Development of the larval parasitoid fly, Exorista larvarum (Diptera: Tachinidae) with emphasis in its movements and attachment strategy to the host tissue

Exorista larvarum (L.) is a polyphagous gregarious larval endoparasitoid of Lepidoptera. Female flies deposit macrotype eggs on the host surface. The newly-hatched larva penetrates the host integument, induces the formation of a primary integumental respiratory funnel and develops through three larval instars. Pupation generally occurs outside the host larval remains. The morphology of larval E. larvarum was studied in its factitious host Galleria mellonella (L.) (Lepidoptera: Pyralidae). Last instar host larvae were exposed for parasitization to female flies and removed when the 3 - 5 eggs had been laid on their body. Parasitized larvae were dissected and fixed every day of the parasitoid development, which lasts 3 - 4 days from egg hatching to pupal stage at 26°C. Specimens were embedded in Histoplast II and sections 5 - 7 µm thick were stained with hematoxylin - eosin and Masson΄s trichrome stain. To obtain sections of better quality, the cryosection and rapid microscopic analysis were also used. Frozen sections were stained with azan or hematoxylin - eosin. Here, we focus on the integument and related structures participating in the movements and attachment process of E. larvarum larva to the host tissue. The tachinid larva penetrates the host using the robust cephalopharyngeal skeleton that undergoes changes during larval development. Using mouthhooks, it feeds on host hemolymph and subsequently on fat body and other host tissues. During larval development the digestive tract gradually becomes more extensive and denotes a great increase in the third instar, which abandons the funnel and migrates in the host body cavity. The tracheal system with prominent respiratory plates is well developed in all larval instars. The larvae are equipped with a thick anal hook and numerous cuticular spines supporting the larva in their attachment to the respiratory funnel. The external morphology of E. larvarum larvae of all instars was studied by electron microscopy. The behaviour of E. larvarum larvae in the host tissues and the escape of the advanced-third instar larvae from the host carcass were documented in vivo using the light microscope.