SWARMING OF MOSQUITOES: Laboratory Experiments Under Controlled Conditions
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 BY 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. PURPOSE 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. 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. SWARM TYPES 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 . . . . 17 . . . . . 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. ---4 9 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 the same few cubic centimeters for minutes on end. But from time to time the little 'box' in which it seems to swarm is shifted a little. When the illumination had reached an intensity of somewhere between 1 and 2 log lux, single individuals left the swarm and began calm, non-swarming flights. In many cases the swarming flight slowed up gradually, the turns and back-and- forth movements becoming less frequent and less distinct until the animal finally left the swarming site in calm flight. Sometimes it returned and made a few swarm-like turns, then left again, while oth, ers calmly sailed right through a swarm without as much as a dip in the flight. Marker Swarms As a rule, when the main part of the stand swarm had reached the middle of the stand, one or more mosquitoes would begin swarming over the marker. This flight was faster and more concentrated than that of the stand swarm. The swarm never seemed to go beyond the edges of the paper, but in height might reach from the bottom of the stand swarm almost to the floor. On occasions when practically all individuals appeared to swarm, they sometimes formed one column of swarm from the ceiling to the floor where it was impossible to distir~guish the two kinds of swarms. When the marker was moved slowly, practically all that were performing the typical marker swarm followed it. If it was moved fast the animals scattered, and some hovered in the neighborhood of the place where the marker had been, while the majority found the marker in its new place and commenced swarming over it. I f the marker was put on edge the swarm would stop,, but the animals would hover in the neighborhood and as soon as the marker was turned again they would resume the swarm. < ~ < < .~ < - BOWING BOBBING J jJ DANCE-LIKE 3TEP.5 $h/ARM/NG Fig. 3. Types of flight over black marker. SWARMING OF MOSQUITOES 19 The main types of flight are shown in Fig. 3. Contrary to the stand swarm, the marker swarm had an orderly, uniform appearance, because all mosquitoes made back-and-forth movements, pointing the head in the long direction of the marker, more often toward the cemer of the cage (Fig. 4). Toward the end of the swarming period single indMduals seemed to conform less with the other swarmers. Sometimes the flight would be slower and no longer strictly back and forth in the long direction of the marker, but instead they would turn the head in any direction and make figure '8' or similar patterns, occasionally proceeding beyond the edges of the marker. This type of activity was called "dance-like steps". I f an individual performing this type of flight is watched closely - - which is fairly easy as he is then in contrast to the rest of the swarmers - - his movements become less and less swarm-like and he sometimes makes small excursions away from the paper in non-swarming flights; when returning to the marker he might make a few swarm-like movements, but more often he will move up and down rapidly a number of times, which type of behaviour in the following will be called "bobbing". Sometimes an individual will make a certain round trip several times and return to the marker to make 'dance-like steps' or to 'bob'. One favorite 'trip' was from the marker (placed to the right of the stand) to the observation window, along the wall to the right-hand corner, then back to the marker, sometimes after a little turn along the solid screen wall (the right-hand wall from the observation window). Sometimes the animals would not take this long to stop swarming; they would rise upward and calmly fly off or simply fly to the food. STAND MARKER Fig. 4. Swarm over the black marker. Arrow indicates the direction of the head. Even when all swarming movements had stopped, the animals looking for resting places would often cross over the marker going from one end of the cage to the other. Whenever this happened, they would 'bob' or make a dive or two at it (this was called "bowing"). The 'bobbing' eventually stopped, but the 'bowing' went on all day whenever an animal passed over the marker. 