Dear Moderator,


I offer the following comments in amplification of Jerry Bromenshenk's
recent note about harmonic radar and bees.

The hand held 'skier' units to which Jerry refers are often loosely
called radars, but in fact they are actually harmonic direction finders
- they tell you roughly in what direction the target is, but crucially,
NOT its range; (RADAR = RAdio Direction And Ranging).  So to locate a
target you have to walk (or run!) along the direction of strongest
signal until you can actually see it.

By contrast, the system that we designed and built, and used with Beth
Capaldi is a true, scanning radar that gives instantaneous position
fixes (target range and bearing) once every three seconds, and so yields
dynamic and geometrically accurate records of the insects' actual flight
paths.   You don't have to guess where a bee is when it's out of sight,
you KNOW its actual position (and ground speed).  The maximum range of
our radar is currently about 900m, so it gives coverage over a 1.8km
diameter circle, i.e. over an area of about 2.5 square kilometers.  We
first described it in 1996  (Riley et al. "Tracking bees with harmonic
radar", Nature 379, 29-30)

An interesting summary of radar and bee studies is given in Norman
Carreck's 1993 article in Bee World, 80 (3), 124-131.   Harmonic radar
has proved particularly useful in investigating flight by bumble bees:
(Riley et al 1999, "Compensation for the wind by bumble bees", Nature
400, 126);  (Osborne et al., 1999  "A landscape-scale study of foraging
range and constancy, using harmonic radar"  J. Appl. Ecol 36, 519-533).
We have also used it successfully to track night-flying moths (Riley et
al. 1998, "Harmonic radar as a means of tracking the pheromone-finding
and pheromone-following flight of male moths", J. Insect. Behav. 11,
287-296).  For a more general overview of radar entomology, see Alistair
Drake's excellent Radar Entomology Web Site
(http://www.ph.adfa.edu.au/a-drake/trews/).

Jerry correctly draws attention to the need to keep the weight of the
transponder as low as possible, and we have produced operational devices
that weigh just less than one milligram (equivalent to about a single
grain of sugar) with this in mind.  However, these lightweight
transponders were mechanically rather fragile, and were often bent and
damaged when being fitted to the bees.  It was for this practical reason
that we often choose to use the heavier and more robust versions
described in Beth's paper.

Unfortunately, all our tranponders are 16mm long (but about 1 tenth of
the optimal required for the hand-held direction finders), so (as Jerry
points out) they have to be removed before returning bees can actually
re-enter their hive.  This is inconvenient, but has not proved an
impossible obstacle in our studies so far.

--
Prof. Joe Riley, M.A., D.Phil., FIEE, C.Eng.
NRI Radar Unit, University of Greenwich, Malvern, UK
Tel: +44-(0)1684-582193  Fax: +44-(0)1684-582984
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