The Sources of Energy for Flight in Mosquitoes

Mosquitoes have three possible sources of flight energy in meals of nectar and of blood and in the reserves carried over from the larval stage. The abundant literature on the flower-visiting habits of male and female mosquitoes has been reviewed by Hocking (1953), who considered that in northern Canada mosquitoes obtain energy for flight almost entirely from nectar. An indication of the extent to which reserves and ingested blood can provide energy for mosquito flight is given in the work described here.

A strain of Culexpipiens form berbericus Roubaud, obtained in Algiers, was found to provide suitable material for study. The mosquitoes were largely anautogenous (i.e. requiring a blood meal for ovary development) and selected for anautogeny, stenogamous (i.e. able to copulate in a small space), man-biting and non-diapausing, and could easily be cultured in the laboratory. The larvae were fed on a thick bacterial infusion which developed over ground dog-biscuit soaked in water.

The mosquitoes were flown on a ‘flight mill ‘similar to that designed by Hocking (1953) to measure thedistances flown by small insects under experimental conditions. A tapering arm of steel shim, 0·004 in thick, revolved about a vertical steel needle which passed through a hole in the arm. The arm was supported on the needle by a glass cone which was secured over the hole, the only friction as the arm revolved coming from the contact of the point of the needle with the glass cone. The tip of one end of the arm passed through a circle of 1 m. circumference and the other end was shortened and balanced with a counterpoise. A mosquito was attached to the tip of the arm by wire 0-008 in. thick. A spot of 42° C. wax was melted on to the end of the wire and this was then melted against the top of the thorax of the mosquito using a cautery of suitable temperature. When the free end of the wire had been fastened to the tip of the arm, small adjustments to the wire placed the mosquito in the normal attitude of flight. The same arm was used in all experiments, each revolution being recorded when the arm blocked light falling on to a photocell connected to an electric counter. The mosquitoes were flown at temperatures of 20–22° C. and the saturation deficiency of the air was not allowed to exceed 15 mm. Hg.

The suspended mosquito would start to fly when its legs lost contact with a surface and if flight stopped before the mosquito was exhausted it could be started again by touching the legs. The distance flown by a mosquito before it was exhausted was used as a measure of the energy available for flight. A mosquito was considered to be exhausted when for some minutes it had been unable to fly more than a few revolutions after brief rests. Most individuals flew readily on the mill and the distances and speeds recorded showed that attachment to the arm did not greatly impede flight. No attempt was made to correct the results for artificial drag so the results recorded, which were used for comparative purposes, indicate the distances flown on the mill.

In experiments in which glucose solution was fed, females aged 40–72 hr. were flown to exhaustion, kept overnight and exhausted again the following day. They were then fed 10 % glucose solution from a graduated, wax-lined pipette. The mosquitoes could generally resume continuous flight within a minute of starting to feed.

The fat body cells of fully grown larvae of Culexpipiens contain discrete droplets of fat and protein and deposits of glycogen (Boissezon, 1930, 1932). These reserves are carried over to the adult stage where they disappear from the fat body as they are used. There is no development of an ‘adult fat body’ of different structure as in the higher Diptera, but feeding on sugar leads to a heavy accumulation of fat and glycogen in the fat body cells. Staining the flight muscles shows the presence of small amounts of glycogen; the apparently heavy staining of fat is confined to the sarcosomes.

Mosquitoes which had been flown to exhaustion were fixed in Baker’s formaldehyde calcium, divided sagitally in two and stained in Sudan IV, or were fixed in Camoy’s fluid, divided and stained with Best’s carmine. No differences could be found in the extent of fat body fat in exhausted and unflown specimens, but the glycogen had almost disappeared from the fat body of exhausted specimens. This suggested that, as in Drosophila, glycogen was the principal source of energy for flight.

