Comparative larval anatomy of the digestive system of three Calliphoridae (Diptera) species that cause different types of myiasis

The Calliphoridae are one of the main Diptera families that include agents of the parasitic disease condition known as myiasis. Parasitism seems to have evolved multiple independent times within the Calliphoridae; consequently, this family includes a diversity of myiasis-causing species, varying in their obligate or facultative habits and in their specific location in the host. Larval morphological studies can provide novel and relevant insights into the biology of those species, as well as on the pathogenesis and evolution of myiasis; however, the anatomy of internal organs and structures —particularly those within the digestive system— has generally been overlooked, despite potentially reflecting parasitic adaptations. We use here non-invasive X-ray micro-computed tomographic techniques to study the anatomy of the digestive system of the third instar larvae of three Calliphoridae species: Protocalliphora azurea, an obligate agent of sanguinivorous myiasis in passerine bird nestlings; Cordylobia anthropophaga, an obligate agent of subcutaneous myiasis in mammals; and Lucilia sericata, a facultative agent of traumatic myiasis in mammals. The three species are relatively uniform in the internal anatomy of their digestive organs, although they differ in the shape and size of the salivary glands —a main source of larval antigens—, which are considerably smaller in P. azurea. Moreover, the three species differ from the larvae of Oestridae, a close family that exclusively includes obligate myiasis-causing species, in the presence of gastric caeca and a crop, which shows a remarkable storage capacity in L. sericata. The observed differences are discussed from a functional perspective and in relation to the type of myiasis caused.