The Use of Laser Speckle Spectroscopy for Subtle Detection of Infantile Ants

Diverse organisms, ranging from microorganisms to mammals, inhabit subterranean environments, living portions or all of their life underground. The opacity of the media in which they reside creates difficulties for monitoring behaviors. Social insects, like termites and ants are of interest due to their surface and subsurface collective behaviors. Thus, it is important to develop techniques for detecting subterranean movements. Diffusing-wave spectroscopy has previously been used for examining dry and cohesive granular dynamics (e.g., shearing, creeping, jamming). This non-invasive technique measures small deformations induced in the medium. One potential application is exploring the mechanical movements of trapped and buried insects, such as ants, which has previously (Spangler, Science, 1967) been linked to collective rescue behavior. To capture delicate movements of different ant infantile stages (i.e., larvae, pupae) buried within ~700 μm glass particles at 1-5 cm depths, we focus a HeNe laser on the medium and digitally capture the backscatter speckle pattern. By evaluating the speckle pattern over time, we categorize movement frequency, length, and intensity by changes in pixel intensity. As the ant’s developmental stage progresses, movement frequency and duration increases. Since ants sense medium vibration through their body via their subgenual organs, we hypothesize that subterranean mechanical cues could be used by ants for non-visual location of nestmates.