'Bowing' has even been seen in animals so young that the antennal fibrillae were still recumbent. In other cases the honey-soaked gauze served as a marker when it had turned moldy and black. This swarm was never very large and was only seen when 20 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN practically all individuals participated in the swarms. In these cases there might also be an individual or two performing over the photocells. The description of the swarms is given here for the "dawn" observations - - for the "evening" observations the same kinds of activity were observed, but in ~everse order. SUNRISE EXPERIMENTS Duration of Twilight It is well known from field observations that there is a correlation between the duration of swarming and the duration of twilight. In most of the experiments the light was turned on gradually, so that full illumination would have been reached in about 30 minutes; but usually the increase was halted for about ten minutes during the maximum swarming at an illumination of about 1.0 log iux. The stand swarms began within a few minutes (average 1.5 minutes) after the start of the "sunrise" at an illumination of 0.60--2 log lux, and ended after a period of 13--22 minutes at illuminations varying from 1.20--1.82 log lux (average 1.60 log lux). The marker swarm began 5--13 minutes and ended 23--28 minutes after "sunrise"; the average illuminations were correspondingly 0.92 and 2.11 log lux. In some of the experiments the increase was faster (20 minutes instead of 29 minutes) and the increase was not halted. In such cases the stand swarm began instantaneously before it was ,possible to measure the illumination, and ended at a higher illumination, 2.02 log lux instead of 1.60 log lux, which seems to indicate that the swarming will continue for a certain time even at illumination which would be inhibitory after a longer flight period. The marker swarm commenced with nearly the same delay (average 7 instead of 8 minutes) at a higher illumination: 1.14 instead of 0.92 log lux, but this type of swarm ended at practically the same illumination (2.10 log lux instead of 2.11 log lux) while the duration in minutes was very different (11 instead of 25 mi- nutes). When the illumination was changed from darkness to full light within a fraction of a second, the mosquitoes reacted by excitedly flying around for a few minutes, after which most of them sought resting places. A few made dance-like steps and short swarms over the marker on and off for some time; but after half an hour or so, normal daytime rest prevailed. If the light is turned .on gradually, even if it is very fast (5--15 seconds) swarming will occur, but only after a delay which depends on the degree of illumination to which the light is turned (Fig. 5). The stand swarm began im- mediately if the level was 1.0 log lux. The delay increased slowly up to 3--4 minutes with light intensities rising to about 3.00 log lux; higher light intensities caused a more pronounced rise. The marker swarm showed a similar relationship to fast light increases, but there was always a delay, even at 1.00 log Iux. The conclusions which may be drawn from these experiments are: (1) Swarming is released by a gradual increase in light intensity, the stand swarm (top swarm) normally occurring with the slightest increase in illumination; the marker swarm after a certain delay. SWARMING OF ~vIOSQUITOES 21 STAND SWARM MINUTES DELAY OF SWARMING 4 o I I I I I I t5 ~5 2.0 2,5 3.O 3.5 LOG LUX NARICs SWARM MINUTE DELAY OF $WARN/NG ol o (3- I I I I I I 1.0 15 20 2.5 30 .I$ LOG L(IX Fig. 5. Ordinates are the delays before the swarming starts when the light is increased in 5--15 seconds to the intensities given as abscissa. (2) The urge to swarm is present before dawn; the change of light releases the habit and also further activates the urge. Therefore, if the light is increased to an inhibitory level before swarming begins, the accumulated motivation will release the swarming after a delay. The higher the light intensity, the stronger the in- hibition and the longer the delay. (3) If the light is changed more slowly swarming will begin during ~he twilight; there is, however, some difference in the light thresholds for the beginning and end of swarming whether the duration of twilight is 20 minutes or less (short twilight) or more than about 25 minutes (normal twilight) (Table I). TABLE I The Effect of the Duration of Twilight on the Sunrise Swarms of Culex fatigans Stand Swarm Marker Swarm Duration of Number of Log Lux Duration Log Lux Light Increase Experiments Start Stop in Minutes Start Stop 29 9 0.65--2 1.60 19 0.94 2.13 11 3 - - 1.90 10 1.14 2.10 (4) The upper limit for stand swarms at normal duration of twilight is 1.6 log lux. During one experiment the increase was stopped at 1.54 log lux instead of the usual 1.05 log lux; the stand swarm stopped during the constant light interval 13 minutes after "dawn", while in the eight other experiments when the light increase was stopped at 1.05 log lux the swarms did not stop until the light change was resumed 15--22 minutes after dawn. Further experiments dealing with this point shall be mentioned below. At short twilights the threshold is 1.9 log lux. It seems that the urge to swarm is probably so strong that the swarming continues into the period with light intensities which normally are inhibitory. The upper threshold for marker swarms is independent of the duration of twilight (2.1 log lux), but the beginning is a little later at short twilights (1.14 log lux) than at normal ones (0.94 log lux). The difference is not significant. (5) The duration of stand swarms is at normal duration of twilight 19 minutes, at shorter duration of twilight it is 11 minutes. 