The fat body is fairly extensively developed in newly emerged adults of Culex pipiens, form berbericus, particularly in the abdomen where fat body lobes lie between the other organs. To find whether the fat body reserves could make an appreciable contribution to flight, females aged 48–72 hr., which had been confined before flight in 3 × i in. glass tubes with moist filter-paper, were flown to exhaustion on the flight mill on each of five successive days without receiving any food. They were returned to the glass tubes between flights. The results, which are summarized in Table 1, series C, and given in full in Appendix I, C, showed that considerable distances could be flown on the flight mill on the reserves alone, and doubtless much greater distances would be covered in free flight. The mean total distance flown was 5339 metres. Nearly half of the total distance was covered on the first day, the distance falling precipitously on the second day and then continuing to decline gradually.

Ten females of C. pipiens form berbericus were fed a total of 0-5035 mg. glucose after being flown to exhaustion and the sum of the distances flown on this glucose was 21,823 m. Details of the flights are given in Appendix II. By comparing the distance flown by unfed females with that of females fed glucose solution, it is found that the flight reserves of an unfed female aged 48–72 hr. must be, very approximately, equivalent to 0-12 mg. glucose, a result which agrees remarkably closely with the calculated reserves of certain species of Aëdes (Hocking, 1953).

Hibernating mosquitoes are known to develop particularly extensive fat bodies containing, in C. pipiens, droplets of fat and glycogen but no protein (Roubaud, 1933). To find the extent of their powers of flight, females of C. pipiens pipiens L were removed from hibernation in a cellar during December, raised to room temperatureand flown toexhaustion. Most individuals were flown within a few hours of being taken from hibernation, nearly all within 12 hr. The mean distance flown on the mill was 4300 m., the maximum 8674 m. and the minimum 2239 m. The mosquitoes flew fairly readily and strongly compared with non-diapausing mosquitoes and speeds up to 1 m./sec. were recorded. This was also the maximum speed found in C. pipiens form berbericus. The females of C. pipiens form berbericus required 0·099 mg. glucose to cover the mean distance flown by hibernating females of C. pipiens pipiens i.e., 4300 m. It was not known whether the mosquitoes had fed on plant juices before entering hibernation. Females of the hibernating generation of C. pipiens pipiens have been seen feeding on plant juices before entering hibernation (Lacour, 1937), but Roubaud (1933) considered that the hibernation fat body could be developed without adult feeding.

In the absence of experimental evidence opinions have varied on the possibility of ingested blood being used as an energy source by mosquitoes. It has been claimed that there is no evidence that insects can utilize proteins to provide energy for flight (Hocking, 1953), yet it has also been stated that in the field all the indications are that females of Anopheles feed on blood alone (Muirhead-Thomson, 1951). However, females of A. aquasalis Curry have since been reported feeding from flowers (Senior White, 1952) and it is well known that Culicine mosquitoes are frequent flower visitors.

To find the effect of blood meals on the duration of flight, the distances flown by unfed females of Culex pipiens form berbericus were compared with those of females fed once, to repletion, on human blood. As mosquitoes kept under only slightly different conditions before flight showed considerable differences in flying ability, the two groups were kept, as far as possible, under identical conditions. The mosquitoes were kept at first in cages of 8 in. side, those fed blood being fed on the day before their first flight. They were first flown at the age of 72–96 hr. as few mosquitoes will take a blood meal before they are 48–72 hr. old. Each mosquito was flown to exhaustion on 5 successive days and between flights the mosquitoes were each kept in a 3 × 1 in. glass tube with moist filter-paper. The results are summarized in Table 1, series A and B, and given in full in Appendix I A, B.

The mosquitoes which had taken a blood meal flew more than twice as far as the unfed females. The mean distance flown by the end of the 5 days was 2422 m. by the unfed mosquitoes and 5063 m. by the blood-fed. There is a slight difference in the mean distances flown by the two groups on the first day but it is not statistically significant. This distance was maintained, even increased, by the blood-fed females on the second day while the unfed females could fly little more than a quarter of the distance they had covered the previous day. The mean distance flown by the unfed mosquitoes continued to fall gradually over the last three flights. The blood-fed mosquitoes flew a considerable distance on the third day, although less than on the first 2 days. This distance fell precipitously on the fourth day as it had done in the second flight of the unfed mosquitoes.

