Molecular and morphological characterization of third instar Palaearctic horse stomach bot fly larvae (Oestridae: Gasterophilinae, Gasterophilus).

The research focuses on the identification of third instar Gasterophilus larva, a type of horse stomach bot fly, by examining molecular markers and physical features. The study explores the reliability of different molecular markers for identification and carries out a comparative morphological study to assist identification.

Molecular Identification of Gasterophilus Larvae

• The researchers tested three different molecular markers to identify the Gasterophilus larvae species. These markers were located near the 5′ terminus of the cytochrome c oxidase subunit I (COI), near the 3′ terminus of COI, and near the 5′ terminus of the large subunit ribosomal RNA.
• The study revealed that the marker near the 5′ terminus of the COI, a commonly used region for DNA barcoding, proved to be the most efficient in species identification.
• However, the DNA barcoding could not differentiate between G. haemorrhoidalis and G. inermis, indicating that molecular methods are not always sufficient to identify certain types of third instar Gasterophilus larvae.

Morphological Identification of Gasterophilus Larvae

• To supplement the molecular identification, the researchers conducted a detailed comparative morphological study using scanning electron microscopy.
• The study included photographs of fresh mature third instar larvae for all species under consideration, including the remarkable green body colour of the third instar G. nigricornis, documented for the first time in detail.
• The research includes two morphological keys; one allows for quick identification while the other focuses on ultrastructural details for a deeper comparative morphological study.

Terminological Changes and Further Applications

• The study proposed changing the term ‘mandible’ to ‘oral plate’ to refer to a pair of sclerites (hardened body parts) of uncertain homology present in the secondary mouth opening of the larvae.
• The research emphasized that DNA barcodes are not a standalone identification tool for third instar larvae of Gasterophilus species. Morphological aspects remain significant in their identification.
• The combined use of molecular and morphological methods provides a reference system for taxonomic identification which may further help in studying host-parasite co-evolutionary investigations, functional anatomy, and phylogenetic analysis of Gasterophilus.
• Better understanding and identifying the Gasterophilus species would also aid in monitoring gasterophilosis, a disease caused by the parasites that can severely affect equid health.