Materials and methods

The hovering flies have been filmed near Sint-Michielsgestel, the Netherlands, from April until August 2009.  The camera is a Casio EX-F1. The camera has been used to film in HD with 30 frames per second and in a high speed mode with 300 frames per second.

The camera is held by hand. Parts of the film with much movement along the line of sight have been discarded. The films have been analysed in small parts of a few seconds, called tracks, using the “Tracker Video analysis and modelling tool” , “Tracker” for short  (http://www.cabrillo.edu/%7Edbrown/tracker/index.html). The tracks are coded by the date and time of  recording the film and a code for a specific track of the film. Track 090831_1656_A10 is track A10 from the film recorded August 31 2009 at time 16.56. The code 10 indicates that the film is made with 300 frames per second and in Tracker 1 out of 10 frames have been used, so the effective framerate is 30 frames per second. The unit of distance is chosen to be the length of the female, approximately 1 cm. The analysis in this paper is using relative distances, transformation from units to cm is therefore not necessary in estimating delay and gain.

The accuracy of position control of male Eristalis nemorum is better than 0.1 unit, terefore it will be necessary to measure the position of male and female with an accuracy of approximately 0.02 units. Only the position of one clearly recongnizable point of the male has been measured, i.e. the centre of the face or the tip of the abdomen.. Due to the fact that the males do not rotate different points only differ in a constant offset. The female may rotate, therefore the choice of position is a little more intricate. For the female two points have allways been chosen, in example the centre of the ocelli and the scutellum. In “Tracker” the mean between ocelli and scutellum has been  calculated, this will be called the “thorax” point.

The optical axis of the camera may not have been completely horizontal, pointing eventually some 10 to 20 degrees below the horizon leading to a contraction in the vertical direction of a few percent. It has not been attempted to correct for this contraction. Calculations have been performed with Matlab.  

 

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