Major differences in the larval anatomy of the digestive and excretory systems of three Oestridae species revealed by micro-CT.

Oestrid flies (Diptera: Oestridae) do not feed during the adult stage, so they depend on an efficient assimilation and storage of nutrients during their parasitic larval stage. We describe the general morphology and provide volumetric data for the digestive and excretory organs of the three larval instars of the nasal bot fly Oestrus ovis L., using micro-computed tomography. The size of the digestive and excretory organs greatly increased across larval instars. In all instars, the two salivary glands were remarkably large and formed a 'glandular band' by coming together, but without lumina uniting, at their posterior ends. The distal region of the anterior Malpighian tubules was greatly enlarged and full of highly radio-opaque concretions. Moreover, the anatomy of O. ovis third-instar larva was compared to that of two species of, respectively, similar and different feeding habits: Cephenemyia stimulator (Clark) and Hypoderma actaeon Brauer. Whereas the general morphology and arrangement of the digestive and excretory systems of C. stimulator was similar to that of O. ovis, some differences were observed in H. actaeon: a swollen anterior region of the midgut, salivary glands shorter and not forming a 'band' and anterior Malpighian tubules narrowly uniform throughout their entire length.

Internal morphological changes during metamorphosis in the sheep nasal bot fly, Oestrus ovis.

During the larval stage, oestrid flies (Diptera: Oestridae) are obligate parasites, whereas during the adult stage they are free-living and do not feed. Like other cyclorrhaphous flies, oestrids undergo metamorphosis inside an opaque puparium, formed by the contracted and hardened cuticle of the third-instar larva. The present study documents the internal morphological changes taking place during metamorphosis of the sheep nasal bot fly, Oestrus ovis L., using non-invasive, micro-CT-based virtual histology and provides quantitative data of volumetric changes in specific organs. Virtual histological sections allowed visualisation of the progression and completion of the apolyses, which delimit the different intra-puparial stages, and the connection to the tracheal system of a large gas bubble, which plays an essential role during early metamorphosis. Overall, our results show that the sequence of morphological and volumetric changes in tissues and organs is similar to those found in other cyclorrhaphous flies, but they also reveal developmental differences that result in an adult vestigial digestive tract. Future studies could develop non-invasive, reliable methods for aging the intra-puparial forms of different oestrid species of veterinary importance, based on both qualitative and quantitative markers, thus improving our knowledge of their development and the efficiency of control strategies.