THE SUSCEPTIBILITY OF COCKROACHES TO PLAGUE BACILLI 

 INOCULATED INTO THE BODY CAVITY. 



By M. A. Barber. 

 (From the Biological Laboratory, Bureau of Science, Manila, P. I.) 



The methods and results of these experiments may be illus- 

 trated by a detailed description of two or three series. 



In one series 26 cockroaches, 11 Periplaneta americana Linn, 

 and 15 Rhyparobia maderx Fabr./ all adults except 2 well- 

 grown nymphs, were inoculated with a virulent strain of plague 

 from a 24-hour culture. Inoculations were made under a magni- 

 fying lens with a very fine pipette of hard glass, the outer diam- 

 eter of the point of which was 0.08 millimeter. The dose, 

 approximately 0.3 cubic millimeter of a thick suspension in salt 

 solution, was estimated in two ways. The cubic contents of 

 the pipette below the dose mark measured with the eyepiece 

 micrometer gave 0.4 cubic millimeter. The dose was delivered 

 on the counting chamber of a Thoma Zeiss counter, the cover 

 adjusted, and the cylindrical drop measured, giving 0.3 cubic 

 millimeter. This quantity, corresponding approximately to the 

 cubic capacity of the pipette, may be taken as approximately 

 the dose delivered. The number of bacilli, estimated by counting 

 dilutions, was approximately two and one-half millions, a quantity 

 far in excess of the fatal dose for guinea pigs,^ and enormous 

 for insects of an average weight of about 1.2 grams. 



Inoculations were made in the leg, in most instances in the 

 basal sclerite of the dorsal surface of the coxa where the chitin 

 is thin enough to permit the easy passage of the pipette. In 

 order to prevent contamination with other bacteria, the surface 

 inoculated was rubbed with a small pledget of cotton moistened 

 with alcohol just before inoculation, and after inoculation again 

 rubbed with the alcohol and the minute wound covered with 

 sterile vaseline. Each insect was put after inoculation into a 

 separate receptacle and left at a room temperature of 25° to 31°. 



' Identifications by Charles S. Banks, Bureau of Science. 

 'This Journal, Sec. B (1912), 7, 251-254. 



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