Ontogenetic Studies on the Immune System of Two Locust Species: Lowsta Migratoria and Schistocerca Gregaria

Abstract

Work presented in this thesis examines the ontogenesis of immunity in two species of locust, Locusta migratoria (the migratory locust), and Schistocerca gregaria (the desert locust). Three haemocyte types (granulocytes, coagulocytes and plasmatocytes) were observed in all stages examined and the differential blood cell count remained constant throughout development. Following antigenic challenge in all developmental stages, there was an increase in the percentage of blood granulocytes, a decline in coagulocytes and less consistent changes were noted in the percentage of plasmatocytes. Autoradiographical studies revealed proliferative activity in haemopoietic tissue following antigenic challenge but not in peripheral haemocytes. Inducible agglutinins were observed in the haemolymph following challenge with vertebrate erythrocytes and bacteria. Agglutinin titres varied with species, developmental stage and the antigen used. Opsonisation studies revealed that haemolymph from immunised Locusta could promote phagocytosis by haemocytes in vitro. Haemocytes of both species were able to phagocytose vertebrate erythrocytes, charcoal and indian ink particles in vivo. The phagocytic response to erythrocytes was stronger than that observed towards inert particles and phagocytic responses were weaker or absent in younger stages. Adult and fifth instar was active in phagocytosis of charcoal and indian ink, but that of younger stages failed to trap foreign particles. Haemocytes of adult and fifth instar nymphs were seen to respond positively in chemotaxis and chemokinesis assays. Positive responses were observed following exposure to erythrocytes and bacteria, and haemocyte capsules formed around cat gut implants also produced chemotaxis and chemokinesis responses. Schistocerca gregaria failed to encapsulate implants and, in general, responses in this species were weaker than those observed in Locusta migratoria.