Ent. exp. & appl. 5 (1962): 14--32. North-Holland Publishing Co., Amsterdam
SWARMING OF MOSQUITOES
Laboratory Exper iments
Under Cont ro l led Cond i t ions
HEDVIG TETENS NIELSEN and ERIK TETENS NIELSEN*) Mols Laboratorium, Femm611er, Denmark
Experiments have been made on the swarming of a number of mosquitoes, especially Culex fatigans, under laboratory conditions and the influence of light intensity, temperature, different color swarm markers; and time factors, both in regard to duration of the change of light and the time lapse between experiments, have been studied.
In connection with field studies of the swarming of mosquitoes it was attempted to produce swarming under controlled conditions in the laboratory.
The experiments reported here are somewhat preliminary as the technique was developed concurrently.
Definition of Swarming In the field, swarming is a formalized flight pattern performed by male mos-
quitoes within narrow spatial limits (NIELSEN & HaEGER, 1960). In a cage the swarming flight includes the same characteristic elements as in nature: persistence of flight, formalized movements and restrictioa to a certain space.
EXPERIMENTS WITH Culex pipiens fatigans WIED. Technique
The cage used for most of the experiments was 130 cm high, 122 cm wide, and 95 cm deep, with white-painted back wall and floor; the ceiling and the middle third of 'the front (the observation window) were of glass; the sides and the rest of the front were of screen wire. On the left side of the front was a sleeve for easy access to the cage, and nearly the whole left side was made into a door.
In the middle of the cage was placed a ring stand 1) (also white) on which three th,ermocouples were attached, one sticking up from the top of the stand, 90 cm above the floor, one on the middle of the stand, and one almost touching the floor. There was in the cage, furthermore, a dish with honey-soaked gauze or cotton and a bowl with water and sand or moist paper towels. A piece of black
*) Contribution No. 113, Florida State Board of Health, Entomological Research Center, Veto Beach, Florida. This investigation was supported by Public Health Research Grant E- 1492, from the National Institute of Allergy and Infectious Diseases, Public Health Service.
2) A horizontal iron plate with a screwed-in vertical rod.
SWARMING OF MOSQUITOES 1.5
paper (24 X 131//2 cm) was attached to a dowel stuck through the screen to the right of the observation window so it could be moved from the outside; during the ,experiments this "marker" was usually placed on the floor to the right of the stand; between the experiments it was either placed in the far right-hand corner up toward the white wall or with a corner tucked in under the foot of the stand.
The cage was placed in a temperature-controlled room without daylight, the temperature of which is given for each experiment. When the lamps which furnished the illumination were lit, the temperature in the cage rose a few degrees and th,e temperature was therefore measured several times during each observation period by means of the thermocouples.
The illumination consisted of four 200-watt incandescent lamps placed above the cage. Changes in illumination were made by means of an automatic control. In this report, the word "twilight", when unqualified, refers to the experimental light-change period; this period started at "sunrise" (first glow of light) with increasing light and at "sunset" (first decrease from full illumination) with decreasing light.
The illumination was measured either with a comparometer placed on the floor of the cage, or with photocells attached in different heights to the frame of the cage. The light could be measured from 0.00--2 log lux, when the lamps were just glowing, to full "daylight" of 3.20 log lux.
The illumination to which the animals were actually exposed is difficult to indicate. The light from a few incarzdescent lamps cannot be compared to the diffused light from the sky as measure~d, in the field.
Not only the spectral distribution is different, but with the light source so close at hand, there is a definite gradient in illumination. The intensities given in "lux" were measured at only one place; the actual illumination varied from 50% of the given values at the bottom, to 170% at the top of the cage.
The animals Culex f~tigans were collected as larvae or pupae in the field and brought into
the temperature-controlled room before emergence; the adult males (usually 30--50) were transferred as soon as possible to the swarming cage. The males usually lived a month or two in the cage and appeared to be healthy and com- fortable. In one case a female happened to be among the males, but no influence on the swarming behavior was noticed - - she remained calm and usually rested on one of the walls throughout the swarming periods. This observation is in agreement with the concept (NIELSEN & HAEGER 1960) that swarming is an independent habit without any specific importance for the sexual behavior.
Non-swarming behavior of male C. fatigans Feeding:
After the swarming in the "morning" some individuals would usually fly down to eat, but by far most would continue to fly about searching for resting places.
The favorite resting place was the "door" or~ the left-hand side of the cage.
16 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN
The most likely explanation for this preference was that the screen on the door was of a slightly larger mesh than was used on the rest of the cage.
The white wall and other exposed surfaces were avoided for daytime resting. The animals were mostly found where there was shadow, especially up under the ceiling, under a slender piece of wood attaching the walls to the ceiling. The sleeve, which was white, was not liked, nor was the black paper even when it curled up in one corner offering a very dark hiding place sheltered from the ceiling light. Toward sunset, especially, some might be found resting on the stand and on the photocells. After the sunrise swarm the animals usually took a long time to find resting places - - but after an hour or so practically all would be quiet. During most of the day they would remain immobile; once in a long while a single individual would fly up and slowly mo>e to another resting place, rarely making any detours but going straight to the new resting place, where he would remain.
For the first few days of adult life they were usually difficult to scare up during the daytime but later they would fly up when the door slammed; or the observer counting them at their resting places would make them fly up. However, they immediately returned to the resting places and within a few minutes none would be moving in the cage.
Swarming of Culex fatigans The antennal fibrillae were extended during the first day after emergence and
kept permanently extended for the rest of their lives. Swarming did not start until they were three days old and 'the first swarms were usually very poor and of short duration, after which time they would form swarms both during increase (sunrise) and decrease (sunset) of the light intensity.
During these laboratory experiments there were two main types of swarming flights exhibited by the mosquitoes which we called stand swarms and marker swarms. They might correspond to the types known from field observations as topswarms and marker swarms (NIELSEN &: GREVE 1950, NIELSEN &: HAEGER 1960) but appear to be somewhat different from these and until more has been learned about swarm types caution should be used in comparing the experimental types with the ones observed in nature.
Stand Swarms The first peroeived glow from the lamps would usually be seen about two minu-
tes after the motor for the light change had been started, and at once animals flying hither and thither could be discerned against the glowing filaments of the lamps (I, Fig. 1). The flight seemed slow, and it is uncertain if this flight is a true swarm. The light intensity rose rapidly during the first few minutes of dawn and as soon as it was light enough to see the animals without the aid of the filaments as background, their flight was definitely swarming. At the same time the animals slowly withdrew from the ceiling and seemed to gather over the top of the stand (II, Fig. 1) and during the next 51/2 minutes (average) part of this
SWARMING OF MOSQUITOES
. . . . . . . . . . . ; :~ : : : :=2G- - -X . . . . . . .
' ~ I ! . . . . . . . . . . I
r . . . .
. . . . . I
Fig. 1. The four types of swarming in the cage (vertical cross-section). I. The first activity seen at dawn (flight just below the ceiling). II. Swarm between the ceiling and stand (the first gathering of the stand swarm). IH. Stand swarm fully developed. IV. Marker swarm.
swarm went further down in the cage at least until the middle thermocouple had been reached, sometimes even lower. The swarm was not performed all around the stand; there seemed to be a preference for swarming between the stand and the observation window, but closer to the stand.
Fig. 2. Stand swarm, zigzag type flight. Arrows indicate direction in which the head was pointing (seen from above).
The swarming flight is a continuous, rapid movement with twists and turns (Fig. 2). The swarmer is only concerned with swarming, showing no interest in food or resting places. Each individual appears to have its own pattern of
18 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN
turns and twists in the flight, and each swarmer remains within