Penetration and encapsulation of the larval parasitoid, Exorista larvarum (Diptera: Tachinidae) in the factitious host Galleria mellonella (Lepidoptera: Pyralidae)

The tachinid fly, E. larvarum is a polyphagous larval endoparasitoid which deposit macrotype eggs on the host exoskeleton. The newly-hatched larvae burrow through the host integument, leaving a dark spot in the point of their penetration as a result of cuticle melanisation. The attachment of E. larvarum larvae to the host tissues and formation of the hemocyte capsule surrounding the tachinid larva were studied in the last larval instar of the wax moth, Galleria mellonella (L). Parasitization was carried out according to the standard laboratory procedure. The parasitized larvae were examined, dissected and fixed every day of the parasitoid development, which lasts 3 - 4 days from egg hatching to pupal stage at 26°C. Standard histological techniques were used. The material was embedded in Histoplast II and sections 5 - 7 µm thick were stained with hematoxylin - eosin and Masson΄s trichrome stain. As the parasitoid first and second instar larvae are more difficult to be sectioned, the cryosection and rapid microscopic analysis were used to obtain sections of better quality. Frozen sections were stained with azan or hematoxylin - eosin. Our histological examination showed that the endoparasitoid penetration induces the host cellular defence resulting into the formation of a hemocyte capsule consisting of a multi-cellular sheath. This enveloping capsule later undergoes melanisation, which is more evident towards the abdominal part of the parasitoid larval body. The tachinid larva uses this encapsulation response, as well as the integument reaction to wounding, to build the respiratory funnel. The encapsulated larva is attached to the respiratory funnel via an anal hook and cuticular spines and remains in contact with the outside air with its posterior spiracles until the third instar (when the funnel is abandoned). The respiratory funnel was also studied by scanning electron microscopy