Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 302]

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296 cultures must be used to produce, in healthy insects of the same or other_ species, a disease characterized by the symptoms and results of the original affection. It is further desirable that (6) second cultures should be prepared from these cases of disease artificially produced, in order that a strict comparison may be made of the bacteria concerned, as they occur both in the bodies of the insects and in artificial culture media. I propose to take up these points seriatim with respect to the white plague of the cabbage worm, giving only an abstract of the facts bearing upon each, premising, how-, ever, that the proof of our proposition is sometimes partly contained in the paragraph relating chiefly to another. This disease is distinguishable with great ease and certainty by conspicuous external symptoms, the color alone of affected larvae being, in fact, entirely characteristic and unmistakable. The natural color of a healthy cabbage worm is a light lively green, sometimes slightly tinged with yellowish, but without any approach to an ashy or milky hue. As the first symptom of the plague, however, the larva commences to turn pale, this paleness increasing more or less rapidly until the color is almost milky white, only slightly tinged with greenish. After death the color deepens to a sooty gray, commonly uniform, but sometimes appearing first about the center of the length of the larva. In the actions of the insect there is little to indicate any change of state except a gradually increasing sluggishness, slowness of movement, and loss of appetite. These are later to appear than the pale discoloration above mentioned, and even shortly before death a larvae may show considerable impatience if roughly handled. When the disease is well developed the caterpillar is very feeble, and will remain motionless for a long time; or if it attempt to crawl where some strength is needed, as horizontally on a vertical surface, it may lose its hold with its jointed limbs and cling only by its central prolegs, the fore and hinder parts hanging limp and helpless at right angles to the remainder of the body. Most commonly an escape of fluid from the vent is among the earlier symptons of the affection, at first greenish or whitish and later a dirty gray or even a chocolate brown. The stomach is almost invariably well filled with food, digestion being evidently suspended during the course of the disease. The color of the fluids of the healthy larva is a very pale transparent green, the blood containing only lymphoid corpuscles in greater or lesser number; but if a proleg of a diseased specimen be snipped off, and a cover glass be pressed against the cut surface, the droplet exuding will be of almost milky whiteness, or in the latest stages of the disease, a dirty gray. If examined under a high power of the microscope this will be found to contain innumerable myriads of very minute sphericles, varying in diameter, according to the individual, from .5 /J. to 1 /A Usually their average size does not surpass .7 /*. It is the infinite multitude of these which gives to the fluids of the caterpillar their milky look, the blood being so thick with them that little else can ordinarily be seen in it.