Insects are the most diverse organisms and often the most abundant animals in some ecosystems. Despite the importance of their functional roles and of the knowledge for conservation, the trophic ecology of many insect species is not fully understood. In this review, I present how stable carbon (C) and nitrogen (N) isotopes have been used to reveal the trophic ecology of insects over the last 30 years. The isotopic studies on insects have used differences in C isotope ratios between C3 and C4 plants, along vertical profiles of temperate and tropical forest stands, and between terrestrial and aquatic resources. These differences enable exploration of the relative importance of the food resources, as well as movement and dispersal of insects across habitats. The 13 C-enrichment (approximately 3‰) caused by saprotrophic fungi can allow the estimation of the importance of fungi in insect diets. Stable N isotopes have revealed food resource partitioning across diverse insect species above and belowground. Detritivorous insects often show a large trophic enrichment in 13 C (up to 3‰) and 15 N (up to 10‰) relative to the food substrates, soil organic matter. These values are greater than those commonly used for estimation of trophic level. This enrichment likely reflects the prevalence of soil microbial processes, such as fungal development and humification, influencing the isotopic signatures of diets in detritivores. Isotope analysis can become an essential tool in the exploration of insect trophic ecology in terms of biogeochemical C and N cycles, including trophic interactions, plant physiological and soil microbial processes.

Summary 1.To investigate whether the role of symbiotic fungi in lignin degradation and food provision differs among fungus-growing termites (four species of Macrotermes , three species of Odontotermes , Hypotermes makhamensis , Ancistrotermes pakistanicus and Pseudacanthotermes militaris ), the chemical composition of fungus combs of different ages and the chitinolytic activity in the gut of termites were analysed. In addition, the carbon stable isotope ratios ( δ 13 C) in old combs, worker termite tissues and fungal nodules (aggregated conidia) were compared. 2. In Macrotermes spp., the carbohydrate : lignin ratio of the combs increased with increased comb age, but it decreased or remained the same in Odontotermes spp., H. makhamensis , A. pakistanicus and P. militaris. In contrast, the chitin : carbohydrate ratio and ash content increased more over time in the combs of Odontotermes spp., H. makhamensis , A. pakistanicus and P. militaris than in the combs of Macrotermes spp. 3. Chitinolytic activity in the gut of workers was higher in O. takensis , H. makhamensis and A. pakistanicus than in M. annandalei . 4. In all species examined, the δ 13 C value increased in the sequence: old combs < termites < fungal nodules. Based on a two-source model of δ 13 C, the contribution of fungi to termite nutrition was considered higher in Odontotermes spp., H. makhamensis , A. pakistanicus and P. militaris than in Macrotermes spp. 5. These results suggest that symbiotic fungi play different roles among fungus-growing termites. In Macrotermes spp., the main role of symbiotic fungi is to degrade lignin, so that the termites can utilize cellulose more efficiently, whereas in Odontotermes spp., H. makhamensis , A. pakistanicus and P. militaris , it is to serve as a food source.

Zygomorphic flowers are usually more complex than actinomorphic flowers and are more likely to be visited by specialized pollinators. Complex zygomorphic flowers tend to be oriented horizontally. It is hypothesized that a horizontal flower orientation ensures effective pollen transfer by facilitating pollinator recognition (the recognition-facilitation hypothesis) and/or pollinator landing (the landing-control hypothesis). To examine these two hypotheses, we altered the angle of Commelina communis flowers and examined the efficiency of pollen transfer, as well as the behavior of their visitors. We exposed unmanipulated (horizontal-), upward-, and downward-facing flowers to syrphid flies (mostly Episyrphus balteatus), which are natural visitors to C. communis. The frequency of pollinator approaches and landings, as well as the amount of pollen deposited by E. balteatus, decreased for the downward-facing flowers, supporting both hypotheses. The upward-facing flowers received the same numbers of approaches and landings as the unmanipulated flowers, but experienced more illegitimate landings. In addition, the visitors failed to touch the stigmas or anthers on the upward-facing flowers, leading to reduced pollen export and receipt, and supporting the landing-control hypothesis. Collectively, our data suggested that the horizontal orientation of zygomorphic flowers enhances pollen transfer by both facilitating pollinator recognition and controlling pollinator landing position. These findings suggest that zygomorphic flowers which deviate from a horizontal orientation may have lower fitness because of decreased pollen transfer.

Abstract:Carbon and nitrogen stable isotope ratios (δ13C and δ15N) have been used to study the structure of food webs. However, few studies have examined how a terrestrial food web can be depicted by this technique. We measured δ13C and δ15N in various consumers of four trophic groups (detritivores, herbivores, omnivores and predators), including vertebrates and invertebrates (14 orders, ≥24 families), as well as canopy and understorey leaves in a tropical rain forest in Malaysia. We found that δ13C and δ15N of the consumers differed significantly among the trophic groups. The predators had significantly higher δ13C than the herbivores, and were similar in δ13C to the detritivores, suggesting that most predators examined depend largely on below-ground food webs. δ15N was higher in predators than detritivores by about 3‰. The comparison of δ13C in plant materials and herbivores suggests that most herbivores are dependent on C fixed in the canopy layers. The vertebrates had significantly higher δ15N and δ13C than the invertebrates of the same trophic group, likely reflecting differences in the physiological processes and/or feeding habits. This study indicates that stable isotope techniques can help better understanding of the terrestrial food webs in terms of both trophic level and the linkage of above- and below-ground systems.