Evidence for intermolecular forces involved in ladybird beetle tarsal setae adhesion

Area of Science:

• Biophysics
• Insect adhesion mechanisms
• Surface science

Background:

• Ladybird beetles (Coccinella septempunctata) can walk on smooth surfaces, including vertically and upside-down.
• Adhesion is often attributed to secretions on their footpads, but the dominant physical forces remain unclear.
• Quantifying the role of secretions is challenging, hindering a full understanding of beetle adhesion.

Purpose of the Study:

• To experimentally determine the primary physical mechanism behind ladybird beetle adhesion.
• To investigate the relationship between surface properties and adhesive forces in ladybird beetles.
• To understand the role of intermolecular forces versus fluid secretions in beetle footpad adhesion.

Main Methods:

• Observation of beetle secretion fluid using inverted optical microscopy and cryo-scanning electron microscopy.
• Measurement of adhesion forces between ladybird beetle footpads and substrates with controlled surface free energies.
• Analysis of the fluid layer thickness between the footpad and substrate.

Main Results:

• The contact fluid layer was found to be very thin (10-20 nm), suggesting intermolecular forces play a significant role.
• Adhesive force was directly proportional to the square root of the substrate's dispersive surface free energy component.
• Adhesion force was not significantly affected by the polar component of the substrate's surface free energy.

Conclusions:

• Intermolecular forces are the predominant physical mechanism responsible for ladybird beetle adhesion.
• Ladybird beetles exhibit enhanced adhesion on surfaces with higher dispersive components, like waxy plant leaves.
• This adhesive capability is likely an adaptive trait for navigating diverse plant surfaces in their natural environment.