INSECTS IN RELATION TO PLANTS 259 



white powdery ring, or halo. The white intersegmental 1 Kinds 

 are made by threads of the fungus just named, and the white 

 halo by countless asexual spores known as conidia, which have 

 been forcibly discharged from the swollen threads that bore 

 them (Fig. 251) by pressure, resulting probably from the ab- 

 sorption of moisture. These spores, ejected in all directions, 

 may infect another fly upon contact and produce a growth of 

 fungus threads, or h-yphce, in its body. The fungus may be 

 propagated also by means of resting spores, as found by Thax- 

 ter, our authority upon the fungi of insects. 



Einpusa aphidis is very common on plant lice and is an im- 

 portant check upon their multiplication. Aphids killed by this 

 fungus are found clinging to their food plant, with the body 

 swollen and discolored. Empusa grylli attacks crickets, grass- 

 hoppers, caterpillars and other forms. Curiously enough, 

 grasshoppers affected by this fungus almost always crawl to 

 the top of some plant and die in this conspicuous position. 



Sporo trie hum, a genus of hyphomycetous fungi, affects a 

 great variety of insects, spreading within the body of the host 

 and at length emerging to form on the body of the insect a 

 dense white felt-like covering, this consisting chiefly of myriads 

 of spores, by means of which healthy insects may become in- 

 fected. Under favorable conditions, especially in moist sea- 

 sons, contagious fungus diseases constitute one of the most 

 important checks upon the increase of insects and are therefore 

 of vast economic importance. Thus the termination (in 

 1889) of a disastrous outbreak of the chinch bug in Illinois 

 and neighboring states " was apparently due chiefly, if not 

 altogether, to parasitism by fungi." Artificial cultures of the 

 common Sporotrichum globulifenini have been used exten- 

 sively as a means of spreading infection. among chinch bugs 

 and grasshoppers, with, however, but moderate success as yet. 



Insects in Relation to Flowers. Among the most marve- 

 lous phenomena known to the biologist are the innumerable 

 and complex adaptations by means of which flowers secure 

 cross pollination through the agency of insect visitors. 