22 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN Influence of Te.mperature on Swarms at Sunrise In the preceding, only experiments at temperatures between 19 ~ and 24 ~ have been considered; there is no indication of influence of temperatures within this interval. In addition to these experiments, two were made at lower temperatures and five at higher ones. At 14.8 ~ and 14.9 ~ the swarmers moved so slowly and wi*h so much less pronounced swarming movements that it was doubtful whether the flight could be called swarming at all. At high temperatures they appeared to swarm with a faster flight than at lower temperatures. In Table II are given the averages of *he times and light intensities at the three groups of temperatures. TABLE II The e]fect o] temperature on the sunrise swarms of C. fatigans iNTo. of Temperature Experi- ments 14.8--14.9 2 19.6----23.0 9 26.5--31.1 6 Stand Swarm Marker Swarm Start Stop Start Stop Min. Log Lux Min, Log Lux Min. Log Lux Min. Log Lux 2 0.15--2 13 1.03 16 1.03 21 1.28 1~ 0.64--2 19 1.58 8 0.92 25 2.11 11/2 0.93--2 15 1.11 5 0.46 22 1.84 There seems to be a delay in the start of the stand swarm with increasing temperature; this cannot be seen in starting time, but the increase in average light intensity at starting seems to indicate it. It must be remembered that the light estimates are rather unreliable at these low intensities, and the changes, both in intensity and in the behaviour of the animals, are so fast that the increase cannot be considered factual. There is, however, no doubt about the end of the stand swarm being influenced by temperature; the difference between the average intensities at medium and at high tempera*ures is 6 times larger than the error of the difference. It is justified to .conclude that the end of the stand swarm is later at medium temperatures than at low or high ones; and as there is but little difference in the beginning, the duration of the stand swarm is longest at medium temperatures. The end of the marker swarm is also later at medium than at extreme temperatures, but also beginning of this type of swarming is influenced by temperature. Contrary to the weak tendency for *he stand swarm to delay the beginning to a higher illumination, the marker swarm begins earlier, at lower light intensity, the higher the temperature. The duration of the marker swarm is very short (5 minutes) at low temperatures, but at medium and high temperatures the duration is the same (17 minutes). The duration of all swarming is 23 minutes at medium temperatures, and 19 and 201/2 minutes at low and high temperatures, respectively. SUNSET SWARMS Stand Swarms Of the experiments including observations at artificial sunset there are two series, with 12 and 3 observations, respectively, which were conducted the same way as the sunrise experiments. SWARMING OF MOSQUITOES 23 Before the start of the sunset observations the animals were usually very quiet, and if, as it sometimes happened, they were frightened up by the entrance of the observers, they would calm down again in a matter of minutes. Therefore, the moment of the first activity varied a great deal. But in any case, whether disturbed by the observer or not, they would begin their activity by making a short flight or by cleaning. This took place some time after the start of the change of light; the average light intensity for the start of the cleaning was found to be 2.52 log lux; the first movement was at 2.82 log lux. Usually only one animal would move or begin to clean, but on a very hot evening (32.1 ~ there were several animals active within the first few minutes of sunset; some were shaking their limbs, others were cleaning, and some flew about. The activity was in no case continuous, and rarely would more than a few individuals participate in it until the start of the swarm. As the light grew dimmer the animals became more and more active. One would begin to fly in wavy circles all around the ca&e and presently begin to make bobs or dance-like steps (see above). At times he would go to rest before resuming the flights again. The bobs and dance-like steps were often performed at other places in the cage than where the swarm usually took place. These flights may be comparable to that phase of the swarming called "ascent" (NIELSEN & GREVE, 1950). These pre-swarming flights would gradually become more and more swarmlike. The swarms were nearly always started by one or two individuals, while the bulk of the swarmers would not arrive until a minute or so later. On all evenings wi*h standard sunset the swarms took the form of a stand swarm; marker swarms were only found in a few cases which will be discussed below. Duration of Twilight In one experiment the light was changed instantly from full "daylight" to "swarming level" (3.2--1.0 log lux) and only two mosquitoes swarmed; they began 8 minutes after the change of light and continued for 3 and 6 minutes. Light change from 3.2--1.0 log lux lasting 21,/2 minutes produced swarms 5 minutes after the first change; these swarms continued until the light change was resumed. From Table III can be seen *he effects of different durations of twilight (full daylight to darkness). TABLE III The E]fect of the Duration of Twilight on the Beginning of Swarming at Sunset Minutes after Start of the Swarming Number of Beginning of Light Duration Experiment Observa- Change of Minutes after Number tions 2.6 Log Lux 0.6 Log Lux Twilight Start of Dusk Log Lux 1--3 3 7 18 11 11 2.11 4--9 5 13 29 16 24 1.56 10--12 3 34 62 28 51 1.76 13 1 55 74 19 70 1.18 14 1 13 28 15 25 1.38 15 1 12 28 16 26 0.98 Average of Experiment number 4--12 1.627 +- 0.066 24 