The milk gland of the sheep ked, Melophagus ovinus: A comparison with Glossina

Structurally, the milk gland of the sheep ked, Melophagus ovinus, is quite similar to that of the tsetse fly, Glossina morsitans. In both insects the highly branched gland consists of two cell layers. An extracellular reservoir associated with each secretory cell initially receives the secreted milk. Milk then passes into the gland lumen through a dense cuticular rete. Gram-negative bacteria, presumably symbionts, are abundant in the lumen. Unlike tsetse, the secretory reservoir of the sheep ked is bi-lobed, and the secretory cell nucleus remains centrally located throughout the pregnancy cycle. Lipid droplets are much more abundant in the cytoplasm of the ked secretory cell, and analysis of larval milk shows 5–6% higher lipid content in the sheep ked. Results of histochemical analysis of ked milk are consistent with the analysis of tsetse milk. Four major milk proteins are detectable with polyacrylamide gel electrophoresis. Changes in abundance of ER and sizes of the secretory cell nucleus and secretory reservoir reflect a dramatic cycle of glandular activity during pregnancy. Unlike tsetse, the sheep ked remains constantly on its host and appears to take frequent, but small, blood meals. This strategy implies that the demand for nutrient storage is less than in tsetse.