迁飞过程中空中虫群的成层机制

Many migrating insect species, such as noctuid moths and ricehoppers, frequently concentrate into horizontal layers in a low level jet coincided with the top of the surface temperature inversion within the stable nocturnal boundary layer, that provides an aerial highway for the long distance downwind migrants. The flight behaviour of Helicoverpa armigera was studied in a simulated wind and temperature field to explore the stratification mechanism of airborne migratory fauna. The different wind profiles were generated by three jet fans to simulate different horizontal speed gradients. The vertical airflow at the edge of the simulated low level jet was measured by a 3D anemometer. The response of the moths to temperature profiles was tested by a self-made heating device in the laboratory. Flight behavior of the moths was closely related to the changing airflow under different wind profiles. The analysis of wind field data showed that high speed airflow had significant suction effect to the tested moths, and that there was obvious negative pressure around the high speed airflow. It was the inward vertical airflow generated by the high speed removing jet that pushed and bound the airborne migrants into the high speed horizontal airflow. In simulated temperature filed, the tested moths exhibited significant actively selection behavior for temperature. The moths tended to select their optimal flight temperature (20~22℃) and the low temperature threshold (13℃) for flight respectively. The moths would prefer to select the maximum temperature if the ambient temperature approximated to their optimal flight temperature, but they would prefer to the flight low temperature threshold when ambient temperatures were obvious lower than their optimal flight temperature. The results illustrated that the actively selection behaviour of the airborne migrants for optimal flight temperature caused their tendency towards the nocturnal inversion layer, and the inward vertical airflow resulted from the negative pressure at the edges of low level jet located on the top of inversion was the key factor to form the sustained layer concentration and aggregation of the aerial fauna. But if environmental temperature significantly lower than optimal flight temperature of insect, the flying insects would prefer to climb up to their flight ceiling, i.e., the height with the low temperature threshold, so that the ceiling layer occurred.