13 

Fig. 5.—Entomophthora radicans Bref. A, caterpillar of Pieris brassicw killed by it; a, 
the hyphe growing out fromit. B, the same at a later stage, and entirely enveloped 
by the fungus. OC, cross-section through B; a, cuticula of caterpillar; b, trachex; c, 
remains of food—all the soft parts of the caterpillar are replaced by mycelium threads, 
which have, at d, penetrated through the skin, and formed spores, ate. D, a, fruit- 
hyyhe; b, basidia; c. spores. EH. single spore. F, a single spore a, producing ”~ my- 
celium thread with secondary sporesb andc. G,a piece of the skin, with germinating 
spores a, penetrating the skin at b, and growing in the interior atc. J. branched my- 
ecelial thread. K, mycelial threads bearing resting spores; a, filled with protoplasm; 
@ emptied; b, developing, and b’ mature resting spores. IL, mature resting spores with 
a thick envelope and fat-drops within. After Brefeld. 
When an insect is found infested with a disease with which 
it is desirable to experiment, the first step is to procure, if 
possible, pure cultures on some artificial substance. This has 
to be done in various ways; sometimes a method that will suc- 
ceed with one disease will utterly fail with another. The or- 
dinary method is to make a dilution-culture as follows: three 
glass tubes, each containing a small amount of nutrient agar, 
are placed in a water-bath and heated until the agar is melted. 
The tubes must have been previously plugged with cotton and 
thoroughly steamed for several days to destroy any living or- 
ganisms present in them. The agar is a jelly-like substance, 
