records of the capture of living females of Anopheles maculipennis on board vessels after their arrival at Auckland from Samarang (Java) and Singapore (Graham, 1939). These localities, in both of which Aedes aegypti and several species of Anopheles are abundant, are now but a few hours by air from this country. Although the chances of insects entering and travelling in aircraft as compared with ships are reduced by reason of the smaller dimensions of the former, the greater speed of aircraft would appear to be a factor augmenting the chances of survival of those insects remaining undetected.

There is already an extensive literature concerning insects and air transport. Since the early days of intercontinental aviation it has been realist that insects might be carried from one country to another by means of aircraft. Kisliuk (1929) seems to be the first actually to record such carriage. He lists several species of insects collected from the dirigible “Graf Zeppelin” at the conclusion of its first flight from Germany to the U.S.A. Great numbers of insects belonging to many different orders have now been collected on board aircraft. Whitfield (1939) reports the finding of 1,960 insects of 146 species in 2,000 aircraft examined at Khartoum. This total is exclusive of house flies, of which about 1,000 were collected; of the 2,000 aircraft 192 harboured potential vectors of disease organisms other than house flies.

Several workers have made observations on the ability of certain insects to survive air travel. Experimental work in this field has centred on the carriage of mosquito vectors of disease organisms. It is recognised that a wide range of insects of medical, agricultural and pest significance might be carried in aircraft, but in general control measures developed for use against mosquitoes in planes would also be effective against other insects. Aëdes aegypti has been singled out for special attention because of its importance as a vector of the virus which causes yellow fever.

Griffitts and Griffitts (1931) released 100 mosquitoes in the cabins of three Fokker trimotor airliners about to depart from San Juan, Porto Rico, on the 1,250 mile journey to Miami, Florida. Most of these insects were Aedes aegypti, but a few Culex quinquefasciatus were included. All had been reared from larvae collected in San Juan. They were allowed to feed from an arm introduced into their cage, and were stained with a 2 per cent, aqueous solution of eosin before the experiment. The average flying time for the journey was nine hours, and the greatest altitude attained was 5,000 feet. One hour and ten minutes were spent at three intermediate landing fields. Careful searching of the machines on their arrival at Miami revealed that 22 living mosquitoes were still aboard. Examples of both species, including both males and females of Aëdes aegypti, were recovered. It was thus demonstrated that mosquitoes can survive an air journey under the conditions of the experiment, and that these insects do not necessarily leave an aircraft at intermediate stops.

Working in the Belgian Congo, Trolli (1932) conducted experiments with caged Mansonioides. Most of these mosquitoes had been

captured in a gorged condition. The first cage was sent by airliner from Léopoldville to Coquilhatville. All but two insects which had become caught in the netting of the cage survived the journey, and were kept alive “for several days” afterwards. This flight took six hours and ten minutes, and the maximum altitude reached was 3.280 feet. A second cage was placed aboard an aircraft the evening before it left Léopoldville for Elisabethville. An overnight stop was made on this journey, which took fourteen hours and thirty-five minutes. The greatest altitude attained was 7,900 feet. On the day following the arrival of the machine at Elisabethville the cage of Mansonioides was examined, after being aboard for four days. All the mosquitoes in this batch were found to be dead. Trolli suggests that shocks on landing, or the effects of the duration and altitude of the flight, might have been responsible for the mortality.

Roubaud (1932) records that 100 specimens of Anopheles maculipennis were enclosed in a wooden box and sent by aircraft from Southern Italy to Paris in mid-winter. Conditions of severe cold were encountered during this flight, particularly when crossing the Alps. The mosquitoes were frozen and quite inert when, they reached Paris. Nevertheless, they all recovered when placed in a warm, moist atmosphere.

Griffitts (1933) and McMullen (1933) give the results of further experiments carried out by the United States Public Health Service. Once again the technique employed was to release stained Aëdes aegypti into the cabins of aircraft. One series of observations was made on insects sent from San Salvador to Brownsville. Texas, in Ford trimotor airliners. There were six intermediate stops on this thirty-one hour journey, including an overnight one in Mexico. The maximum altitude attained was 14,000 feet. Eight per cent, of the mosquitoes liberated in the cabins of twelve aircraft at San Salvador were recovered at Brownsville. Both males and females were taken, many of the latter being gorged with blood. Some of these were seventeen days old when they reached Brownsville, and had taken their first blood-meal fifteen days before the flight.

Other observations proved that Aëdes aegypti could survive the flight from San Salvador to Miami, a journey with ten intermediate stops, including three overnight ones. Four out of a total of seventy mosquitoes from five to twelve days old were recaptured after a journey of seventy-nine hours and forty-five minutes on this route. Stained mosquitoes were also recaptured after the fifty-five hour flight from Cristobal, Canal Zone, to Brownsville, and the thirty-four hour one from Cristobal to Miami. The greatest altitude reached during these flights was 7,500 feet.

Griffitts and McMullen thus demonstrate that Aëdes aegypti is able to survive periods of almost cighty hours in aircraft and can withstand the effects of flight at 14,000 feet. Furthermore, they show that mosquitoes a fortnight old can survive an air journey as well as newly-hatched insects. This indicates that Aëdes aegypti which have lived long enough to become infected with the yellow fever virus have as good a chance of surviving journeys by aircraft as have newly-hatched mosquitoes.

