338
E. B. EDNEY
the respiratory surfaces if the spiracles have to be kept open, as Mellanby
(1942) pointed out.
H. OXIDATION WATER AND THE MIGRATION OF LOCUSTS
There is no doubt that oxidation water forms an important component
of the water balance of desert arthropods as it does of desert mammals,
and this has important ecological consequences. The point is well illustrated by the work of Weis-Fogh (1967) on the desert locust, Schistocerca,
and we shall now consider this.
The ventilation rate of a resting locust is about 30 ml of air/gm/hour,
but this increases to about 320 ml/gm/hour through the thorax alone
when the insect flies. Most of this ventilation results from the thoracic deformation caused by flight movements.
Such a high level of ventilation leads to a high rate of water loss, and
the question arises as to how a locust can remain in water balance during
long migratory flights. The rate of metabolism during level flight is known.
It is about 65 cal/gm/hour, and since fat is the fuel, this results in the
liberation about 8.1 mg of water/gm/hour. In level flight, at an ambient
temperature of 30°C in air at a relative humidity of 60%, a locust's thoracic temperature in the absence of radiation is about 36°C and it loses
water at the rate of 8.0 mg/gm/hour so that it is in positive water balance.
In direct sunshine under a radiation load of 1.25 cal/cm
2
/minute, the
insect's temperature is about 4.0°C higher than ambient, so that evaporation of water would be more rapid, and we see that whether or not it remains in water balance depends upon (1) the radiation intensity, (2) the
relative humidity, and (3) the ambient temperature. Weis-Fogh (1967)
has calculated a series of curves which show these relationships conveniently (see Fig. 11), and which lead to the following conclusions,
among others.
Unless the relative humidity is above 90% or so, continuous flight at
30°C ambient temperature in sunlight will lead to negative water balance.
On the other hand, in the absence of solar radiation, and in air at 25 °C,
a relative humidity of only 35% is sufficient to maintain water balance.
Under true desert conditions it is, therefore, advantageous for locusts to
fly high where it is cooler. After allowing for the decrease in air density
and of barometric pressure at higher altitudes, Weis-Fogh (1967) concludes: "A decrease in temperature from 35°C at ground level to 23°C
at 3 km altitude should permit sustained flight at 35% relative humidity
even in sunshine. At ground level this amount of water corresponds to
only 18% relative humidity as in a true desert. Provided that a swarm
of locusts is lifted to a height of 2-3 km by means of thermal upcurrents
E. B. EDNEY
the respiratory surfaces if the spiracles have to be kept open, as Mellanby
(1942) pointed out.
H. OXIDATION WATER AND THE MIGRATION OF LOCUSTS
There is no doubt that oxidation water forms an important component
of the water balance of desert arthropods as it does of desert mammals,
and this has important ecological consequences. The point is well illustrated by the work of Weis-Fogh (1967) on the desert locust, Schistocerca,
and we shall now consider this.
The ventilation rate of a resting locust is about 30 ml of air/gm/hour,
but this increases to about 320 ml/gm/hour through the thorax alone
when the insect flies. Most of this ventilation results from the thoracic deformation caused by flight movements.
Such a high level of ventilation leads to a high rate of water loss, and
the question arises as to how a locust can remain in water balance during
long migratory flights. The rate of metabolism during level flight is known.
It is about 65 cal/gm/hour, and since fat is the fuel, this results in the
liberation about 8.1 mg of water/gm/hour. In level flight, at an ambient
temperature of 30°C in air at a relative humidity of 60%, a locust's thoracic temperature in the absence of radiation is about 36°C and it loses
water at the rate of 8.0 mg/gm/hour so that it is in positive water balance.
In direct sunshine under a radiation load of 1.25 cal/cm
2
/minute, the
insect's temperature is about 4.0°C higher than ambient, so that evaporation of water would be more rapid, and we see that whether or not it remains in water balance depends upon (1) the radiation intensity, (2) the
relative humidity, and (3) the ambient temperature. Weis-Fogh (1967)
has calculated a series of curves which show these relationships conveniently (see Fig. 11), and which lead to the following conclusions,
among others.
Unless the relative humidity is above 90% or so, continuous flight at
30°C ambient temperature in sunlight will lead to negative water balance.
On the other hand, in the absence of solar radiation, and in air at 25 °C,
a relative humidity of only 35% is sufficient to maintain water balance.
Under true desert conditions it is, therefore, advantageous for locusts to
fly high where it is cooler. After allowing for the decrease in air density
and of barometric pressure at higher altitudes, Weis-Fogh (1967) concludes: "A decrease in temperature from 35°C at ground level to 23°C
at 3 km altitude should permit sustained flight at 35% relative humidity
even in sunshine. At ground level this amount of water corresponds to
only 18% relative humidity as in a true desert. Provided that a swarm
of locusts is lifted to a height of 2-3 km by means of thermal upcurrents