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Technique.
Four 9 × 9 × 6 inch cages were made for this study. The sides and roof of each were of mosquito netting, the floor and framework of white-enamelled wood. A calico feeding-sock was attached to an end wall so that captive mosquitoes could be fed with blood from a hand introduced into the cage. One hundred newly-hatched Aëdes notoscriptus, seventy-five females and twenty-five males, were transferred to these cages on May 21, 1946. On the following day the females were given a blood-meal and the males were allowed to feed from wads of cotton-wool soaked in sugar-water.
All four cages were distributed within the fuselage of a Douglas Dakota aircraft at Whenuapai airfield on the morning of May 23. No. 1, containing thirty females and ten males, was placed on a cabin seat. Nos. 2 and 3, each holding fifteen females and five males, were located beneath this seat. These three cages were fastened to the cabin wall, so as to be subjected to the full effect of the vibration of the aircraft. Cage 4 also contained fifteen females and five males, and was strapped to the upper part of the wall of the toilet compartment. Just after midday that same day the Dakota left Whenuapai on the first stage of a 12,452 mile return flight to Japan with mail for the New Zealand occupation force. This route was at the time the longest being regularly flown by a single machine anywhere in the world.
The female Aëdes notoscriptus were given a blood meal once every three days during the journey. Griffitts (1933) and McMullen (1933) show that under normal circumstances female mosquitoes may feed from crew and passengers at least between the stages of a long flight. Griffitts and Griffitts (1931) and Griffitts (1933) record instances of these insects biting during flight. On the other hand, male mosquitoes are normally unable to feed on board aircraft as they do not suck blood but live on various plant juices. Accordingly, the male Aëdes notoscriptus were not supplied with food after the take-off from Whenuapai.
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Reactions of Mosquitoes During Air Travel.
None of the caged insects showed undue activity when the motors were started or during taxi-ing. All of them commenced to fly in agitated fashion when the motors were fully opened out just prior to the take-off run. Their behaviour returned to normal within five minutes after the plane's becoming airborne. The abnormal activity at take-off was attributed to the effect of the sudden pronounced increase in vibration of the fuselage at that time.
A similar marked increase in activity was observed each time the motor was throttled back and the flaps put down when coming in to land. This activity was maintained in some cases until five minutes after the aircraft had halted on the tarmac. It was attributed first to the effect of the pronounced increase in vibration of the fuselage on landing, secondly to that of the final cessation in vibration after the motors had been running steadily for several hours. A possible contributory cause was the marked rise in air temperature experienced during the descent and landing.
Hourly observations of the behaviour of the mosquitoes in all four cages were made during each stage of the flight. The insects behaved normally and made voluntary flights from time to time at altitudes below 10,000 feet and temperatures in excess of 10° C. When altitudes ranging from 10,000 feet to 11,500 feet were attained, and temperatures were still above 10° C., the mosquitoes no longer made voluntary flights and were noticeably sluggish in their movements. They made short flights in response to the stimulus of a sharp tap on their cage. Sluggishness in this case appeared to be due solely to the effects of the reduced air pressure at altitudes above 10,000 feet.
A similar sluggishness with a complete lack of voluntary flights was apparent at altitudes below 10,000 feet when temperatures ranged from 8° C. to 10° C. As behaviour at such altitudes was normal when the air temperature was above 10° C., this reaction was ascribed to the effects of reduced temperature alone.
Sluggishness at altitudes above 10,000 feet when the air temperature ranged from 8° C. to 10° C. was put down to the combined effects of reduced air pressure and temperature.
At temperatures below 8° C. there was no flight response to the tapping stimulus regardless of the altitude. When such temperatures were maintained for any length of time the insects became inert, lying on their backs on the floor of their cage. Some still made occasional feeble movements of the legs and head.
There was no observable differences between the reactions of male and female Aëdes notoscriptus to the various conditions of vibration, reduced air pressure, and reduced temperature encountered.
In all cases mosquitoes which had become sluggish or inert made a complete recovery during the descent preparatory to landing. In the case of inert mosquitoes this recovery became apparent as soon as the temperature rose above 8° C. Sluggishness disappeared when altitudes below 10,000 feet and temperatures above 10° C. were reached. Complete recovery was usually effected within five
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minutes of regaining favourable conditions of air pressure and temperature.
During the journey the cabin heating system was defective and temperatures varied more than is usually the case. The reading at Cage 1 on the cabin seat ranged from 5° C. to 27° C., and averaged 17.5° C. The behaviour of the occupants of this cage was abnormal on seven of the sixteen stages of the journey. On four of these stages the mosquitoes were sluggish when the aircraft'was at altitudes of 10,000 feet and above, although the temperature averaged 22° C. Sluggishness was apparent during two stages at altitudes above 10,000 feet and at an average temperature of 10° C. On the remaining stage all the mosquitoes were inert for four and a-half hours when the temperature averaged 5° C. and the altitude was 8,000 feet.
