68 



Minnesota Plant Diseases. 



body of an insect there is considerable chance for aeration on 

 account of the large number of air-tubes which traverse the 

 insect body. This very probably accounts very largely for the 

 popularity of the insects as fungus hosts. Of these fungi 

 the insect molds are very abundant as is also the "burnt wood'' 

 fungus known as the caterpillar fungus, and these two groups 

 of fungi are responsible for most of the disease epidemics of 

 insects. 



Plant lice have been known to suffer from attacks of both 

 of these fungi. The common housefly and its relatives are de- 

 stroyed in enormous numbers every fall by an insect mold 

 causing a disease commonly known as fly cholera. Such flies 

 are seen clinging to window panes or the ceiling or walls of a 

 room, surrounded by a dim circular haze or halo of the fungus 

 spores which have been forcibly snapped off from fungus 

 threads and caught on the glass. Of course most of the spores 

 have been thrown off into the air where they may float about 

 until they come into contact with another fly. The fungus 



continues to form spores as 

 long as there is available 

 food material in the insect 

 body. When spores alight 

 on an otherwise healthy fly 

 a fungus thread is pro- 

 duced which may make its 

 way through the skin to the 

 inside of the body and 

 there continue to grow. 

 The mycelium soon causes 

 the death of the insect and 

 later comes again to the 

 surface to produce its 

 spores. Other insect molds have been known to attack com- 

 mon house fly relatives. The mosquito may also prove a prey 

 to fungus diseases and attempts have been made to fight 

 it by aiding the spread and dissemination of those par- 

 ticular insect-molds which are parasitic upon it. One of the 

 most remarkable groups of insect parasites are the beetle 

 fungi, relatives, perhaps, of the black fungi. They are found 



FIG. 30. Beetle fungi attached to an insect. 

 The black spots at the base are the at- 

 taching organs. Highly magnified. After 

 Thaxter. 



