Is a urea cycle present in insects?

1. The presence of appreciable activity of the urea-cycle enzymes in the tissues of Sarcophaga ruficornis, a carnivorous dipteran insect, all through its life-cycle appears significant in view of their total absence barring arginase (L-arginine ureohydrolase, EC 3.5.3.1) in the phytophagous lepidopteran eri silkwork Philosamia ricini at any stage of development. Further, the variation of all these enzymes all through its development suggests the possibility of the operation of the Krebs-Henseleit urea cycle in this carnivorous insect. 2. The almost parallel behaviour of arginase and ornithine delta-transaminase (L-ornithine-2-oxo acid aminotransferase, EC 2.6.1.13) in both the insects suggests another important role of the former in proline biosynthesis in insects. 3. High proteolytic activity accompanied with significant protein depletion and simultaneous increase in arginine is suggestive of the degradation of proteins and peptides.