RESEARCH ARTICLE A quick tongue: older honey bees dip nectar faster to compensate for mouthpart structure deterioration Jianing Wu 1,2, * , ‡ , Yue Chen 3, *, Chuchu Li 3 , Matthew S. Lehnert 4 , Yunqiang Yang 3 and Shaoze Yan 2 ABSTRACT The western honey bee, Apis mellifera L. (Hymenoptera), is arguably the most important pollinator worldwide. While feeding, A. mellifera uses a rapid back-and-forth motion with its brush-like mouthparts to probe pools and films of nectar. Because of the physical forces experienced by the mouthparts during the feeding process, we hypothesized that the mouthparts acquire wear or damage over time, which is paradoxical, because it is the older worker bees that are tasked with foraging for nectar and pollen. Here, we show that the average length of the setae (brush-like structures) on the glossa decreases with honey bee age, particularly when feeding on high- viscosity sucrose solutions. The nectar intake rate, however, remains nearly constant regardless of age or setae length (0.39±0.03 μgs -1 for honey bees fed a 45% sucrose solution and 0.48±0.05 μgs -1 for those fed a 35% sucrose solution). Observations of the feeding process with high-speed video recording revealed that the older honey bees with shorter setae dip nectar at a higher frequency. We propose a liquid transport model to calculate the nectar intake rate, energy intake rate and the power to overcome viscous drag. Theoretical analysis indicates that A. mellifera with shorter glossal setae can compensate both nectar and energy intake rates by increasing dipping frequency. The altered feeding behavior provides insight into how A. mellifera, and perhaps other insects with similar feeding mechanisms, can maintain a consistent fluid uptake rate, despite having damaged mouthparts. KEY WORDS: Nectar intake rate, Glossal setae, Dipping frequency, Adaptive behavior, Feeding habits INTRODUCTION The underlying physical mechanisms by which organisms acquire and transport liquids for feeding is of significance to a wide variety of disciplines (Gillett, 1967; Kim and Bush, 2012; Yang et al., 2014). Several fluid-uptake mechanisms have been described, which often depend on material properties, including morphology, chemistry and physiology (Kim and Bush, 2012; Crompton and Musinsky, 2011; Lehnert et al., 2013; Harper et al., 2013). Fluid- feeding insects are of particular interest because they have mouthparts that are adapted to acquire and transport nanoliter amounts of liquids (Kim et al., 2011; Lehnert et al., 2017; Hischen et al., 2018). The western honey bee, Apis mellifera L. (Hymenoptera), for example, rapidly dips floral nectar using a tongue (glossa) covered with brush-like setae (Snodgrass, 1956; Simpson and Riedel, 1964; Krenn et al., 2005; Wu et al., 2015). The first drinking model to elucidate the viscous-dipping feeding mechanism of A. mellifera simplified the glossa as a bald rod (Kim and Bush, 2012). Subsequently, Yang et al. (2014) proposed a model that considered the effects of the setae (erectable, brush-like structures on the glossa) and used experimental data to validate theoretical predictions on volumetric flow rate and energy intake rate. Considering that the back-and-forth movements of the glossa occur at a frequency of ∼5 Hz (Li et al., 2015) (similar to a sewing needle), we hypothesize that the high-intensity work and fast dipping frequencies cause wear or damage to the glossal setae, which could result in the gradual deterioration of nectar-loading capabilities. This situation, however, creates a paradox because it is the older worker honey bees (i.e. those likely most prone to setae damage) that forage pollen and nectar (Amdam and Omholt, 2002). We hypothesize that if mouthpart damage does occur, A. mellifera employ a method of mechanistic or behavioral compensation to overcome the structural wear of the glossa in order to maintain optimum fluid uptake rates (Abrams et al., 2015). MATERIALS AND METHODS Western honey bee rearing and colony maintenance Approximately 2000 western honey bees, A. mellifera, were collected from Guangzhou, China (22°N, 112°E), where no specific collecting permits were required, and were housed in a hive with drones and a queen. The entire system was maintained at 25°C at 50% humidity, and bees were fed a 35% (w/w) sucrose solution and an inorganic salt solution (Kim et al., 2011) (Fig. 1). Pupae were removed from the hive and placed into a container (28–30°C). Upon adult emergence, individual A. mellifera workers were color coded with a unique mark on the tergum (solution composed of acetone and oil painting dye) that was used to identify their age (Huang et al., 1991). Mouthpart morphology Fifteen-day old adult A. mellifera were removed from the hive and randomly placed into beakers (170 mm×270 mm) with either 35% or 45% (w/w) sucrose solution. The sucrose solutions were based on sucrose concentration measurements from nectar (acquired with a polarimeter, Autopol IV) collected from three species of plants (Sophora japonica, Physostegia virginiana and Paulownia tomentosa) located near the bee hive. Each beaker was provided with 10 ml of their respective solution daily. The glossa of each honey bee was measured every 2 days using a light microscope (Eclapse 90i) at 4× magnification. During each measurement period, two individuals were randomly selected and placed into 100% ethanol for dehydration and further studied with scanning electron microscopy (SEM; FEI Quanta 200). Because setae Received 16 August 2019; Accepted 7 October 2019 1 School of Aeronautics and Astronautics, Sun Yat-Sen University, 510006 Guangzhou, PRC. 2 Division of Intelligent and Biomechanical Systems, State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, 100084 Beijing, PRC. 3 School of Engineering and Technology, China University of Geosciences (Beijing), 100083 Beijing, PRC. 4 Department of Biological Sciences, Kent State University at Stark, North Canton, OH 44720, USA. *These authors contributed equally to this work ‡ Author for correspondence (wujn27@mail.sysu.edu.cn) J.W., 0000-0003-0902-4466 1 © 2019. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2019) 222, jeb212191. doi:10.1242/jeb.212191 Journal of Experimental Biology