Insectivory and social digestion in Drosophila

It has long believed that Drosophila larvae feed almost entirely by ingesting yeast and possibly other microorganisms that are associated with fermenting fruits or other vegetable matter. However, we have discovered that the larvae of a number of Drosophila species can consume such diverse substrates as insect tissues, including the exoskeleton. Experiments reported here, which include raising sterile dechorionated eggs to adulthood on adult carcasses under axenic conditions, show that larvae can consume complex chitinous substrates directly without the assistance of microorganisms. We show that Drosophila larvae are able externally to digest amylose, cellulose, and chitin, without coming into physical contact with them. We conclude that not only do Drosophila larvae produce enzymes enabling them to digest a wide variety of substrates, but also these enzymes are egested onto the substrates so that at least some digestion, especially of large polymers, takes place externally. Finally, we suggest that the phenomenon of external digestion explains both the previously unexplained massiveness of Drosophila salivary glands and their chromosomes and the tendency of larvae to cluster, which may also be true of other dipterans.