In nature, highly diverse mouthparts allow liquid-feeding insects to exploit a remarkable range of food sources including nectar, sap, honeydew, and blood. (Labandeira, 1997, Kim and Bush, 2012, Krenn, 2019). For instance, the lapping proboscises of honey bees, structured with a protrusible hairy glossa, can collect viscous fluids through capillarity (Wu et al., 2015, Wu et al., 2019, Shi et al., 2020; Wei et al., 2020a, 2022). The piercing-sucking proboscises found in plant sap-sucking insects or blood feeders, typically composed of interlocked elongated components and a sharp tip, are specialized to penetrate the soft medium and ingest the inner fluid (Lee et al., 2009; B. H. Kim et al., 2011; Kim et al., 2013). The sponging proboscises of flies, which are short but retractable structures with fleshy labellum, can absorb liquids from porous liquid sources (Lehnert et al., 2017). In moths and butterflies, the siphoning proboscises formed by highly-elongated slender galea can capture nectar in the very deep corolla of flowers (Kingsolver and Daniel, 1979, Tsai et al., 2014, Lee et al., 2014, Lehnert et al., 2016).

Beetles, the largest insect group with over 380,000 described species (Zhang et al., 2018, Evans, 2023), adopt general mouthparts to collect different foods spanning from solids to liquids (Ichiishi et al., 2019, Valdez, 2019, Hao et al., 2020). The liquid food encompasses a range of natural sources including nectar, sap, and juice of fruits (Stork, 2018, Krenn, 2019). The concentrated natural liquid may provide higher energetic rewards for beetles, but the accompanied rise in viscosity would pose challenges for insects to collect (W. Kim et al., 2011; Lois-Milevicich et al., 2021). Specifically, for the sucking pattern, insects like moths and butterflies could be severely hindered by the viscous drag when ingesting concentrated liquid, as their sucking pumps in the heads can hardly offer sufficient driving force to overcome the viscous resistance. In contrast, rhinoceros beetles can thrive on highly viscous fluids with adults feeding on sap exuding from wounds or surfaces on oak, maple, and ash trees, where the liquid can be extremely viscous (Johns et al., 2014, Wolton and Luff, 2016, Jensen et al., 2016, del Sol et al., 2020). The chewing mouthparts of rhinoceros beetles are commonly considered specialized for handling solid food (Chapman and de Boer, 1995, Paul and Gronenberg, 1999, Krenn, 2019), while their drinking mechanism for liquid food, especially concerning high-viscosity liquids, remains elusive.

In this study, we examined the feeding behavior of the adult rhinoceros beetles, particularly focusing on their capacity to ingest viscous liquids. Utilizing the approach that combines experimental methods such as morphological characterization, flow visualization, and fluid-structure coupling simulation, we elucidated the beetle drinking strategies of manipulating their mouthpart apparatus to facilitate the collection of viscous liquids. Our studies could offer a new perspective on how insects with chewing mouthparts are able to collect various liquids in nature to better survive from interspecific competition or environmental constraints.