39 2 Polydesmus complanatus. [July, 
As mutual assistance is imperative with those who desire 
to turn natural science to account, I drop a friendly line to 
my new correspondent, offering to co-operate with him in his 
researches, and to render him any aid that lay in my power. 
Expe(5tation stood tiptoe for a day or so, and then there 
comes a little box by the post, enclosing some carefully 
hoarded fragments of the insedf in question, with further 
intimation that live examples would follow by the carrier. 
A glance at the enclosure showed me that my correspondent 
was correct in his assertion that the marvellous insedt was 
of the myriapod class, and on reflection it occurred that the 
only party who was likely to have bestowed much attention 
on this order would be Prof. Westwood, and to him I decided 
to refer the matter. 
A few days went by, and I received a reply to say that the 
insedts supposed to be the cause of the potato disease were 
without doubt the Polydesmus complanatus , one of the Myri- 
apoda. The Professor, however, deprecated their being 
supposed the cause of the disease, and added that he had no 
doubt but that they might be found in diseased potatoes, 
just as a score of other inseCts of different kinds, concerning 
which the French writer GuCrin gave an account, in 1845, 
in his “ Memoire sur les Acariens, les Myriapodes, les 
InseCtes et les Helminthes, descrivees jusqu’ ici dans les 
pomme de terre malades.” The Polydesmus , I should perhaps 
observe, is a pale whity-brown inseCt, which must be familiar 
to most potato-growers. Though a centipede in form, it has 
only seven joints to its feelers or antennas, and it is there- 
fore more nearly akin to the wood-louse and cylindrical 
myriapod than to the brown centipedes and Scolopendras of 
the tropics, which have fourteen. 
The potato disease, as is well known, is due to a fungus 
or mildew called Peronospora infestans , and I gather the fol- 
lowing particulars concerning its spread from an article by a 
relative of mine, Dr. Galbraith, of the Stirling Natural 
History and Archaeological Society, who has made a spe- 
ciality of the matter : — The spores or capsules of this mildew 
are of two kinds, and they have the marvellous property, 
when moistened, of absorbing the moisture and bursting, by 
which means numerous living seeds — if it be permissible to 
so translate the learned term applied to them — are set free. 
These living seeds or zoospores move about by means of 
their fringes or cilia, and germinate within from an hour and 
a half to three hours after the spores are sown. One of the 
two kinds of spores or seed capsules hybernates or remains 
dormant throughout the winter season, and these propagate 
the disease from year to year. 
