Patterns of Fuel Utilization by the Thoracic Muscles of Adult Worker Ants. the Use of Lipid by a Hymenopteran

-1. Patterns of fuel utilization in the thoracic muscles of three species of ants have been established. 2. The thoracic muscles of Formica ulkei exhibit a typical Hymenopteran metabolic organization, relying exclusively upon carbohydrate oxidation for the provision of metabolic energy. This species feeds upon honeydew. 3. Pooonomyrmex californicus, a granivorous ant, exhibits a metabofic organization unprecedented for a Hymenopteran species. Its thoracic energy metabolism is based upon lipid oxidation. 4. Atta colombica, a fungus feeder, can metabolize both carbohydrate and fat, a versatility which is not typical of Hymenoptera. 5. It is concluded that patterns of fuel utilization in insects are not determined by phylogenetic inertia, but are selected to accommodate the activity patterns, feeding ecology and dietary regime of the species. I N T R O D U C T I O N Carbohydrates and lipids are the storage fuels most frequently utilized to sustain muscular activity. In insects it is generally recognized that there exist three common patterns of fuel utilization by flight muscle (Bailey, 1975; Crabtree & Newsholme, 1975; Sacktor, 1975). Diptera and Hymenoptera sustain flight through the oxidation of carbohydrates, whereas flight metabolism in Lepidoptera is supported predominantly by the oxidation of fatty acids. Orthoptera are able to make efficient use of both carbohydrates and fats. In addition to these three common patterns of fuel utilization, a few insects have evolved a metabolic organization which exploits proline as a fuel for flight (Bursell et al., t974; Hansford & Johnson, 1975; Khan & deKort, 1978). Previous studies of patterns of fuel utilization in insects have emphasized phylogenetic correlations. However, it should be noted that the species which have been employed to establish phylogenetic correlates represent very different ecotypes. Thus, the carbohydrate specialists which ha',e been identified, all of which are Diptera and Hymenoptera, are species which feed frequently on nectar or other sugar solutions. The fat specialists, all of which are Lepidoptera, are all species which feed rarely if at all as adults, and which must survive on fuel reserves accumulated as larvae. Finally, the generalists capable of utilizing both carbohydrates and lipids, the Orthoptera, are voracious polyphagous species which are capable of long migratory flights. This study explores patterns of fuel utilization in the thoracic musculature of three species of ants having very different dietary habits. Formica ulkei feeds extensively on honeydew, and therefore has a diet rich in carbohydrates and very low in lipid and protein. Pooonomyrmex californicus feeds on seeds, a resource which is particularly rich in lipids, and less rich in carbohydrates and protein. Atta colombica is a fungus-grower which subsists on the liquid contents of the mycelium of a symbiotic fungus. This fluid is rich in carbohydrates and protein, and low in lipids. Both F. ulkei and P. californicus also scavenge dead insects, but it is likely that this high-protein, high-lipid food is largely diverted to the developing brood (Lange, 1971). The objective of this investigation was to establish whether all three of these ant species exhibit fuel utilization patterns typical of other Hymenoptera, or whether the organization of their energy metabolism might reflect their diverse dietary habits. The patterns of metabolic organization were estabfished by determining the maximum activities of certain enzymes of constant proportion groups indicative of major metabolic pathways. The enzymes assayed were citrate synthase (CS, EC 4.1.3.7), glyceraldehydephosphate dehydrogenase (GAPDH, EC 1.2.1.12), 3-hydroxyacylCoA dehydrogenase (HOAD, EC 1.1.1.35), glycerophosphate dehydrogenase (GDH, EC 1.1.1.8) and lactate dehydrogenase (LDH, EC 1.1.1.27). M A T E R I A L S A N D M E T H O D S