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN 1/1 experiments 1--3 the duration of twilight was very short (11 minutes) and the swarming began at a higher illumination than in the following; in these experiments the light was measured by means of the comparometer instead of the photocell, and the difference may have been caused by the difficulties in the calibration of the instruments. The difference in experiments 4 9 and 10--12 is not significant, and it is justified to say that the difference in the duration of twilight between 13 and 35 minutes does not influence the light intensity at which the swarming starts. This light intensity of 1.627 log lux occurs, of course, at different times after the start of the light decrease, and as the swarming in all cases ends at or just after total darkness, flae duration of the swarming (as is well known from field observations) is correlated with the duration of twilight. It is remarkable that the swarming at sunset starts at the same level as it normally stops at sunrise: 1.576 0.063 and 1.627 q- 0.066 log lux, the difference being 0.051 0.092 log lux. Experiment number 13 is not included in the calculations because a mechanical error in the sunset machine gave a very slow decrease in light for nearly one hour, after which the decrease followed rapidly with a twilight of 19 minutes. The swarming was here postponed to an illumination of 1.18 log lux. In experiment 14 the temperature was 28.1 ~ and the swarming began at 1.38 log lux. This is a rather low intensity but not outside the normal range. However, in experiment 15 the temperature was 32.1 ~ and there was considerable delay in the beginning of the swarming, it being postponed to a light intensity of 0.98 log lux. This strongly indicates a delaying effect of high temperature, so the two experiments, 14 and 15, were accordingly excluded from the calculations of the 'normal' swarming at temperatures ranging from 19.8 ~ to 25.0 ~ Marker Swarms Marker swarms in the "morning" occurred at all temperatures with a marked increase in the duration of the swarms at high temperatures. In the evening this type of swarming only occurred at temperatures above 25 ~ . At 22--23 ~ some individuals would bow to the marker and at 20 ~ very few paid any attention to it at all. In other experiments than those quoted here these temperature limits for the sunset marker swarms were found to hold true. ENDURANCE EXPERIMENTS During the experiments reported above, it was found that the end of the sunrise swarms and the beginning of the sunset swarms took place at the same level, 1.6 log lux. The importance of this threshold shall be discussed below. The end of the sunset swarms and, in most cases, the beginning of the sunrise swarms occur at such low light intensities that observations are difficult. For other species such a lower limit has been found in the field. NIELSEN & GREVE (1950) found Aedes cantans starting in the morning at about the same level the evening swarms stopped, and PROVOST (1958) determined the lower threshold for swar- ming of Psorophora confi, nis to be 0.30--2 log lux, which is below the illu- mination on a night with full moon. On such a night the swarming was actually found to continue over an hour after its expected termination. SWARMING OF MOSQUITOES 25 In many experiments the light change was arrested during the swarming period for about 10 minutes to give the observer a better opportunity to study the in- tricate swarming movements when the swarm was at its best, around 1.0 log lux. Four sunrise experiments were made in order to see how long the swarms would continue when the light was kept constant at 1.0 log lux with different temperatures. At 24.3 ~ the main swarming over the stand lasted 47 minutes after the start of the swarming, but single individuals continued with short spurts of swarming until the light was raised to full daylight. At 14.8 ~ and 30.4 ~ the stand swarm stopped completely after 9 and 11 minutes of swarming, respectively, while the swarm at 21.1 ~ lasted 28 minutes. The marker swarm lasted longest at 24.3 ~ (70 minutes); at 14.8 ~ it stopped after 9 minutes, while at 30.4 ~ and 21.1 ~ the swarms lasted 48 and 47 minutes, respectively. This might indicate that the marker swarm is much less affected by high temperatures than the stand swarm, and both are highest developed at 24.3 ~ and poorest at 14.8 ~ . During four of the sunset experiments the light change was stopped all night at different levels, with a temperature ranging from 21--23 ~ At 1.68 log lux, just above the normal threshold for swarming, only a few swarmlike movements were observed, but with constant light of 1.38, 1.08, and 0.78 log lux, respectively, swarming went on until 12 hours later when the light was changed in the usual 2.00 ~. log. lux when swarm stops 1.80 1.60 1.40 M,4 RKER ,5~//A RM 1.20 5TA/YD ~'WA R/#' 1.00 0 .80 0 .60 0 .80 LO0 I.?_O 1.40 Io 9. lux durin 9 the nigh~ Fig. 6. Correlation between the i l lumination during the night and il lumination when the swarming stops in the morning. 