Ultrastructural investigation of the melanization process in Culex pipiens (Culicidae) in response to a nematode

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The present account is an ultrastructural investigation on the formation of a melanized sheath around third stage infective juveniles of Neoaplectana carpocapsae Weiser within the larval hemocoel of Culex pipiens L.

The host reaction began almost immediately after the nematode entered the body cavity of the mosquito larva. Within 1 hour, a homogeneous matrix or deposit surrounded the nematode. After being in the host for 1–2 hours, minute pigment granules formed within the homogeneous matrix, and they eventually enlarged and coalesced to form a layer of melanin around the parasite.

The normal definitive capsule appeared in 5–10 hours and was composed of an inner melanized region, a middle non- or slightly melanized homogeneous layer and an outer region containing tracheole elements and cellular debris.

It appeared that the homogeneous matrix in which the melanin was eventually produced did not arise from blood cells, but from components in the noncellular portion of the hemolymph that coagulated out on the surface of the nematode. This reaction could then be considered an example of humoral melanization in contrast to the more common hemocytic melanization involving the host's blood cells.

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