Visually-guided, upwind turning behaviour of free-flying tsetse flies in odour-laden wind: a wind-tunnel study

Abstract. To test the hypothesis that tsetse flies use visual input from the apparent movement of the ground to assess wind direction while in flight, Glossina morsitans morsitans Westwood females were video- recorded in a wind-tunnel as they entered, in cross-wind flight, a broad plume of simulated host odour (C0₂ at c. 0.05%). The tunnel (2.3 times 1.2 m wide) generated winds up to 0.25 m s^-1 and had a strongly patterned floor that could be moved upwind or downwind to increase or decrease the visual input due to wind drift. Flight tracks were analysed for speed, direction relative to the wind, and angle of turn. Mean groundspeeds were c. 1.8 m s^-1. In control measurements in still air (with or without odour) flies turned 50:50 'upwind': 'downwind'. With a 0.25 m s^-1 odour-perme- ated wind, 79% turned upwind, and c. 70% left view flying upwind. When the floor was moved at 0.25 m s^-1 upwind (to mimic the visual input from the ground due to a 0.5 m s_^-1 wind), the strength of this response increased. If instead the floor was moved downwind, faster than the wind speed (to mimic the visual input due to a wind from the opposite direction), 59% turned downwind and c. 70% left view flying downwind, and thus away from the source (though progressing 'upwind' in terms of the visual input from apparent ground pattern movement). Upwind turns were on average significantly larger than downwind turns. It is concluded that tsetse navigate up host odour plumes in flight by responding to the visual flow fields due to their movement over the ground (optomotor anemotaxis), even in weak winds blowing at a fraction of their groundspeed.     

Flight behaviour of tsetse flies in host odour plumes: the initial response to leaving or entering odour

ABSTRACT. Free-flying, wild male and female Glossina pallidipes Aust. and G. m. morsitans Westw. were video-recorded in the field in Zimbabwe as they entered or left the side of a host-odour plume in cross-wind flight, or as they overshot a source of host odour in upwind flight (camera 2.5 m up looking down at a 3 times 2.5 m field of view at ground level). 80% of cross-wind odour leavers turned sharply ( turns 95^o), but without regard to wind direction (overshooters behaved essentially the same except that nearly 100% turned). Many fewer flies entering a plume cross wind turned ( c . 60%), and when they did they made much smaller turns ( 58^o); these turns were, however, significantly biassed upwind ( c . 70%). All three classes of fly had similar groundspeeds ( 5.5–6.5 m s^_1) and angular velocities ( 350–400^o s^-1). Clear evidence was obtained of in-flight sensitivity to wind direction: significantly more flies entering odour turned upwind than downwind, and odour losers turning upwind made significantly larger turns than average. The main basis for the different sizes of turn was the different durations of the turning flight, rather than changes in angular velocity or speed. No evidence was found of flies landing after losing contact with odour.     

Positive anemotaxis by Varroa mites: responses to bee odour plumes and single clean‐air puffs

Abstract.The stimuli and mechanisms mediating host location and host choice by the bee mite, Varroa jacobsoni (Oudemans), are currently unknown. It is shown that Varroa can use single clean‐air puffs and bee‐odour plumes in a wind tunnel as directional cues. Varroa turned nearly straight upwind in response to single 0.1‐s puffs of clean air directed at 90° to the their anterior‐posterior axis. They turned significantly further to their left side (104°) than to their right (76°), but showed no difference in latency to initiation of the turns (means of 63.3 ms vs. 62.6 ms, respectively). They also followed bee‐odour plumes in a wind tunnel. When released in odour and control plumes mid‐way between the plume's origin and the downwind end of the tunnel, mites responding to bee‐odour walked upwind in, or along the edge of, the odour plume with 38% making contact with the odour delivery tube; mites in clean air did not walk upwind along the air stream, and none made contact with the air delivery tube. Walking speeds were not different between the bee‐odour and control groups (0.28 vs. 0.29 cm s^–1); there were also no differences in the turning rates (96.85 vs. 97.16 deg s^–1 and 388.08 vs. 379.18 deg cm^–1, respectively). Under all conditions, mites walked in a zigzag fashion.     

