Humoral encapsulation in Diptera (Insecta): comparative studiesin vitro

The humoral encapsulation reaction of larvae ofChironomushas been reproduced and studiedin vitro. This reaction is activated by entomogenous fungi and certain foreign bodies as well as by nematodes. The different stages of encapsulation and the duration of the process have been determined. In the larvae of a given species of Chironomidae encapsulation does not vary with age or sex. In contrast, the duration of the process differs according to the species of Chironomidae and is modified by temperature or by parasitism of the host by Mermithidae.The capsules contain melanin. Their formation is prevented by glutathion, an inhibitor of phenoloxidase.The humoral type of encapsulation also occursin vitroin the haemolymph of larvae of the nematocera families, Culicidae and Chaoboridae. However, the reaction of these insects differs in the amount, distribution and pigmentation of the capsular material.The results demonstrate similarities between humoral and cellular encapsulation.

Humoral encapsulation in Diptera (Insecta): defence reactions of Chironomus larvae against fungi

Humoral encapsulation is an effective defence reaction against fungal pathogens. The development of injected spores in the haemocoele of Chironomus larvae may be completely (Aspergillus niger) or partially (Mucor hiemalis) prevented by this reaction. The encapsulation proceeds very rapidly; within 5 min of injection most of the spores are enclosed in a solid capsule. Disintegrating blood cells may participate in the formation of capsule substance, but this is not usually the case. Encapsulation also occurs within the cuticle against invading hyphae of Beauveria bassiana. Histochemical tests show that the capsule substance does not consist of polysaccharides; some tests for proteins and all tests for melanin were positive. The significance of melanin formation and the biochemistry of the phenoloxidase system in insects is discussed in detail. The data presented lead to the conclusion that humoral encapsulation is based upon an activation of phenoloxidases and that the capsule substance represents a polyphenol—protein complex. In contrast to current opinion, the formation of melanin in cellular as well as in humoral encapsulation is not considered to be a secondary process, independent of the actual encapsulation procedure. Instead, the authors interpret the presence of melanin as an indication of the activity of phenoloxidases which cause the formation of a capsule substance on the surface of the parasites.