What is the Smallest object a bat can detect

I was recently asked how small an object can a bat detect.  Not as simple a question as it seems, with over 1200 species to choose from, and many of them have never had their performance levels measured.  We will also ignore for the time being frugiverous, nectarivorous and carnivorous bats for example, but concentrate our attention on small laryngeal echolocating insectivorous bats.

Don Grifin and Alan Grinnell showed that Myotis lucifugus could detect 1mm wires at a distance of three to six feet, based on when they changed the rate of emission of pulses and when they turned away from a barrier of wires.  Behavioural studies show that when fruit flies are released in a bat cage the bats will pursue and hunt them and that they can detect insects of 1-4mm diameter at a distance of 35cm.  The absolute levels of obstacle avoidance have been determined for several species of bats and have shown that they can detect wires with a diameter 0f 0.06-0.1mm, some 20-280 time smaller than the wavelength of the sounds they were using.  Indeed it has not been possible for the experimenters to measure the level of sound being reflected by such small wires.

Other experiments have shown that bats can discriminate between triangles that differ in size by just 17%, giving a difference in echo sound level of just 1-3dB.  They can also be taught to discriminate between circular, square and triangular targets or between cylinders, cubes and pyramids or between objects made of wood, plexiglass or plastic.  Surface textures can also be recognised and discrimated between.  However, many of these tasks are trained performance tests and may not reflect what the bat does in nature, although they do show the physical limits of the echolocation systems they are using.

Reflectivity of an object is dependent on its size compared to the wavelength of the sound being reflected, and at 50kHz that is around 6mm.  However, the reflectivity cut-off is not abrupt but declines steeply as the object gets smaller than a wavelength across and the bat compensates for this by having extremely sensitive and well tuned hearing, as well as producing sounds of very high intensity - so loud that he has to have a mechanism to turn off his hearing as the sound is produced to avoid deafening himself with his own cries.

Bat can also use additional cues to help them locate and identify prey.  A cloud of midges may be difficult to detect other than very close to, but from a distance, the reflectivity of the entire swarm can be significant albeit as a large diffuse blob.  Glints from the movement of insect wings ahve also been shown to be significant to bats in prey identification and capture and gleaning bats may stop echolocating altogether, relying on prey generated sounds to home in on their target.