The effect of wind speed on the flight responses of tsetse flies to CO2: a wind-tunnel study

Abstract. Female Glossina morsitans morsitans Westwood were video-recorded in a wind-tunnel as they entered, in cross-wind flight, a broad plume of CO₂ (a component of host odour). At a wind speed that corresponds with peak catches in the field (c. 0.6 ms-¹) odour produced both significant upwind turning responses (in-flight anemotaxis) and kinetic responses (reduced flight speed and increased sinuosity (m-¹). At a wind speed of c. 0.2 ms-¹ flies displayed anemotactic, but not kinetic, responses to odour. At very low wind speeds (0.1ms-¹) neither upwind turning responses nor kinetic responses to odour were detected. The results are discussed with regard to current theory of host-location by tsetse.     

Detection of short pure-tone stimuli in the noctuid ear: what are temporal integration and integration time all about?

The interception of a pheromone filament induces flying moths to surge briefly nearly straight upwind; in the absence of pheromone moths cease upwind progress and zigzag crosswind. We tested males of the almond moth, Cadra cautella (Lepidoptera, Pyralidae), in a low-turbulence wind tunnel in wind velocities of 20, 40 and 80 cm s⁻¹. A mechanical pulse generator was set to produce plumes either with same pheromone pulse frequency (pulse generation frequency of 2.9 Hz, interpulse distances from 7 cm to 28 cm) or plumes with same interpulse distance across the three wind velocities (interpulse distance of 14 ± 2 cm, pulse generation frequency of 1.7–5.0 Hz). In plumes of similar pulse frequency, the faster the speed of the wind the slower the ground speed of flight. However, in plumes of similar interpulse distance, ground speed remained relatively constant independent of the wind speed. A `realized' frequency of pulse interception for males flying along the various combinations of pulse frequencies and wind velocities was calculated using the males' average airspeed and the spatial distribution of pheromone pulses in the plume. Realized frequency of pulse interception ranged from 1.3- to 3.0-fold higher than the frequency of pulse generation. The flight tracks of males reflected the regime of realized pulse interception. These results suggest that upwind flight orientation of male C. cautella to pheromone in different wind velocities is determined by the flux of filament encounter. Accepted: 3 September 1997     

Wind as a factor influencing flower-visiting by Hadena bicruris (Noctuidae) and Deilephila elpenor (Sphingidae)

Abstract. 1. In a low-speed wind tunnel, male as well as female moths of Hadena bicruris responded to floral odours with positive anemotaxis. Hitherto, such orientation has only been demonstrated for male moths in response to pheromones.
2. H.bicruris had a maximum flight speed of 4.5–5.4ms^-1 and stopped its flower visiting at a wind speed of 2.5–2.8 m s^-1.
3. Deilephila elpenor had a similar maximum flight speed (4.5— 5.1 ms^-1), but it continued visiting flowers up to wind speeds of 3.0–5.0 ms^-1.
4. Apart from mechanical resistance during flight and flower visits, wind might have adverse effects on the energy budget and on evaporative water loss.     

Effects of light levels and plume structure on the orientation manoeuvres of male gypsy moths flying along pheromone plumes

The upwind zigzag flights of male gypsy moths (Lymantria dispar L.; Lepidoptera: Lymantriidae) along narrow, ribbon‐like and wide, turbulent plumes of pheromone were examined in a wind tunnel at light levels of 450 and 4 lux. Under all conditions tested males flew upwind zigzag paths. In 450 lux, males flying along turbulent plumes had the highest ground speeds and the widest crosswind excursions between counterturns, compared to slow flight and a narrow zigzag of males along a ribbon plume. In a turbulent plume, males flew more slowly and had narrower zigzags in 4 than in 450 lux. Across most treatments of plume structure and light level, the rate of transverse image flow and the frequency of counterturning remained relatively constant. The effects of light levels on orientation are not readily reconcilable with a model in which moths in low light levels would head more towards crosswind, thereby enhancing the rate of transverse image flow and the perception of wind‐induced drift.     

