STATE HORTICULTURAL SOCIETY. 33 



In studying the diseases of insects I have sought answers to 

 three questions: First, Is this disease contagious? for, if not, it is 

 prohably beyond our control for practical purposes. Second, If so, 

 what is the character of this contagion-' and third, Tan it be used 

 protitahly for the artificial destruction of our insect enemies? 



There is a common affection of our native caterpillars with w hich 

 all who attempt to rear these insects to the perfect stage are more or 

 less familiar, liecause it is very likely to break out in their breeding 

 cages, often with most destructive effect; but it seems hitherto to 

 have been but rarely noticed among caterpillars living in a state of 

 freedom, and the literature of economic entomology is almost silent 

 upon it. I shall endeavor to show you, from studies which T have 

 made this fall, that this is a true contagious disease, the same in 

 fact as one of the most destructive diseases of the silk-worm; that 

 it occurs in a most virulent form among injurious insects in a state 

 of nature and in the open air; that its germ or virus is a minute 

 bacterium infesting the alimentary canal; that this can be cultivated 

 easily in organic infusions; and that when so cultivated it may be 

 used to set up the disease among healthy insects, and may be easily 

 ]ireserved from season to season and from year to year. 



As the disease which I studied proved to be of the same nature 

 as one of those affecting the silk-worm, differing only as we should 

 expect it to differ when appearing in insects of other species, T will 

 })reface my account of my own observations by a brief summary of 

 the discoveries of Pasteur on this disease, — called facJierie by the 

 French, and sc/ilajfsuclit by the Germans. 



This is now the most destructive disease of the silk-worm, and 

 is said by high authority to damage the silk-})roducing countries of 

 Europe at the rate of hundreds of millions annually. During the 

 last ten years it has reduced the average income of the silk-worni 

 ])reeder twenty-five per cent.. :tnd was the ]n'ineipal cause of an enor- 

 mous falling off" in the silk product of 1879, which did not amount 

 to more than a fourth of an average yield. 



The external symptoms of this aifection are not at first especiall}^ 

 noticeable. The caterpillar loses its ai)petite. digests its food imper- 

 fectly, wanders away from its feeding-jjlace, becomes sluggish and 

 slow in its movements,' and afterwards shrunken in aspect, and soft 

 and flabby to the touch. Sometimes it gradually blackens, as if from 

 decay, but in other cases presents hardly any of the visible ap])ear- 

 ances of disease until death overtakes it. The dead worms ra])idly 

 soften, and in a day or two are mere rotten skins, filled with a grey- 

 ish brown fluid which swarms with bacteria. 



If the juices of the alimentary canal of a diseased worm are ex- 

 amined microscopically, they are invarialtly found to contain innu- 

 merable myriads of a minute organism, a plant of the simj)lest pos- 

 sible structure — each a perfectly simple cell, oval in outline usually 

 connected end to end in pairs. ))ut sometimes in short strings of from 



3 



