Localization of phenoloxidase in the midgut of Periplaneta americana parasitized by larvae of Moniliformis moniliformis (Acanthocephala).

In cockroaches infected with Moniliformis moniliformis, the melanogenic enzyme phenoloxidase (PO) was histochemically localized in the posterior midgut and in haemocytes. Midguts were incubated with either 3-hydroxytyramine-HCl (dopamine) or 3-(3,4-dihydroxyphenyl)-L-alanine (dopa), and the resulting electron-dense reaction products of PO activity were found to be homogeneously distributed in the cytoplasm of both midgut cells and haemocytes. Following experimental infection, the first acanthors that reached the outer surface of the gut elicited a haemocyte response similar to that observed during wound healing. Larvae that remained attached to the gut became melanized or developed successfully. PO activity gradually decreased as the course of infection proceeded (10-50 days post-infection) but was apparently not inhibited in either midgut cells or haemocytes that were closely associated with the parasites. PO was lacking in the midgut cells of uninfected cockroaches. The results of the present study are discussed with respect to the defence reactions of the host and the survival mechanisms of the parasite.