In plants, UDP-rhamnose is required for primary cell wall polysaccharides and various Lrhamnose–containing natural organic compounds such as flavonoids, terpenoids, and saponins and is synthesized through a de novo pathway from UDPD-glucose. Although a salvage pathway for UDP-rhamnose remains to be identified in plants, there is evidence for such a pathway because UDP-glucose pyrophosphorylase catalyzes the formation of various UDP-sugars from monosaccaharide-1-phospates at the end of the salvage pathway. Melon fly is a phytophagous insect whose larvae feed on the pulp of gourds, fruits vegetables and fruits such as papaya and mango. Hence, it was feasible that a unique salvage pathway was active in this insect. On the other hand, we found from our current analyses that the dipterose levels increased in the pupal stages even though pupae do not feed. This result suggests that the melon fly synthesizes UDP-rhamnose from other UDP-sugars through a de novo pathway and then uses these products as substrates for the synthesis of dipterose. Previous studies have reported that a number of insects have bacterial endosymbionts that can have a mutualistic relationship with their hosts, providing them with nutrients such as amino acids and vitamins, or that involves intracellularly parasitizing and negatively affecting them. However, there are no reports of bacterial endosymbionts that have achieved a mutualistic relationship with the melon fly, which feeds mainly on cucurbitaceous plants but not on plant sap or blood. Moreover, there is no melon fly which infected with reproductive manipulators such as Wolbachia to be able to mass-produce sterile insects throughout the year. These results suggest that a novel polysaccharide composed of a variety of sugars including L-rhamnose is synthesized by the melon fly itself without the effect of bacterial endosymbionts. Plants and fungi are now known to have various bioactive polysaccharides that induce cytokine and NO production by macrophages. The cell walls of plants and fungi predominantly contain various polysaccharides comprising species-specific monosaccharides. Although previous studies have reported that high-dose treatments of macrophages with many of these polysaccharides activate the innate immune response, we show from our current data that a very low concentration of dipterose can do this at a similar potency to LPS, an immunoBAY-60-7550 PDE inhibitor stimulator and major component of the cell membrane of gram-negative bacteria. The polysaccharide structure is an important determinant of the activation of innate immune cells such as macrophages. Our current findings suggest that dipterose has a characteristic structure that is a potent stimulator of mammalian macrophages. Activation of the innate immune response by polysaccharides is triggered by their recognition by PRRs such as TLRs. Although TLRs recognize structures that are conserved among various pathogens, TLR2 and TLR4 have been well characterized as sensors that recognize ligands containing carbohydrate moieties such as peptidoglycans, LPS, and natural polysaccharides.