Effect of the fine-scale structure of pheromone plumes: pulse frequency modulates activation and upwind flight of almond moth males

Abstract. The effects of plume intermittency and volume on behavioural and flight responses to pheromone of male Cadra cautella (Walker) (Lepidoptera: Phycitinae) were investigated in a pulling wind tunnel. The fine-scale structure of turbulent pheromone plumes was mimicked and manipulated using a pulser device that generated continuous ribbon plumes or intermittent plumes with defined pulse frequency and volume. As pulse frequency increased from 0.6 to 5 Hz and injected volume increased from 0.5 to 5 mls^-1, males flew progressively higher air and ground speeds, turned less frequently, and steered smaller course angles, resulting in straighter flight tracks. The faster the frequency of pulses and the greater the volume of the plume, the higher the proportion of males responding, the shorter their latencies, and the less time spent in the behaviour. Flight tracks of male C.cautella to point sources of pheromone depend on the frequency of filaments encountered.     

Effects of carbon dioxide, acetone and 1-octen-3-ol on the flight responses of the stable fly, Stomoxys calcitrans, in a wind tunnel

Abstract. The flight behaviour of Stomoxys calcitrans (L.) in odour plumes containing carbon dioxide, acetone or l-octen-3-ol was assessed from video recordings. A downwind bias was evident in clean air, whereas all three test chemicals elicited upwind anemotaxis. Response thresholds were ∼0.006% for CO2, between 0.001 and 0.01 μg/l for acetone, and ∼0.0002 u.g/1 for l-octen-3-ol. Sinuosity (° cm^-1) and angular velocity (° s^-1) increased with C0₂ concentration, but velocity (cm s"¹) decreased. Similar, but less clear effects were observed for acetone and l-octen-3-ol.     

Modulation of whitefly take-off and flight orientation by wind speed and visual cues

The effect of different wind speeds on take-off and flight orientation of the sweetpotato whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae), was studied in the presence of a green visual stimulus which reflected 550 ± 10 nm light, or a white stimulus of the same intensity. When the white light was present, take-off was negatively correlated with wind speed. Analysis of the flight tracks of whiteflies in 0, 15 and 30 cm/s wind with the white light present showed that flight was not directed toward the stimulus in zero wind, and that insects were carried downwind as the wind increased. Net displacement downwind was significantly slower than the wind speed, indicating that B. tabaci can control its rate of displacement relative to its surroundings, and is not always passively transported by the wind. In the presence of the green visual stimulus, take-off and flight behaviour of B. tabaci was markedly different to that observed in the presence of the white light. Taking off was more likely and whiteflies made upwind orientated flights, landing on the illuminated section of the screen when it reflected green light. At all wind speeds tested, the mean ground speeds of B. tabaci were approximately 20 cm/s whether the insects were flying upwind or downwind. This uniformity of ground speed regardless of the changing effects of wind-induced drift in different directions strongly suggests that whiteflies actively control their ground speed using visual flow fields in a manner similar to all other flying insects examined thus far.     

Wind speed effects on odour source location by tsetse flies (Glossina)

Abstract. Tsetse flies (mainly Glossina pallidipes Aust.) were captured by various means at sources of artificial host odour in Zimbabwe and Kenya. Their rates of arrival and flight directions were compared with simultaneous data on the wind's speed and direction, on time-scales ranging from 1 s to 30 min. It was predicted that because increasing wind speed up to 1 m s^-1 straightens out the airflow (Brady et al. , 1989) it will straighten out odour plumes, make them easier to navigate, and should therefore increase the rate of arrival of flies at an odour source. In the event, the relationship proved to be more complex, with both positive and negative correlations of arrival rate on wind speed. It seems there is a bimodal relationship: odour source finding is positively related to increasing wind speed in weak winds up to ∼0.5 m s^-1 (presumably as the odour plume straightens out), but is negatively related to increasing wind speed in strong winds above ∼1.0 m s^-1 (presumably due to increasing turbulence breaking up the odour plume).     

