MICROBIAL DISEASES OF INSECTS 947 



succeeded not only in growing normal insect blood cells in vitro but in 

 observing in vitro the formation of typical polyhedra from the blood of 

 healthy M. americanum and Porthetria dispar larvae fed with filtered 

 polyhedral virus (Berkefeld "N" filter used). He found that it was 

 impossible to infect the blood directly with the virus. The virus had 

 to be given "a start" within the insect itself. Later stages of the virus 

 however, find the conditions suitable on the tissue culture slides. 



Glaser observed from his studies that normal blood cells have a 

 normal tendency towards crystalline disintegration, thus it is not 'sur- 

 prising that crystals (polyhedra) are found within the degenerating 

 nuclei in a series of insect diseases. 



The ability of insect tissue to grow well seems to vary to a slight 

 degree according to the species of the insect. 



Glaser (1918) in studying immunity principles in insects found that 

 while entomological text-books emphasize the importance of phago- 

 cytosis by blood cells called amebocytes, in ridding the insect body of 

 foreign matter, in reality these insect blood cells are visibly rather 

 passive. Experiments on various insects with bacteria pathogenic 

 for the species, showed that the normal blood did act antagonistically 

 toward the organisms introduced, but the riddance w r as not accom- 

 plished by hungry amebocytes; the only movement shown by these 

 blood cells in vitro consisted in cell division. The antagonistic sub- 

 stances are extracellular and, therefore, in the blood plasma or serum. 

 Microscopic observation with grasshopper, army worm and gipsy 

 moth caterpillar blood showed that w r hen apparent phagocytosis oc- 

 curred the infecting bacteria seemed to bore their way into the cyto- 

 plasm. This may have been due to surface tension, however, and 

 might be called phagocytosis if the word is used in a broad sense. On 

 the culture slides, the quantity of the blood is not sufficient and meta- 

 bolism is lowered, so that antagonistic substances are not formed so 

 rapidly nor so abundantly as is the case within the body of the insect. 



The blood of several grasshoppers (Melanoplus femur-rubrum) 

 which had been immunized to Bacillus poncei by the injection of o.i cc. 

 of a twenty- four hour broth culture w r as successfully used in demon- 

 strating the presence of a specific agglutinin. The blood of uninfected 

 grasshoppers failed to show this phenomenon. The presence of bac- 

 tericidal substances in immune insect serum was also proven. Actively 

 immunized grasshopper blood showed a high degree of antagonism 



