Insects employ a variety of cues to find and identify resources. One of the most intriguing questions about insect behaviour is how chemical cues influence communication and orientation. When studying mosquitoes, research is directed at understanding how human-emitted odours influence orientation from a distance and landing on the host, and how repellents interfere with these behaviours. This has an immense value if we think of a way to prevent malaria mosquitoes from detecting human hosts. Field and laboratory studies are combined to investigate how odour disperses in wind, how the structure of the odour plume influences the mosquito’s flight orientation, and whether other stimuli (for example, visual) interact with chemical stimuli to modulate orientation. Many of these studies employ wind tunnels where scientists manipulate the sensory inputs and record maneuvers with 3D video.
When studying insect flight, data collection often consists of manual scoring of take-off and landing times, or video recording of flight in either the horizontal or vertical plane (2D), using a single camera. Until recently, nearly all of the recordings of flight paths were conducted in horizontal planar view. Many studies have been conducted with moths and it has been assumed that their height of flight above ground is held fairly constant, while they range mostly laterally, often assuming a zigzag path.
However, measuring the flight in three dimensions provides a more complete description and analysis of flight patterns than in two dimensions. In the case of malaria mosquitoes, where researchers aim at manipulating the odour plume (which by definition is a three-dimensional object), 3D analysis is indispensable to understand their orientation in space and response to visual or olfactory stimuli. In a laboratory situation, it is possible to control the overall structure of the odour plume and determine whether the mosquito is clearly within or outside it (the plume’s ragged boundary remains an area of uncertainty). When a mosquito enters an odour plume, its measured change in orientation may be attributed to odour contact.
In October last year, entomologists gathered to attend the conference of the Asia-Pacific Association of Chemical Ecologists (APACE). Lucas Noldus, Managing Director of Noldus Information Technology presented Track3D, a new video-based system for automated tracking of insects in a 3D space. The Track3D system records the flight of an insect in a test chamber or wind tunnel, visualizes the trajectory in 3D and calculates a large number of movement parameters. The system consists of tightly integrated hardware and software components. Insect flight is recorded using two synchronized video cameras and stored in digital video files. From these files, the insect’s position in 2D is acquired by EthoVision XT video tracking software. Subsequently, the Track3D program – after 3D calibration using a specially designed calibration frame – converts the 2D coordinates from each camera view into one set of 3D coordinates (Figure 1). This track can be visualized in 3D, played back, rotated, and zoomed in/out. The software also calculates a large number of flight parameters, including distance moved, tortuosity, velocity (absolute and ground speed), heading and turn angles relative to the different planes.
In another talk, Professor Ring Cardé (University of California Riverside) addressed the question of how navigation is influenced by odour plumes, this time in different species of moths and mosquitoes. Using Track3D with a wind tunnel setup, his group found that, for example, female Culex quinquefasciatus mosquitoes rely primarily on CO2 for take-off and upwind orientation, whilst they seemed to employ odours from human skin at close range for landing. When analyzing the 3D flight track, it appeared that this species responded to foot odours by slowing down and flying directly less upwind.
Ongoing studies on malaria mosquitoes
The Track3D system was developed in close collaboration with the research groups of Prof. Willem Takken (Laboratory of Entomology) and Prof. Johan van Leeuwen (Experimental Zoology Group) of Wageningen University, The Netherlands. Willem Takken is a world expert in the behavior of malaria mosquitoes and other vectors of human and veterinary diseases. Johan van Leeuwen is an international authority on motion capture, 3D movement analysis, and biomechanics in a wide range of animal species.
The Track3D system has been extensively tested and validated in the Laboratory of Entomology of Wageningen University, with research on the behaviour of the nocturnal malaria mosquito Anopheles gambiae. Responses to different human host cues involved in the foraging behavior of mosquitoes were studied by quantifying flight track characteristics in a wind tunnel. The insects were tracked while navigating through a plume of host-emitted cues under nocturnal conditions. This quantitative analysis of nocturnal host-seeking mosquitoes is a new step in the development of effective monitoring and preventive techniques for the control of malaria. It also confirms the value of 3D analysis of insect flight behavior (see an example in Figure 2).
Beeuwkes, J., J. Spitzen, C.W. Spoor, J.L. van Leeuwen, and W. Takken (2008). 3-D flight behaviour of the malarial mosquito Anopheles gambiae s.s. inside an odour plume. Proceedings of the Netherlands Entomological Society Meeting 19: 137–146.
Spitzen, J., Spoor, C.W., Kranenbarg, S. et al. (2008) Track 3D: visualization and flight track analysis of Anopheles gambiae s.s mosquitoes. Proceedings of Measuring Behavior 2008 (ed. by A. J. Spink, M. R. Ballintijn, N. D. Bogers et al.), 133–135. Noldus Information Technology, The Netherlands.
*) Dr. Fabrizio Grieco is a Behavioral Research Consultant at Noldus Information Technology bv, Wageningen, The Netherlands. Email: firstname.lastname@example.org.