Effects of pheromone plume structure and visual stimuli on the pheromone-modulated upwind flight of male gypsy moths (Lymantria dispar) in a Forest (Lepidoptera: Lymantriidae)

The pheromone-modulated upwind flight ofLymantria dispar males responding to different pheromone plume structures and visual stimuli designed to mimic trees was video recorded in a forest. Males flying upwind along pheromone plumes of similar structure generated tracks that were similar in appearance and quantitatively similar in almost all parameters measured, regardless of the experimentally manipulated visual stimuli associated with the pheromone source. Net velocities, ground speeds, and airspeeds of males flying in point-source plumes were slower than those of males flying in the wider, more diffuse plumes issuing from a cylindrical baffle. The mean track angle of males flying in plumes issuing from a point source was greater (oriented more across the wind) than that of males flying in plumes issuing from a transparent cylindrical baffle. Males flying in point-source plumes also turned more frequently and had narrower tracks overall than males responding to plumes from a cylindrical baffle. These data suggest thatL. dispar males orienting to pheromone sources (i.e., calling females) associated with visible vertical cylinders (i.e., trees) use predominantly olfactory cues to locate the source and that the structure of the pheromone plume markedly affects the flight orientation and the resultant track.     

Directional control by male gypsy moths of upwind flight along a pheromone plume in three wind speeds

By attaching a reflective strip to the thorax, we documented with video recordings in a wind tunnel the spatial orientation of male gypsy moths, Lymantria dispar, as they flew along a plume of sex pheromone. In wind speeds of 61, 122, and 183 cm s⁻¹, moths flew very similar tracks along a pheromone plume. Moths aimed their thrust closer to upwind in increasing wind speeds using a roll maneuver. As a result, the orientation of their visual flow field, represented by the slip angle (the angular distance between the direction of flight and the longitudinal body axis), remained relatively constant. We propose that directional control during self-steered zigzagging is achieved by rolling, thereby maintaining a set slip angle. Following a roll at the apex of a turn that aligns the moth with its preferred slip angle, a moth banks toward a cross wind leg. By banking moths may maintain a stable image flow at oblique angles to their longitudinal body axis. Accepted: 16 July 1998     

风洞内粘虫飞翔行为与气流的关系

利用自制的昆虫飞翔实验风洞 ,系统观测了在风洞条件下粘虫在不同流速实验气流中的起飞行为与飞翔行为。结果表明 ,微风能刺激粘虫起飞 ,试虫表现明显的偏爱迎风 (或稍偏一点角度 )起飞的习性 ,飞翔时亦多采取迎风 (或稍偏一点角度 )的姿势。试虫在气流中的实际位移是昆虫飞翔位移与气流位移的矢量和。当气流速度小于 2 m/ s时 ,逆风向位移占多数 ;而气流速度为 3～ 4 m/ s时 ,94 .8%的试虫为顺风向位移。     

To what extent can the tiny parasitoid wasps,Eretmocerus mundus,fly upwind?

Body miniaturization in insects is predicted to result in decreased flight speed and therefore limited ability of these insects to fly upwind. Therefore, tiny insects are often regarded as relying on passive dispersal by winds. We tested this assumption in a wind tunnel by measuring the burst speed of Eretmocerus mundus (Mercet), a beneficial parasitoid wasp with body length <1 mm. Insects were filmed flying upwind towards a UV light source in a range of wind speed 0–0.5 m/s. The Insects flew towards the UV light in the absence and presence of wind but increased their flight speed in the presence of wind. They also changed flight direction to be directly upwind and maintained this body orientation even while drifted backwards relative to the ground by stronger winds. Field measurements showed that the average flight speed observed in the wind tunnel (0.3 m/s) is sufficient to allow flying between plants even when the wind speed above the vegetation was 3–5 folds higher. A simulation of the ability of the insects to control their flight trajectory towards a visual target (sticky traps) in winds show that the insects can manipulate their progress relative to the ground even when the wind speed exceeds their flight speed. The main factors determining the ability of the insects to reach the trap were trap diameter and the difference between insect flight speed and wind speed. The simulation also predicts the direction of arrival to the sticky target showing that many of the insects reach the target from the leeward side (i.e. by flight upwind). In light of these results, the notion that miniature insects passively disperse by winds is misleading because it disregards the ability of the insects to control their drift relative to the ground in winds that are faster than their flight speed.     

