MOULD FUNGI AS EXCITING AGENTS OF DISEASE. 638 



spread can take place, while the latter is impossible as 

 soon as other species of plants, which are not liable to 

 the infection, are mixed with the individuals of the kind 

 that are threatened. It is evident that by taking into 

 consideration all these numerous factors which exercise 

 an influence on the occurrence of epidemics, the rise 

 and fall of the parasitic diseases of plants can be 

 explained, and that at the same time these observations, 

 which can be made on plants with great certainty, must 

 be of great advantage in enabling us to understand the 

 epidemics of animals and man with which they offer 

 numerous analogies. 



Among animals it is chiefly the invertebrata, and, Mould fungi 

 among these, the insects, which are attacked by para- on animals! 

 sitic mould fungi. The infection always occurs by the 

 penetration of the rnycelial threads into the external 

 uninjured skin ; in the case of empusa radicans it has 

 been demonstrated, as the result of experiments, that 

 infection never occurs from the intestine. In some In insects, 

 cases the fungus may enter almost anywhere; for ex- 

 ample, laboulbenia can penetrate through the back, 

 head, legs, or wings of flies. In other cases the place of 

 entrance is limited ; empusa muscae, for example, can 

 only enter at the abdomen of the fly, and in the case of 

 isuritB the infection of the caterpillars occurs through the 

 stigmata of the tracheae. In some cases the parasitic 

 fungi cause very little inconvenience to their host ; thus 

 laboulbenia muscse may exert no injurious action, but 

 the insects attacked usually die. The mycelium threads, 

 when they have penetrated into the body, grow through 

 the muscle and fatty tissue, and usually give off conidia 

 in the blood, which grow to form an extensive my- 

 celium ; during this process there is an almost entire 

 consumption of the structures of the animal attacked. 

 Such an energetic growth is only conceivable when the 

 nutrient conditions which the animal body offers are 

 particularly favourable to the fungus, when the animal 

 cells assimilate with very little energy, and when there 

 is a more or less complete absence of reaction. These 

 conditions appear to be present in the insects which are 



