoscriptus after Exposure to Artificially Produced Conditions of Reduced Air Pressure, Reduced Temperature, and Extreme Vibration, and Under Starvation Conditions. Aëdes notoscriptus of both sexes were exposed to reduced air pressure for varying periods, and after their return to normal atmospheric pressure were kept alive as long as possible. The average room temperature throughout these experiments was 15° C.

Ten males and the same number of females 18 days of age were fed on sugar-water, and were then exposed to a reduced air pressure equivalent to that of the atmosphere at 30,000 feet for twenty-four hours. After restoration to normal atmospheric pressure these insects lived for periods ranging from twenty-eight to sixty-seven days. The average age of these mosquitoes at death was fifty-four days. A similar batch of mosquitoes this time four days of age were exposed to a simulated altitude of 30,000 feet for eight hours each day for a week. These insects lived for periods ranging from twenty-two to ninety-three days after restoration to normal atmospheric pressure. Their average age at death was sixty-four days.

Other batches of Aëdes notoscriptus were held at 40,000 feet for periods of twelve, twenty-four and forty-eight hours. The average age of these mosquitoes when subjected to the reduced air pressure was eight days, and at death fifty-four days.

The average life-span of eighty-four male controls was seventy-four days, and of 112 females, seventy days. Thus exposure to reduced air pressures up to that corresponding to an altitude of 40,000 feet for periods of up to forty-eight hours has little significant effect on the life-span of Aëdes notoscriptus.

A further series of experiments on the survival of mosquitoes at various air pressures was carried out at 2° C. Ten thirty-four-day-old males and the same number of females were exposed to a temperature of 2° C. at atmospheric pressure for twenty-four hours. They survived for from thirty to fifty-two days after the experiment, and their average age at death was seventy-five days. A similar batch of twenty-seven-day-old mosquitoes were kept at this temperature and pressure for forty-eight hours. Members of this batch lived for from thirty-four to forty-one days after the experiment, and their average age at death was sixty-eight days. Thus exposure to freezing temperatures for periods of up to forty-eight hours also has no significant effect on the life-span of Aëdes notoscriptus.

Batches of mosquitoes were maintained at 30,000 feet and 40,000 feet at −2° C. for twenty-four to forty-eight hour periods to determine the effects of exposure to combined reduced air pressures and temperatures. Once again, there was no significant shortening of the life-span of the insects tested.

Subjection to intense vibration at atmospheric pressure and at 30,000 feet did not shorten the life-span of Aëdes notoscriptus. Ten males and ten females five days of age were placed in a tube which was vibrated in the apparatus already described. During the eight hours for which these insects were held at 30,000 feet they were continually jarred off their feet and prevented from resting. Nevertheless, they lived for from forty-four to sixty-seven days after the experiment and averaged sixty-two days of age at death. Room temperature throughout this experiment averaged 14·5° C.

Finally, a batch of male and female Aëdes notoscriptus were subjected to intense vibration for twenty-four hours at normal atmospheric pressure and a temperature of −2° C. Throughout this experiment the inert mosquitoes were shaken together at the bottom of the vibrator tube. These insects were all twenty-eight days old when tested. After the experiment they lived for periods ranging from forty-one to sixty-one days, and their average age at death was seventy-nine days.

Thus the life-spans of Aëdes notoscriptus males and females subjected to various degrees of reduced air pressure, reduced temperature, and vibration, did not differ significantly from those of control insects. Under normal present-day conditions of air transport, insects would seldom have to undergo such extreme conditions of reduced pressure as Aëdes notoscriptus survived in these experiments. Furthermore, it is most unlikely that conditions paralleling those of the experiments will ever be maintained in aircraft for longer periods than those for which they were maintained in the laboratory.

An estimation was made of the length of time for which Aëdes notoscriptus can live without food. Ten females thirty-five days old were allowed to gorge themselves on blood, after which they were not given any more food. These insects survived for from four to eight days, the average survival period being six days.

Ten newly emerged males and the same number of newly emerged females were never allowed to feed. The males lived for