Butterflies and moths (Lepidoptera) have elongated, tubular mouthparts termed a “proboscis”, which is used for drawing up fluids. The proboscis is composed of two c-shaped strands (galeae) that come together to form the hollow, straw-like conduit. Previous research has inferred that fluid rise only occurs via action of the sucking pump in the insect’s head, which generates the pressure differential for fluid rise. Here, we provide evidence that capillarity might play an important role in the feeding process. We split the mouthparts of three lepidopteran species and fed them a solution with fluorescent microspheres, dissected them, and observed their gut with confocal microscopy to determine feeding ability. We also straightened and dried proboscis halves and dipped them into fluids to observe capillary rise speeds. We found that butterflies did not feed when their mouthparts were split, possibly because the sucking pump was disabled, which could be necessary for swallowing. Fluid rise occurred in the split proboscises of all species; however, the speed was unexpectedly slowest in the species with a middle-sized conduit. We hypothesize this is due to the enlarged chemosensilla that might impair fluid entry. We suggest that capillarity might have played an essential role in feeding in ancestral Lepidoptera; however, as proboscis length increased in derived species, so did the reliance on a sucking pump to transport fluids.