Adaptive leg morphology of the small hive beetle, Aethina tumida Murray (Coleoptera: Nitidulidae)

Social insects host a variety of inquilinous beetle species. Many of these species display morphological adaptations that allow integration into host colonies. Our study used scanning electron microscopy to compare the leg morphology of nitidulid beetle species exhibiting varying levels of integration into honey bee (Apis mellifera) colonies. The following beetle species were included in our study: Aethina tumida (highly integrated); Aethina villosa, Lobiopa insularis, Glischrochilus fasciatus, Carpophilus dimidiatus, and Epuraea luteola (non-integrated accidentals found infrequently in colonies); and Amphicrossus ciliatus, Stelidota geminata, Carpophilus hemipterus, and C. humeralis (presumably non-integrated). The SEM images revealed the presence of well-defined grooves in the femora of A. tumida, broadened and flattened tibiae, and dense and undulate tarsal setae. No other species displayed this combination of leg characteristics. These characters presumably are adaptive for A. tumida to avoid defensive honey bee encounters by enabling the retractions of its legs completely under the body, by placing almost the entire flattened tibiae into the femoral grooves. This “turtling behavior” yields the beetle’s legs and vulnerable venter inaccessible to honey bee attacks, while keeping the beetle firmly attached to the substrate.