Hicks and Chand (1936) placed caged batches of Aëdes aegypti in the care of pilots of airliners leaving Karachi. The insects were given a blood meal immediately before departure, and were provided with raisins from which to feed en route. Fifty-six mosquitoes in five lots of two cages each were put aboard Hannibal class airliners leaving Karachi for England. One cage of each batch was located in the pilot's cockpit or in the forward baggage compartment. The other was tied in the rear of the fuselage behind the cabin. Although most of the insects survived the first day's journey, all were dead when Bagdad was reached thirty-six hours after leaving Karachi. These deaths were attributed to the effects of insecticidal spraying, and to the possibility that severe draughts might have affected the occupants of the cage in the rear of the fuselage. Later batches of mosquitoes were placed on board Douglas airliners leaving Karachi for Amsterdam. The longest survival period recorded in these experiments was six and a-half days. The distance covered in this period was 9,580 miles, from Karachi to Amsterdam and back through Karachi to Jodhpur.

Sicé (1939) states that adult female Anopheles gambiae were enclosed in test-tubes and sent by aircraft from the French Sudan to Marseilles. A large proportion of these mosquitoes reached France alive, and laid eggs which developed normally in artificial breeding places at ordinary summer temperatures. The second generation insects were perfectly normal. It was thus demonstrated that air travel does not adversely affect the reproductive powers of Anopheles gambiae.

There is little published information regarding the actual behaviouristic reactions of mosquitoes to air travel. With regard to the activity of these insects during air journeys, Griffitts and Griffitts (1931) state that the radio operator of one of the aircraft in which their experiments were carried out was bitten on the face at an altitude of 3,000 feet. In some cases aircraft in which Aëdes aegypti had been liberated have had to be sprayed out with insecticide because those on board were troubled by the attacks of mosquitoes (Grifitts, 1933). Unfortunately there is no record of the altitudes at which these insects were troublesome. Captain A. B. Klots mentions his being bitten on the leg by a culicine mosquito, Psorophora ferox (Humbolt) at about 8,000 feet altitude during a flight from Natal to Belem, Brazil (U.S.A.A.F., 1946). McMullen (1933) records an interesting observation made by Newman during a flight from Cristobal. Newman states that when the motors were started some mosquitoes settled on the cabin roof. These insects did not budge during the flight, but began to fly again when the motors were shut off on landing. Griffitts and Griffitts (1931) observe that on one occasion, immediately after landing at Miami on a flight from Porto Rico, two Aëdes aegypti flew out from beneath a seat and attempted to bite.

The earliest investigation into the effects of reduced air pressure on insects seems to have been that carried out by Bert in 1877 (Hitchcock and Hitchcock, 1943). Bert records that poplar beetles (Chrysomelidae) recovered from twenty hour exposures to reduced

pressure of 9 cm. and 4 cm. although inert and apparently lifeless during the experiment. These reduced air pressures are approximately equivalent to those of the atmosphere at 50,000 to 60,000 feet.

Whitfield and Lefroy (Whitfield, 1939) carried out experiments with varying degrees of vacuum in 1924. They used a specially built iron pressure cylinder in this investigation. Among the insects tested were Musca domestica, larvae of Lepidoptera, and adult Tene-brionidae (Coleoptera). Whitfield states that vacuums (the degrees of which were lost) were maintained for periods of up to twenty-four hours and were then suddenly released. None of the insects concerned showed any signs of distress.

Hicks and Chand (1936) experimented on the effects of reduced air pressure on Aëdes aegypti, using an apparatus designed for testing altimeters. As the insects were enclosed in a glass-fronted chamber in this apparatus, it was possible to study them during their exposure to reduced air pressures. Both males and females were observed during these tests, the females being fully fed beforehand. The experiments were carried out at air pressures equivalent to those at altitudes of 5,000 feet and 10,000 feet. In one test two females appeared to be sluggish in their movements when held at 10,000 feet for one hour. When the apparatus was shaken they seemed less willing to fly than usual. No such effect was noticed in any of the other experiments. In all cases the insects appeared normal on the following day.

Rouband's observations on the effects of reduced air temperatures on mosquitoes travelling in aircraft have already been discussed (p. 95).

It appears that no experiments have been carried out to determine the effects of vibration on insects in aircraft. Magath (1945), however, states: “It has now become evident that, owing to the vibration of the plane which prevents delicate insects like mosquitoes from resting during the trip and because of the dryness and high degree of ‘ventilation,’ most delicate insects are unable to survive a long trip. Even though they do, many appear to be incapable of carrying on their life cycle.” These remarks are at variance with the published results of earlier workers already discussed. Quite apart from the demonstrated fact that mosquitoes can and do survive long air journeys, it will be remembered that Newman (McMullen, 1933) observed Aëdes aegypti resting on the cabin roof of a Ford trimotor aircraft throughout a flight. Vibration during flight was thus insufficient to prevent these mosquitoes from resting. The degree of vibration of the fuselage of modern aircraft is at least no greater than that of obsolescent trimotor machines.

It will be convenient at this stage to summarise the findings of the investigators whose work is outlined above.

Thus insofar as mosquitoes are concerned little has been experimentally demonstrated beyond the facts that these insects can survive long air journeys and do not necessarily leave aircraft at intermediate stops during such journeys. There is very little information concerning the reactions of mosquitoes to the aircraft environment. In fact the literature contains only three direct observations on the behaviour of mosquitoes during air travel, those of Griffitts (1931), Newman and Klots. Furthermore there is but scanty information as to the ability of these insects to establish themselves in a strange environment after surviving flights in aircraft.

At the commencement of the present investigation the following points were seen as being most in need of study:—