CELLULAR HEMOLYMPH RESPONSE OF SIMULIUM-VITTATUM (DIPTERA, SIMULIIDAE) TO INTRATHORACIC INJECTION OF ONCHOCERCA-LIENALIS (FILARIOIDEA, ONCHOCERCIDAE) MICROFILARIAE

Hemolymph cellular changes in Simulium vittatum Zetterstedt in respons e to intrathoracic injection with Onchocerca lienalis Stiles were char acterized by increased numbers of tissue fragments that contained cell s embedded within a noncellular matrix. Cell numbers within the matrix fragments increased both for sham and microfilariae-injected flies, i ndicating a blastogenic response to injection alone. Morphological cha racteristics of the fixed, Giemsa-stained cells within these tissue fr agments were most similar to those previously described for prohemocyt es. The total population of freely circulating hemocytes collected 24 h after injection also responded to injection alone. Differential cell counts showed a complex pattern of changes that was influenced strong ly by increased numbers of prohemocytes at 24 h in microfilariae-injec ted flies. Fat body fragments collected in hemocoel perfusates at 24 h were fewer when flies were maintained at 27 degrees C than at 21 degr ees C. More fat body fragments were collected from microfilariae-injec ted flies than from control and sham-injected flies held at 27 degrees C. Injection of 1.25-5 mu g of lipopolysaccharide per Ay did not elic it similar hemolymph-associated matrix tissue and fat body changes, in dicating that S. vittatum response to microfilaria infection and bacte ria infection are likely to involve different mechanisms.