The differences between the mean flight distances of blood-fed and unfed mosquitoes was 2641 m., a distance which would be covered by exhausted mosquitoes after feeding on approximately 0·061 mg. of glucose. A meal of 3 mg. of blood would contain only 0·003 mg. of hlood sugars so it is clear that digestion products of the blood, and not blood sugars alone, were providing energy for flight. In C. pipiens form berbericus development of the ovaries is normally dependent on blood feeding, most females being anautogenous, and fully grown eggs are found in the ovaries five days after the females have fed. The ovaries of females which had been flown to exhaustion on the 5 days following blood feeding were found to contain the usual number of fully grown eggs, showing that both energy for flight and material for the growth of the eggs can be obtained from ingested blood.

Histological examination of the animals showed that after a single flight to exhaustion the glycogen deposits in the flight muscles and fat body were almost depleted. Twenty four hours after exhaustion there were slight deposits of glycogen in the flight muscles of unfed specimens but practically none in the fat body. In specimens fed blood before the flight to exhaustion, slight deposits of glycogen appeared in the flight muscles after a twenty-four hour rest and considerable quantities of glycogen were to be found in the fat body.

From the nature of the reserves remaining in exhausted mosquitoes it has been inferred that glycogen, not fat, is the immediate source of flight energy in these insects, while the many accounts of mosquitoes of both sexes feeding on nectar have shown this to be an important source of flight energy. The Lepidoptera, however, which also feed on nectar have been shown to use fat and not glycogen as the source of energy for flight (Zebe, 1954). Drosophila is known to use glycogen in flight and its fat reserves have been considered to provide energy for activity apart from flight; it was suggested that the fat could not be mobilized rapidly enough to provide energy for flight (Wigglesworth, 1949). In mosquitoes the fat reserves are more extensive than the glycogen reserves and it has been shown that during diapause the amount of fat gradually decreases, the other solids remaining constant (Buxton, 1935). This lends support to the suggestion that fat provides energy for activity apart from flight in Díptera but there is no direct evidence to support the suggestion that fats cannot be mobilized rapidly enough to provide energy for flight and it is now known that insects which consume fat during flight show similar rates of uptake of oxygen to the Diptera (Krogh & Weis-Fogh, 1951; Zebe, 1954).

The reserves of unfed females of C. pipiens form berbericus were able to provide energy for flights of considerable length, and the digestion products of blood meals were also found to provide flight energy. Mosquitoes which were first flown a day after feeding on blood flew considerable distances for 3 days. In most cases the blood was fully digested before the third flight, a time for digestion similar to that recorded in other mosquitoes (Fisk & Shambaugh, 1952; West & Eligh, 1952; Weitz & Buxton, 1953), so it appeared that reserves were available for flight i day after the blood meal had been digested and thereafter the flight distance fell off.

Glycogen was found to disappear from the fat body during flight in the mosquito Culex pipiens form berbericus Roubaud, while no change was observed in the fat deposits. The fat body reserves were able to sustain a flight of considerable length and ingested blood could also be used as a source of flight energy.

It is a pleasure to thank Prof. V. B. Wigglesworth, F.R.S., for his helpful criticism of this work. The electronic circuit was designed by Dr J. W. L. Beament. I am most grateful to him for this, for the loan of equipment and for his help in maintaining it.

Distances flown by females of Culex pipiens form berbericus. The mosquitoes were flown to exhaustion on five successive days. Distances in metres

Distances flown by females of Culex pipiens, form berbericus fed 10% glucose solution after being flown to exhaustion. Distances in metres

Distances flown by females of Culex pipiens pipiens taken from hibernation. Distances in metres