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Conditions governing the distribution of insects in the free atmosphere



Conditions governing the distribution of insects in the free atmosphere



Canadian Ent 77(1): 7-15



Laboratory expts. to determine effects of decreases in air pressure, temp. and humidity, comparable to those of high altitudes, were conducted on several spp. of insects. A fan-equipped pressure-temp. chamber permitted any combination of air pressure and temp. Humidity was controlled by exposing the air flow to various salts. Field-collected adult winged insects of both sexes, representing diurnal, crepuscular and nocturnal spp., were used. The duration of exp'tl. conditions was limited to 5 hrs. on the assumption that an insect's stay at heights above 1 km. is transient. The following tests were made: I. Pressure (1) was lowered at varying rates from surface pressure at 1000 millibars to a minimum of 50 mb. with temp. constant at +24[degree] C; (2) was lowered to 50 mb. at each 10[degree] interval between +24[degree] C and [long dash]6[degree] C; (3) was lowered and raised rapidly to simulate pressure ranges traversed by vertical currents. II. Temp. (1) was lowered to 0[degree] C with constant pressure at 1000 mb.; (2) was lowered from +24[degree] C to the lowest temp. producing immobility at each of 100-mb. intervals between 1000 and 400 mb.; (3) was established at 0[degree] C, [long dash]10[degree] C, and [long dash]25[degree] C, the insects being dropped in from room temp.; they were left until after they were frozen, then thawed and dropped in again; the process was repeated 5 times to simulate temp. ranges traversed by vertical currents (in all the above tests, rel. humidities were held at 30, 60, and 80%); (4) was established at 0[degree] C, [long dash]10[degree] C, and [long dash]25[degree] C, and the insects after being coated with water were dropped in, kept until frozen, thawed, re-coated with water and again exposed, with 5 repetitions. III. Relative humidity (1) was varied in the pressure and temp. expts. above; (2) was varied in 10% steps between 92 and 25% at room temp. and pressure. Observations were made on the insects while in the chamber. Activity was stimulated by an electric buzzer attached to the cage in which the insects were confined, and controlled from the outside. In the pressure expts. the changes producing inactivity and insensibility were recorded. In the temp. and humidity expts., changes producing minimum flight and min. activity were noted. Down to pressures equivalent to an altitude of 1.46 km. there was a distinct increase in activity in Diptera and Coleoptera which was shown to be a distinct barotaxis, due entirely to lowered air pressure. Inactivity was produced at pressures equivalent to from 12.46 to 18.06 km., varying with the species. Since insects rarely ascend beyond 4 km., it was concluded that pressure may be disregarded as a limiting factor in the atmospheric distribution of spp. Resistance to pressure changes was not impaired by lowered temps. or humidities. All spp. withstood the rapid changes in expt. I (3). In the temp. expts. min. flight occurred at from +14[degree] to +1.1[degree] C; min. activity at from +7[degree] to + 0.5[degree] C. Temps. of 0[degree] C had no harmful effect at various humidities, except that Homoptera did not withstand low temps. for any length of time. Repeated freezing and thawing were withstood by all but the Homoptera, although exposure of > 5 min. at subzero temps. was sometimes fatal, those of 10 min. always so. All except Chrysopids survived low temps. when coated with water. Lowered humidities at low pressures and temps. had no special laboratory effects.

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Accession: 022257180

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DOI: 10.4039/ent777-1


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