322 Lesions Produced by the Bite of the Black-fly 
5. The toxic agent is rendered inactive or destroyed by hydro¬ 
chloric acid in a concentration of 0.25%. 
6. The toxic agent is most abundant in the region of the ana¬ 
tomical structures connected with the biting and salivary apparatus 
(head and thorax). 
7. The toxic agent is not affected by a 0.5 % solution of sodium 
bicarbonate. 
8. The toxic agent is not affected by exposure to dry heat at 
ioo° C. for two hours. 
9. The toxic agent is destroyed or rendered inactive in alkaline 
solution by a typical hydrolytic ferment, pancreatin. 
10. Incomplete experimental evidence suggests that the activity 
of the toxic agent may be heightened by a possible lytic action of 
the blood serum of a sensitive individual, and that the sensitive serum 
itself may contain the toxic agent in solution. 
These results, as far as they go (omitting No. 10), accord with 
Langer’s except on the point of alcoholic solubility and the effect 
of acids. The actual nature of the toxic agent in the black-fly is 
left a matter of speculation. 
The following working theories have suggested themselves to 
the writer. First, the toxin may be, as Langer believes in the case 
of the bee, an alkaloidal base, toxic as such, and neutralized after 
injection by antibodies produced for the occasion by the body. In 
such a case the view that a partial local fixation of the toxin occurs, 
which prevents its immediate diffusion, is acceptable. Through 
chemotactic action, special cells capable of breaking up the toxin 
into harmless elements are attracted to the scene. Their function 
may be, on the other hand, to neutralize directly, not by lysis. 
This would explain the role of the eosinophiles in the black-fly lesion. 
If their activities be essential to the destruction or neutralization 
of the toxin, one would expect them to be most numerous where 
there was least reaction. This would be at the site of a bite in an 
immune individual. A point of special interest for further investiga¬ 
tion, would be the study of such a lesion. 
Second, it is conceivable that the injected saliva of the fly does 
not contain an agent toxic as such. It is possible, that like many 
foreign proteins, it only becomes toxic when broken down. The 
completeness and rapidity of the breaking down depends on the 
number of eosinophiles present. In such a case immunity should 
again be marked by intense eosinophilia. 
