908 MICROBIAL DISEASES OF INSECTS 



ing by their pseudofeet. Worms when dead appear so very life-like 

 that it is necessary to touch them in order to make sure that they are 

 not living. From this appearance comes one of the names of this 

 disease "morts-blancs." 



After death they become soft in a short time and assume a blackish 

 color in twenty- four to forty-eight hours. The body is then filled 

 with a brownish fluid swarming with bacteria. Hundreds of worms 

 in this condition show no polyhedral bodies (characteristic of pebrine) . 

 A glance is all that is necessary to distinguish worms dead of flacherie. 



CAUSAL ORGANISM. In the silk-worms as well as in culture Streptococcus bomby- 

 cis forms short chains of small cocci, 0.89/4 in diameter; the chains are from 5.01/1 to 

 H-99AI long; stain well with anilin dyes and are Gram-positive. 



In gelatin plate cultures, colonies are small, round, yellowish-gray, sharply con- 

 toured, finely granulated interior, gelatin not liquefied. Subsurface colonies have 

 the same characteristics. Gelatin stab cultures are dull white, not liquefied. Agar 

 colonies are small, round with a slightly undulating contour, deep brown in color, 

 finely granulated, moist. 



It is opalescent on glycerin agar and on ordinary agar when first isolated. In 

 broth at 37 a marked turbidity is manifested after twelve hours without flocculence; 

 after long standing the broth becomes clear. Potato cultures show an iridescence 

 which later becomes a light gray. Strept. bombycis develops in milk without curdling 

 it. It is a facultative anaerobe; the temperature optimum is 37 but it develops well 

 at 20 also. The streptococcus retains its vitality for a long time in culture. It is 

 destroyed at 6s-7o in fifteen minutes. 



METHODS OF INFECTION. Infection of the silk-worm takes place by 

 means of food infected either with the excrement of sick individuals 

 or with the dust of infected silk- worm nurseries of the year preceding. 



When silk- worms show all the symptoms of flacherie, if they develop 

 into moths the eggs laid by these moths are always infected. If any 

 of the forms in which the silk-worm exists during its life cycle becomes 

 infected it is sure to die before the cycle is completed. 



Certain environmental conditions favor the rapid development of 

 flacherie; high humidity due to an approaching storm or to keeping the 

 worms enclosed in a practically air-tight cage prevents the transpira- 

 tion which is so necessary to the worm after the fourth molt. Too many 

 worms together often favors the progress of the disease. 



CONTROL. Pasteur instituted the following means of producing 

 healthy strains of the silk- worm; a small portion of the digestive cavity 

 of a moth was abstracted with a scalpel, mixed with a little water and 



