FUNGI CULTIVATED BY INSECTS 451 



mycelium, or vice versa, whereupon they germinate, the hyphae 

 fuse, diploidization results, and mushrooms are developed. 



Craigie (1931) showed that Puccinia helianthi and P. graminis 

 may be diploidized by the agency of insects. The pycniospores 

 of these rusts are haploid. Diploidization occurs only if pycnio- 

 spores from one pycnium are transferred to another of opposite 

 sex, whereupon the process is initiated by fusion of a germinating 

 pycniospore with a receptive (flexuous) hypha that projects from 

 the pycnium. Insects may be essential agents in the transfer of 

 pycniospores, and such transfer is an essential condition in the 

 development of dicaryotic aeciospores. Subsequent findings with 

 other rust fungi substantiate these observations. Spermatization 

 of certain ascomycetes also is known to result from insect trans- 

 fer of spermatia. 



FUNGI CULTIVATED BY INSECTS 



Much of our knowledge on this topic involves "ambrosia" 

 beetles (timber-boring Scolytidae, including engraver beetles and 

 bark beetles that tunnel and breed in bark and sapwood), leaf- 

 cutting ants, and termites. Such relationship of insect and fungus 

 is termed an ectosvmbiotic one bv Buchner (1930). By ectosym- 

 biosis, in this instance, is meant an association in which the fungus 

 occurs chiefly outside the body of the insect. 



Beetles and fungi. Many species of Scolytid beetles are asso- 

 ciated with fungi; the better-known ones belong to Scolytus, 

 Dendroctonus, Ips, and Hvlurgopinus. Their relationship with 

 specific fungi seems none too well understood in most cases, al- 

 though the phenomenon of fungus-insect association was first ob- 

 served about a hundred years ago. As indicated in Buchner's 

 treatise (1930), Thomas Hartig in 1844 recognized that the am- 

 brosia of Xyle bonis dispar in Ahms cor data is a fungus, which he 

 named Monilia Candida. Subsequent investigations have shown 

 that there are many other species of ambrosia fungi. Leach 

 (1940) emphasizes, however, that ". . . taxonomic studies of am- 

 brosia fungi are conspicuous by their absence." 



Ambrosia fungi in general permeate the wood and enter into 

 the burrows and brood galleries made by the beetles. The my- 

 celium and spores that protrude into the galleries are eaten by the 