26 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN way for a sunrise observation. There was a clear correlation between illumination during the night and the light intensity at which swarming stopped (fig. 6); the higher the light intensity during the night the higher the threshold for the cessation of swarming. In these long-lasting swarms there were actually only few of the mosquitoes flying at any time; most of them would be resting on prominent places not normally used for resting, such as a photocell high in the cage. We have recently found Psorophora ferox males in the field performing swarms all day. The swarming took place in openings under palmetto fronds, low over the ground, and individuals would temporarily rest on palm fronds and straws jutting out in the swarming arena, from which they would join the swarm from time to time. Sometimes one would retire to an ordinary resting place and not rejoin the swarm any more; other animals resting in the ordinary places would from time to time join the swarm and the resters in the swarming place. Both Culex and Psorophora seemed to prefer certain resting places, all of them, .even to the degree of pushing one another off a favorite perch. The disturbed animals would at times join the swarm, but mostly they would fly to seek another resting place. SWARMING WITH SHORT INTERVALS All the experiments reported above were made with intervals of approximately 12 hours between light changes. After one postponed sunrise the animals were nearly impossible to wake when four hours later the evening observations began. The water and honey had to be replenished but the animals did not s'tir, and one even had to be pushed away from the dish. He was barely able to move. On four days the light was changed with intervals from ~ hour to five hours. These days were interspaced with days with normal 12-hour intervals. The ex- periments were all carried out with a twilight lasting 21 minutes from full light to complete darkness and vice versa. The light was measured with the comparo- meter. The upper threshold for "normal" swarming at the stand was 2.1 log lux in these experiments instead of the previously found (see above) 1.6 log lux; whether this difference is caused by the short duration of twilight or by an error in calibration of the instrument is unknown. With intervals of five hours and shorter, there was a considerable reduction in swarming. There was no pronounced difference in swarming between experiments with 1 to 5 hour intervals, but the only two experiments with 45 minute intervals (one sunrise and one sunset) showed a pronounced reduction. In the stand swarm the number of individuals - - usually 40 to 60 - - was more or less reduced; in 9 of the 14 observations the number at the maximum swarming was between 1 and 10, in 5 cases there were many; no certain correlation could be found between the number of swarming individuals and the preceding light or dark interval; however, on the day with most experiments (8 experiments between 08h and 19h) there were many (30--5O) swarming at the first and the last experiments (sun- rise and sunset, respectively); the rest of the sunrise had 15, 2, and 1 animals participating, the sunsets had 4, 4, and 2 participants. At sunrise the swarms were usually so much delayed that the beginning could be readily observed and the SWARMING OF MOSQUITOES 27 light measured; the average of seven observations was 0.64---2 log lux. The normal swarms started too early to get a light measurement; the sunrise stand swarm stopped at an average of 1.94 log lux after a duration of 12 minutes, while the swarms following short intervals only lasted an average of 41/2 minutes and stopped at 1.16 log lux. The reduction of the stand swarm at sunset was similar. The swarming began when the light had dropped to 0.69 log lux, while the controls started at an average of 2.11 log lux. In four cases the swarming ended after darkness, as it normally does, but at five other sunsets following short 'days' the activity stopped when the light went out. In minutes the average duration was reduced from 14 minutes in the control experimen.t to 5 minutes in the experiments with preceding short intervals. The marker swarms were poorly developed in all experiments; at sunrise there were swarms after all normal nights while after short nights swarming occurred at one sunrise only, when one individual swarmed for one minute at the beginning of the light change and another one swarmed for a similar period at the end of the 'dawn'. At sunset also only few individuals swarmed over the marker in the control experiments. The data available from 12 observations, 3 of which are controls with long days, is that instead of the normal start at 1.45 log lux, the swarming is delayed to 0.52 log lux and the duration of the swarm is reduced from 11 to 5 minutes. Culex fatigans in a black cage The observations given above were all made in a white cage with a black swarm marker. A few preliminary experiments have been made in the same cage as above, but the floor and the walls were covered with black paper. The stand was covered with black masking tape, the food was placed on the black cloth, and the sleeve was also exchanged for a black one. A white marker, exactly the same shape and size as the black one, was placed by the stand. In most cases the mosquitoes showed no interest in the marker but on three occasions they avoided it, when it was held under the normal stand swarm. The light intensities (measured with the comparometer) were 0.6 and 1.1 log hlx for two sunrise observations and 1.0 