282 Transactions British Mycological Society. 
Immediately after making Experiment VII, I was obliged 
to leave England to return to my duties in Canada. My investi- 
gations upon the finding of fungi by slugs were thereby brought 
to an end. 
SLUGS AND MUSTARD GAS. 
There can be but little doubt that the stimulus which comes 
from the fungi to the slugs and which guides these animals on 
their foraging expeditions is gaseous in nature. It has been 
recently shown by Dr Paul Bartsch of the Smithsonian Institute, 
Washington, that Limax maximus is extraordinarily sensitive 
to certain gases. A few years ago a number of slugs of this 
species, which were under observation in his home, escaped 
from the box in which they had been confined. Their behaviour 
in the furnace room showed that they were sensitive to the 
fumes coming from the furnace and, in response thereto, made 
characteristic movements of their tentacles. After the United 
States entered the War and the need for a gas detector arose 
in connection with the fighting at the front, Dr Bartsch recalled 
his furnace-room observations. A very brief period of experi- 
mentation then revealed the extraordinary sensitiveness of 
Limax maximus to mustard gas, and such startling results were 
obtained that within two hours after the first experiment had 
been made, the Allied forces were advised by cable of the 
possibilities of using the slug as a gas detector. Dr Bartsch 
found that the tentacles of Limax maximus are sensitive to a 
dilution of I in 10,000,000 of mustard gas, and that they make 
characteristic responses indicating the degree of dilution. Dr 
Bartsch also found that man reacts to a dilution of I in 4,000,000. 
Limax maximus, therefore, is much more sensitive to the presence 
of mustard gas than man*. If Limax maximus is thus so extra- 
ordinarily sensitive to one gas, we have every reason for be- 
lieving that it is extraordinarily sensitive to other gases, par- 
ticularly those which emanate from its food substances such as 
fungi. If we assume such a sensitiveness, it is not difficult to 
imagine how it is that Limax maximus finds its way unerringly 
over a distance of many feet to the fruit-bodies of Phallus, 
Boletus, Russula, etc., which it devours with such avidity. The 
sense of smell in slugs, like that in dogs, is doubtless much more 
acute than in human beings. 
* Paul Bartsch, Our Poison Gas Detector and How It was Discovered, 
Abstract of an Address delivered on Feb. 7, 1920, to the Biological Society 
of Washington, U.S.A. I read a paper entitled ‘‘ Upon the Chemotactic Attrac- 
tion of Fungi for Slugs’ at Chicago on Dec. 30, 1920, before the Ecological 
Society of America. Subsequently Dr R. F. Griggs called my attention to 
Dr Bartsch’s work, and then Dr Bartsch kindly sent me an abstract of his 
Address. 
