Visual spatial memory in a hoverfly
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We have filmed maleEristalis sp. in their normal environment performing a sequence of behaviour which demonstrates that they have a memory of their position in space relative to visual landmarks in their surroundings. They hover stably in mid-air, periodically leaving their hovering station to chase passing insects. After the chase is over they return to approximately the same position in space.
The following findings indicate that they use visual cues to guide their return. The size of the “home” they return to depends on the proximity of landmarks, being more circumscribed the closer the landmarks are to the home (Figs. 3 and 4). The fly on its return decelerates as it approaches home (Figs. 7–10) and generally stops short of its previous hovering position (Fig. 6). The accuracy of the return is unaffected by the length of the previous chase (Fig. 5).
The fly seems able to return directly to its home from any direction (Fig. 10), though we have not been able to film the complete return after long chases. During the return the fly normally faces in the direction it is flying, though in the last stages it is not uncommon for it to be oriented at an angle of 40–60° to its line of flight. When hovering or flying slowly the fly changes orientation by rapid saccade-like movements and between saccades the orientation is kept very constant (Figs. 1, 2 and 10).
Flies will congregate and hover near to a single mobile marker placed in the middle of an empty lawn and, after a chase, return to a position close to it. If the marker is moved while flies are hovering near-by they follow the movement of the board if it approaches them (Fig. 12), but not if it retreats (Fig. 13). Thus by moving the board away it is possible to change the distance between the fly and the board. In these cases (Fig. 13) the fly remains stationary in its old position until it chases an insect, after which it returns not to this position, but to its original position with respect to the board before this was moved. This shows (1) the board must provide a cue guiding the return, and (2) the homing reflex does not operate constantly when the fly is hovering to correct for small involuntary displacements from a preferred position, but it is only turned on at certain times, for instance after a chase.
Blowflies perform a variant of the same behaviour: they bask on sunny walls waiting for passing insects which they chase, afterwards returning to approximately the same position on the wall (Fig. 16). Their resting site tends to shift slightly after each chase (Fig. 17), and an analysis of this drift indicates that they up-date their memory of home on each return.
In the Discussion we propose a model of how flies might use their memory of the position and form of landmarks as seen from their home to guide their return.
KeywordsSpatial Memory Original Position Change Orientation Prefer Position Normal Environment
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