Minnesota Plant Diseases. 69 



on the legs and wing covers of flies and particularly of water 

 beetles. They are highly specialized as to their abode, often 

 occurring constantly on a certain joint of one leg. This defi- 

 niteness of position is explained in the spore distribution, as 

 the plant sexes are often separated,, growing on dififerent plants, 

 and the sexual cells of the fungus are brought together dur- 

 ing the breeding acts of the insects. These beetle fungi are 

 not, as far as is at present known, harmful to the insect 

 which they inhabit. In form they are very minute and visible 

 only by the aid of strong lenses. They usually have the shape 

 of little broom brushes and are attached by a blackened disk. 



The butterflies, particularly in their caterpillar stages, are 

 also common prey for the insect mold. Perhaps more com- 

 monly, however, they are attacked by the fungus known as the 

 caterpillar fungus, a member of the black fungus group. This 

 fungus has learned to produce a variety of spores, each special- 

 ized for a certain purpose. Cylindrical spores are produced 

 upon orange-colored fruiting bodies in the autumn. When a 

 spore falls on a caterpillar it sends out germ-threads which can 

 eat their way through the covering of the caterpillar and enter 

 the body cavity. Here the threads immediately form long nar- 

 row spores which are pinched off into the fluid of the body 

 cavity and can move around easily, thus rapidly spreading the 

 fungus. These spores germinate immediately and more spores 

 are formed. Meanwhile the threads produced by these spores 

 branch profusely and soon permeate all parts of the insect 

 body-cavity and invade the various organs, finally working 

 their way even into the muscle fibers. The caterpillar gradually 

 becomes sluggish and finally dies. After death, the fungus 

 continues to grow and to appropriate the insect substance for 

 food. At fist the threads are very thin and are thus able to 

 work their way with more ease through all parts of the body. 

 As soon, however, as the threads become very numerous they 

 grow thicker and lay up nutrition as storage material in the 

 form of oil and fungus starch. Finally the threads have com- 

 pletely absorbed all of the insect's soft parts, filling the chitin- 

 ous covering, and retain in their densely compacted form, the 

 exact shape of the larva — not only in the external form but in 

 the form of the internal organs. In other words a mummy 