log lux for a sunset observation. When the light rose to 1.1 log lux, some even stopped swarming. Only in one case interest was shown in the marker: two or three swarmed over it after the stand swarm was finished - - the swarm lasted only a very short time. The temperature on that occasion was 25 ~ at the start of the sunset, and rose to 29 ~ at the end (on the other days the temperature usually fell), the average temperature being 28 ~ for the whole experiment. Other markers were tried: a very bright yellow one was as much disliked as the white one. A "hot" red one was in most cases treated like the white and the yellow, but twice the animals made dance-like steps over it at a light intensity of 0.8 and 0.7 log lux. They swarmed over both the red and the yellow on the day when they swarmed briefly over the white one. Best in the dark cage was, strangely enough, a dark blue marker. They never avoided it as they did the other markers, and they frequently made dance-like steps over it at 0.8 and 0.7 log lux (at the same time as they did over the red marker), but on two evenings 28 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN they swarmed over it. The first evening was the one when the temperature rose during the experiment. They swarmed from 1.5 and 1.6 log lux and stopped both evenings at 0.2 log lux. The stand swarms in. the blackened cage did not vary much from the ones in the white cage. The light change lasted 43 minutes varying between 38--46 minutes. The full "daylight" was 2.96 log lux. Swarming began 6--7 minutes after the dawn when, in 5 out of 9 experiments, it was possible to measure the light (0.80--3 log lux.). They stopped 19 minutes after dawn at an average of 1.47 log lux. The sunset swarms (twilight of 25 minutes) started at 1.37 iog lux, 29 minutes after the start of the light change, and stopped 42 minutes after the start of the light change at a light intensity too low to measure, though in two cases the last swarmer appeared to stop at 0.08--3 log lux and 0.88--3 log lux. In all cases, they stopped before the last glow faded away. OBSERVATIONS ON CULEX NIGRIPALPUS THEOB., ANOPHELES QUADRIMACULATUS SAY, AND PSOROPHORA HOWARDI I COQ. A few experiments has been made with other mosquitoes than Culex fatigans. Culex nigripalpus swarmed in the black cage and began in the morning at an average light intensity of 0.72--2 log lux and stopped at 1.32 log lux. Only 2 sunset experiments were made during which the swarms started at 0.63 log lux and'l.13 log lux and stopped at 0.23--3 and 0.08--3 log lux, resp. There were only one to three swarmers out of about 200 individuals present in the cage. They avoided the white marker and when it was removed the swarm increased to 10 individuals. Anopheles quadrimaculatus were placed in the usual swarming cage in which everything was white except the black marker. As soon as the first glow appeared they would begin to fly under the ceiling, presumably extending the antennal fi- brillae while on. the wing. Between 4---14 minutes later they would form stan~d swarms lasting 4---33 minutes. They apparently continued until the light was suddenly turned up to full "daylight". They did not swarm over the black marker, but sometimes bowed to it. Psorophora howardii did not swarm in the cage but between 32--46 minutes after the start of the 60-minute sunset some would extend the antennal fibrillae and 40--45 minutes after the start of light change a flight would begin, but there was nothing in this that indicated swarming behaviour although sometimes the flight would be somewhat wavy. No sunrise experiments were made with this species. AEDES TAENIORHYNCHUS WIED. A. taeniorhynchus did not swarm in the small cage hitherto mentioned and was therefore tested in a larger cage (4.9 m X 2.4 m, and 2.1 m high), made of cellotex with glass ceiling. In the east end of this cage was a doorway with a curtain and a very small ar~teroom with a door. The mosquitoes always got out even with these precautions. In the opposite end of the cage were two small windows. The daytime illumination was furnished by 8 fluorescent lamps plus SWARMING OF MOSQUITOES 29 normal daylight; sometimes the ceiling light from the assembly room in the Entomological Research Center, which housed the cage, was also used. The light change was effectuated by gradually drawing light-proof curtains over the glass ceiling. In the morning the natural light in the assembly room was used, and when the curtains were fully opened, the overhead lamps would be put on. In the evening the overhead lamps were left on while the curtains were slowly closed. One of the four curtains would still be about a centimeter or two open when the rest of the curtains were completely closed. This small opening was sufficient to give the "swarming" light. When the curtains had been closed except for the small opening, the overhead lamps would be turned off, either all at once or in two steps. Different variations of these ways of changing the light were tried. On simulated mornings, if the light came on all at onoe there would be no swarm but the animals would fly up in the ceiling - - especially in the part where the light came through. Some of them extended the antennal fibrillae but within half an hour they had them retracted again. If the gradation of light was made