The occupants of Cages 2 and 3 beneath the cabin seat experienced a temperature range of 4.5° C. to 26.5° C., the average being 15.5° C. Their behaviour paralleled that of the mosquitoes in Cage 1. Sluggishness was apparent on two additional occasions when the temperature fell to 10° C. at altitudes of 8,000 feet and 8,500 feet.
During flight the temperature in the unheated toilet compartment ranged from 3° C. to 25° C., and averaged 13° C. On eleven of the sixteen stages the mosquitoes in Cage 4 behaved abnormally. During four of these stages the insects were sluggish at altitudes of 10,000 feet or more, while the temperature averaged 17° C. For a total of eleven hours on three stages they were inert at an average temperature of 8° C., at an altitude of 8,000 feet. They were also inert for a further nine hours on two more stages, when at altitudes above 10,000 feet and an average temperature of 6° C.
Iwakuni airfield, Japan, was reached on the tenth day after leaving New Zealand. This airfield is situated on the Inland Sea in the south-western part of the island of Honshu. The temperature range inside the aircraft during the five day stay at Iwakuni was 16° C. to 23° C. All the cages were left in their positions during the stay, and the occupants were fed as usual. Immediately before leaving on the return journey twenty-five males of Culex pipiens pallens, a very common mosquito in this part of Japan, were collected in a disused hanger near the airfield. The female of this species is a vector of the virus of Japanese B encephalitis (Hsiao, 1946). These insects were placed in Cage 1 together with the surviving female Aëdes notoscriptus. They were not fed at the time of collection or during the return journey, so as to simulate the conditions of accidental transportation as closely as possible. No females of the Japanese mosquitoes were brought back to New Zealand, in case some accident might have led to their escape and the establishment of the species in this country.
The return flight to New Zealand was accomplished in just over three days. The Culex pipiens pallens males were observed to show the same reactions as Aëdes notoscriptus to the effects of vibration during take-off and landing, and to those of reduced air pressure and temperature.
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Survival of Mosquitoes During and After Air Travel.
Sixty per cent, of the original seventy-five female Aëdes notoscriptus survived the eighteen-day journey, in which time 12,452 miles were covered. There was no significant difference between the percentage of survivors in the cages in the heated cabin and the unheated toilet compartment. The range of temperatures to which these insects were exposed during the flight and at airfields where stops were made, was 3° C. to 34° C. This latter reading was made inside the fuselage while on the ground at Morotai (Netherlands East Indies) during the return journey.
The twenty-five male Aëdes notoscriptus, which were not fed after the departure from New Zealand, survived for periods ranging from two to seven days. Their average survival period was four days. The range of temperatures to which they were subjected in the aircraft was 7° C. to 33° C.
Seventy-two per cent, of the twenty-five male Culex pipiens pallens taken aboard at Iwakuni survived the three-day journey to New Zealand. The range of temperatures to which they were subjected during the journey was 5° C. to 34° C.
All the surviving caged mosquitoes of both species were fed immediately after returning to Whenuapai on June 9. These insects were now transferred to a laboratory at Victoria University College in Wellington. Here they were kept supplied with food, cotton-wool pads soaked in sugar-water being pressed against the netting of their cages. From time to time the females were given access to a blood-meal as well.
The life-span of the seventy-five female Aëdes notoscriptus averaged thirty-seven days, ranging from two to 104 days. These mosquitoes spent their third to twenty-first days inside the aircraft, on the flight to Japan and back. Over the whole survival period the range of temperatures in the laboratory was 6° C. to 19° C., the average being 13.5° C. It must be mentioned that some of the females which died during the flight were small, weakly insects, presumably originating from undernourished larvae. The average life-span of those insects which survived the full journey was sixty-one days.
The feral male Culex pipiens pallens lived for an average of forty-four days after collection, the survival period ranging from two to 121 days. The first three days of this period were spent in the aircraft. Although the exact ages of these insects were not known, it is most likely that the great majority were newly-hatched when collected. They were all captured resting on a concrete wall immediately above the surface of a small pool in which their larvae were abundant. The average survival period of the males which survived the air journey, was fifty-seven days. Thus the majority of these Japanese mosquitoes successfully withstood the effects of an air journey of 6,226 miles. Although collected under summer conditions in the Northern Hemisphere and transferred in a matter of three days to winter conditions in the Southern Hemisphere, they lived for an average of six weeks after their journey. Over the whole survival period of the Culex pipiens pallens the laboratory temperature ranged from 6° C. to 20.5° C., and averaged 14° C.