Flight responses of tsetse flies (Glossina) to octenol and acetone vapour in a wind-tunnel

Abstract. Female Glossina morsitans morsitans Westwood were video-recorded in a wind-tunnel as they entered, in crosswind flight, a broad plume of either octenol or acetone (two components of ox odour). Both odours produced upwind turning responses (in-flight anemotaxis) to a range of concentrations, with thresholds at around 10^-8mg1^-l for octenol and 10^-6mg1^-1 for acetone. Kinetic responses were unaffected by octenol at low concentrations, but flight speed was significantly reduced and sinuosity (^om^-1) and angular velocity (^os^-1) significantly increased by concentrations at or above those in ox breath; for acetone, these effects were apparent but inconsistently related to concentration. It is concluded that octenol and acetone vapour are used by tsetse flies to locate hosts by upwind anemotaxis, probably combined with kinetic responses. The behavioural basis for the 'repellency' of high octenol concentrations in the field is discussed in the context of the virtual loss of upwind anemotaxis to octenol at the highest concentration tested in the tunnel (30 × ox breath).     

Wind-induced water movements in the South Basin of Windermere

SUMMARY. The results of direct current measurements in the South Basin of Windermere are presented and related to wind history and horizontal variations in near-surface water temperature. Currents were measured by Lagrangian methods using drift-bottles and depth-specific drogues. Except when sudden calms followed strong winds, internal seiche movements had little effect on horizontal transport in the epilimnion. The most important factor governing mass water movement was direct wind forcing. Variations in wind speed accounted for 93% of the variation in near-surface current speed.
For wind speeds between 100 and 500 cm s⁻¹ the wind factor (current speed/wind speed) decreased linearly with wind speed. At wind speeds above 500 cm s⁻¹ the wind factor remained relatively constant around 1 %.
Coriolis effects deflected near-surface currents 4–38° to the right of the wind. The degree of deflection was strongly correlated with the relative depth D_E / D _* (where D_E was the depth of the epilimnion and D _* the depth of frictional resistance). The influence of the Coriolis force also produced pronounced rotations, with depth, of the wind-driven current. The circulation pattern within the epilimnion was broadly that of a distorted conveyor belt moving at some angle to the wind axis. The strength of the transverse circulation was greatest at low wind speeds with a deep thermocline.
Richardson-number calculations suggest that the thermocline generally acts as a low-friction boundary between a turbulent epilimnion and a relatively quiescent hypolimnion. Horizontal variations in water temperature, although ranging from only 0.2 to 1.0°C per km, served as a good indirect 'tracer' of the circulation pattern.     

Optomotor regulation of ground velocity in moths during flight to sex pheromone at different heights

ABSTRACT. Males of two species of moths ( Grapholitha molesta (Busck) and Heliothis virescens (F.)) were flown in a sustained-flight tunnel in horizontal pheromone plumes. The up-tunnel velocity of the moths increased with increasing height of flight and for G.molesta was independent of tunnel wind velocities. Use of moving ground patterns verified that the height of flight above the ground was the factor related to the changes in up-tunnel velocity. Even though up-tunnel velocity increased with increased flight height, angular velocity of image motion did not. Males appeared to use visual cues from the ground pattern and from other sources to determine their up-tunnel velocities. The relationship of preferred retinal velocities to optomotor anemotaxis is discussed.     

Wing kinematics of avian flight across speeds

To test whether wing shape affects the kinematics of wing motion during bird flight, we recorded high-speed video (250 Hz) of four species flying in a variable-speed wind tunnel. The birds flew at intervals of 2 m s⁻¹, ranging from 1 m s⁻¹ up to their respective maximum flight speed, which varied from 14 to 17 m s⁻¹ depending on the species. Kinematic data obtained from two synchronized, high-speed video cameras were analyzed using 3D reconstruction. Three species with relatively pointed, high-aspect ratio wings changed wingbeat styles according to flight speed (budgerigar, Melopsittacus undulatus ; cockatiel, Nymphicus hollandicus ; ringed turtle dove, Streptopelia risoria ). These species used a wing-tip reversal upstroke, characterized by supination of the distal wing at mid-upstroke, at equivalent airspeeds ≤7 to 9 m s⁻¹. In faster flight, they used a swept-wing upstroke, without distal wing supination. At mid-upstroke at any speed, wingspan in these species was greater than wrist span. In contrast, at all steady flight speeds, the black-billed magpie Pica hudsonia with relatively broad, low-aspect ratio wings, used a flexed-wing, feathered upstroke in which wrist spans were equal to or greater than wingspans. Our results demonstrate that wing kinematics vary gradually as a function of flight speed, and that the patterns of variation are strongly influenced by external wing shape.