still more subtle by manipulating the blinds in the assembly room in addition to the overhead blinds, the animals became much more active and they swarmed by the faint light coming through the two observation windows in the cage. The swarms increased in size at the first increase in the light intensity but three minutes later they no longer swarmed over the marker, a black piece of paper somewhat larger than the one used in the smaller cage. There was no concentration of the swarm although they had both a stand and a small tree over which to perform. The best swarm was obtained when all curtains over the cage were open and all but one of the blinds in the assembly room were closed. Three minutes later all curtains but one were suddenly fully opened. The result of this light change is given in Table IV. TABLE IV Swarms of Aedes taeniorhynchus in 'walk-in' cage. These swarms were initiated by a sudden increase in light intensity 7/VIII 8/VIII Type of Minutes After First Light Change Minutes After First Light Change Swarm Start Stop Start Stop I --* 235--321 ---* 222--295 II 3 47 5 21 III 9 43 5 51 I Swarm over a cage containing newly hatched individuals. II Swarm over head of observer. III Free swarms. * Swarms started before the first light change. During the sunset experiments the mosquitoes extended their antennal fibrillae when the light intensity was very low (after all curtains were closed except the one which was opened a couple of centimeters). One evening, when only the natural light in the assembly room was on, the animals extended their fibrillae 30 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN before the curtains were closed. They also did this on the very first night of the experiment (at an age and time at which, in nature, they would have migrated). There was no gathering of a swarm on any occasion but a humming could be heard for some time after the extension of the fibrillae had started. The pitch of the humming would change with the slightest noise. Sometimes a noise would make them intensify the humming and sometimes it would make the humming stop. Only two mosquitoes were seen to swarm at "sunset". One performed over the marker in the light from a 150-watt lamp placed on the floor on the other side of the entrance curtain. The other performed when the observer shielded an animal from the glare of the lamp. The humming might have been swarming, but it cannot have been a united swarm. It is perhaps more probable that the animals were buzzing the ceiling and making other similar kinds of flights. The light intensity was so low that the observer could not see the animals without the aid of a flashlight. It seemed thus that only a shock, such as a sudden jump in the light intensity, broke their inhibition against swarming in a cage. Their period of activity after the shock was at a much higher level than at other experiments; all colors could be distinguished and the animals were seen without any difficulty. DISCUSSION To summarize the results it can be said that swarming is a behavior, the urge for which might always be present in the males, but it is only released by a gradual change of light to a certain level. The swarming behavior is present during a certain range of light - - but is always inhibited at darkness and at full "daylight". The gradation of the light change needed to produce swarming varies with the type of swarm. For the dawn swarms - - which normally begin at the first light - - an instant light change to full daylight will not release the habit. The dusk swarms, however, remain inhibited for the majority of the animals if the change from full daylight to ,,medio-swarming light" is obtained in less than 21/2 minutes. Gradual light changes within 5 to 15 seconds will release the swarming at dawn after a delay which becomes greater the higher the light level. One dusk ob- servation seemed to indicate a similar delay in these swarms. A light change lasting 20 minutes from start to end, with a "twilight" lasting 11 minutes, causes an immediate start of the morning stand swarm, reaching an inhibitory light level at about 2.0 log lux. At a 40-minute light change (20--26 minutes "twilight") swarms start immediately - - or right after the first glow of the lamps; but the inhibitory level is reached already at 1.6 log lux. The inhibitory levels for the dusk swarms are prac6cally the same as for the dawn swarming. The reason for this might be that the 11 minutes "twilight" is too short lasting for the animals to exhaust their desire to swarm within the limits of the normal thresholds, in which case they may extend the threshold at the upper end of the light scale. If the light level is kept constant within the threshold for swarming, the swarms continue until the light change is again resumed, - - further proof that the swarming behavior is released within a certain range of light. SWARMING OF MOSQUITOES 31 Temperature plays a definite rote for the swarms, and the two types of swarms have somewhat different reactions to temperature. Very low temperatures (15 ~ are practically inhibitory - - that is, only few animals become active and their flight becomes less swarm-like and very slow. The activity begins a little earlier and ends much earlier than at higher temperatures. Also at high temperatures (27--31 ~ the sunrise