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Insect Migration

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Migration Within the Boundary Layer

A layer close to the ground at which air-speed increases above the wind-speed is

known as Boundary Layer. This varies from insect to insect due to their independent

airspeeds. An insect can orient and progress to any particular direction within the

boundary layer and at high altitudes it will prefer downwind [16].

A butterfly species (Asciamonuste) flies at low altitudes of only 1–4 m above

the ground levels and is capable of flying against the wind speeds of around 10

km/h. Generally the direction of its migration will be from north or south. The trend

of migration in such insect flight is fixed, and they follow landmarks like roads,

coastlines etc. Such migration has been reported in queen wasps and bumblebees

(Bombus) during spring season.

Beetle Melolontha also migrates within the boundary layer, and its initial orien-

tation is visual and gets supplemented by orientation of sun or pattern of polarized

light from sky [17].

Daytime Migration Outside the Boundary Layer

Thermals arising from the ground due to heat during daytime and the air will be

turbulent. Fu et al. [18] confirmed long distance insect migration in airflow due to

increase in ground temperatures. Many smaller insects are carried by these thermals

above their boundary layers followed by the transportation due to winds. In this

method an insect can be lifted to a height of above 3000 m from the ground levels.

However, the migration of insects under very low illumination levels remains to

be explained [4]. Chapman et al. [19] also summarized that the insect migration

above 12 m remains to be elucidated fully. But during 1947–1948 some radar engi-

neers reported the recorded echoes, suspected to be produced by insects from lower

atmosphere.

Crawford observed two wavelengths (3.2 and 1.25 cm) at lower atmosphere and

attributed them to be due to insects and birds [20].

Migration at Night

Insect migration during night was observed by using radars as in the case of grasshop-

pers, locusts and moths. These migratory insects takeoff in larger groups after sunset

and otherwise they climb up to a distance of 1000 m. This height varies with species

to species. By flying so high these insects are allowing themselves to get out of

boundary layer and migrate with downwind. During downwind some insects will

have common orientation and it results to a faster displacement with wind speed

allowing the insect to travel even 100 km or more (viz. leafhoppers, moths) [5, 21].