938 MICROBIAL DISEASES OF INSECTS 



of the two months that it takes a larva to become a moth, the pebrine 

 disease is alternately sudden or insidious; it bursts out or disappears, 

 it hides itself within the chrysalis and reappears in the moth or the eggs 

 of a moth which has seemed sound. 



CAUSAL ORGANISM. The causal organism for this disease is Nosema bombycis, 

 a protozoon belonging to the Micros poridia. The spores find their way from the 

 caterpillar by means of the dejecta or through the disintegration of dead forms to 

 other silk worms. Some of the parasites find their way into the ovary, produce 

 spores, pass through the pupal and imaginal stages of the host into the next genera- 

 tion of silk-worms. The spores are often regarded as pebrine corpuscles. 



In the worms suffering from pebrine, corpuscles or polyhedral bodies, first noted 

 by Pasteur, are found in all tissues and all fluids of the body, even in the material 

 from which the silk is made; naturally they are also found in the dejecta of the worms. 

 These same bodies are found in and on the infected eggs, pupae and moths and in 

 innumerable quantities in the dust of the infected nurseries; they are easily rec- 

 ognized microscopically. 



These polyhedral bodies are now known not to be the etiological factor, but are 

 most probably an effect of the disease. Glaser and Chapman* have found them to 

 be nucleo-protein crystal-like degeneration products of the insect blood cells, and 

 not organisms. They contain iron and phosphorus. Crystals simulating the 

 original polyhedra are obtained on dissolving polyhedra in alkali dialyzing out the 

 alkali, and evaporating the protein solution. Before their nature was known, how- 

 ever, elimination of all eggs containing these bodies resulted in the suppression of 

 the disease. 



METHODS OF INFECTION. A very common method of infection is 

 due to the habits of the silk-worms crawling over one another. When 

 a worm moves across a diseased worm, its claws cut through the tegu- 

 ment and become contaminated; in its progress it inoculates other worms 

 by means of its soiled fangs. The greatest source of contagion, however, 

 is the excreta which fall on the food of the worms. Luckily this infec- 

 tious material on being exposed to light and air, becomes rapidly at- 

 tenuated. However, the causal organism is not so attenuated when 

 within the egg; it passes the winter in a latent state and develops along 

 with the worm, multiplying within its body and altering more or less 

 profoundly the conditions of existence. 



CONTROL. If moths are not seriously diseased, their eggs will 

 always furnish several healthy larvae and if these are isolated as soon 

 as they hatch out and are kept and bred under sanitary conditions, a 



*Glaser, R. W. and Chapman, J. W. The nature of the polyhedral bodies found 

 in insects. Biol. Bui. Marine Biol. Lab. Woods Hole, Mass., 30, pp. 367-390, 1916. 