stand swarm ends at a lower light intensity. Two sunset experiments point to a similar lowering of the threshold at high temperatures. There are no particular differences between experiments performed at 19--25 ~ but the swarming seemed to be best at these temperatures in that the light range within which the swarming was performed was the largest. The dawn marker swarms are somewhat differently affected by temperature than the stand swarms. The swarming range of light at the lowest temperature was only 0.25 log lux, while it was 2.88 log lux for the stand swarm; and the starting threshold was much lower than at other temperatures. At temperatures of 19--23 ~ the range was 1.19 log lux, but it was better yet at 27--31 ~ with 1.38 log lux (2.94 and 2.10 log lux for the corresponding stand swarms). There were no marker swarms at the sunset experiments except at temperatures above 25~ below 25 ~ there were dance-like steps or similar flights and at low temperatures there was no interest in the marker at all. In other words the stand swarm is performed within the largest range of light at 19--24 ~ while the marker swarm is best at temperatures above 25 ~ but is present during the dawn at all tested temperatures, and is inhibited below 25 ~ during dusk. An experiment where the light was kept constant at 1.0 log lux, but the temperature was varied, gives further proof that the temperature of 24 25 ~ is more favorable for the swarming habit than temperatures of 15 ~ 21 ~ or 30 ~ for both types of swarming, since at this temperature the mosquitoes showed signs of contirming the swarm, while at the other temperatures they stopped after a certain length of time, longest at 21 ~ shortest at 15 ~ The period of time between light changes needed for "normal" swarms is more than five hours. Below five hours there are fewer individuals swarming and the normal thresholds for the reaction to light no longer hold true but the beginning of the swarms is delayed and they stop sooner so that the light interval in which the swarms are performed is much shorter than is normally the case. The present experiments do not show any significant difference between a 45-minute period or a 5-hour one between light changes. During the dawn experiments there was no marker swarm after a short interval. CONCLUSION The best swarms of Culex fatigans can be produced by exposing the animals to a slow light change (more than 20 minutes) at a temperature of 24 25 ~ after a period of more than 5 hours since the last light change. By 'best swarms' is meant (1) swarms where practically all individuals will participate, (2) swarms which are performed at the largest range of light intensity, i.e., swarms for which 32 HEDVIG TETENS NIELSEN & ERIK TETENS NIELSEN the inhibitory thresholds are furthest apart. Swarms of Culex nigripalpus were within the same thresholds of l ight as those of C. fatigans, although the latter swarmed more readily. Aedes taeniorhynchus apparently was more inhibited than the Culex species, and their swarming behavior was only released after a shock such as sudden change of light. Acknowledgements. Our best thanks are due to Mr. Wi l l iam F. Wood for his ingenious automatic l ight changer, to Mr. Gordon Evans for the l ight comparo- meter, to Mr. Frank S. Evans for his valuabl.e assistance in measuring the 'mete- orological' factors during the sunrise experiments, to Mrs. N ina Branch, who identif ied all the animals, and to Dr. M. W. Provost for encouragement and lin- guistic help. ESSAIMAGE DES MOUSTIQUES - - EXPERIENCES DE LABORATOIRE EN CONDITIONS DEFINIES Des experiences sur l'essaimage des moustiques males (surtout Culex ]atigans) ont &~ faites au laboratoire. La cage dans laquelle se formaient les essaims Stair placSe dans une chambre dont l'~clairage, la tempSrature et l'humidit~ pouvaient ~tre rSgl~s. Les essaims qui comprenaient le plus grand hombre d'individus et qui duraient le plus longtemps, Staient obtenus quand le changement d'~clairement (cr~puscule artificiel) duralt plus de 20 minutes et quand la tempSrature ~tait de 24--25 ~ apr~s une duroc de plus de cinq heures depuis le demier changement. Dans ces conditions artificielles les moustiques faisaient des essaims ~ous les "matins" et tousles "soirs". Le commencement de Fessaimage le soir, et sa fin le matin avaient lieu un mSme ~clairement (1,6 log lux). Si l'~clairement ~tait tenu infSrieur ~ 1,6 log lux, les essaims duraient toute la nuit jusqu'~ ce que la lumi~re soit relev~e. Quelques observations sur Culex nigripalpus et Anopheles quadrimaculatus indiquent que les d~liminations de l'$clairement permettent l'essaimage. Chez C. nigripalpus ils Staient presque les m~mes que celles de Culex ]atigans. Psorophora howardii et A~des taeniorhynchus ne font pas d'essaimage dans la petite cage mais la derni~re esp~ce fair un essaim darts une cage plus grande aprSs un ~clairement subit. LITERATURE CITED NIELSEN, ]~. T. & GREVE, H. (1950). Studies of the swarming of mosquitoes and other Nematocera. Bull. ent. Res. 41: 227--258. NIELSEN, E. T. & HAEGER, J. S. (1960). Swarming and mating in mosquitoes. Misc. Publi. Ent. Soc. Amer. 1 : 71--95. PROVOST, M. W. (1958). Mating and male swarming in Psorophora mosquitoes. Proc. lOth Int. Cong. Entom. (1956) Montreal, 2: 553--561. Received [or publication : July 24, 1961.