1990] HARRIS — THY RIDOPTERIX 65 
NOTE ON THE BAG WORM, THYRIDOPTERIX. 
BY J. ARTHUR HARRIS, COLD SPRING HARBOR, N. Y. 
IN an interesting memoir on the constriction of twigs by the Bag Worm, Dr. 
von Schrenk ' has presented certain data which seem to me to deserve consideration 
from a somewhat different point of view. 
Dr. von Schrenk found that the insect attaches the bags almost without fail 
only to one-year-old twigs. Whether this is done because these are the twigs upon 
which the insect has been feeding, or whether because of their size he was unable to 
determine. But he did find, by an interesting experiment, that they would not form 
bands around very large twigs, but glued them to one side instead, although they 
always formed the band entirely around smaller twigs. 
On page 176 Dr. von Schrenk remarks: ‘The first analysis of the measurements 
made deals with the size of the bands in their relation to the size of the twigs to which 
they were attached....On Table I the bands taken from maple twigs are arranged 
according to their width and according to the diameter of the twigs. — It will be noted 
that the largest number of bands occurred on twigs 3 mm. in diameter, and that there 
is a regular falling off towards both sides; in other words, the insect seems to select 
twigs having a diameter of about 3 mm.” 
Now I think there are two points which may be given especial consideration 
from this table. These are the questions: — 
a) Is there any selection of twigs according to size ? 
b) Is there any relationship between the size of the twig and the width of the 
band formed by the insect ? 
It is question a) which Dr. von Schrenk has in mind when he writes of “the size 
of the bands in their relation to the size of the twigs,” I think. Possibly he had the 
second problem in view as well, but he does not draw especial attention to it. 
Now as to question a) it seems to me that unfamiliarity with biometric methods 
has led Dr. von Schrenk into a slip in his consideration of this point. So far as T can 
see his data do not enable us to determine whether there has been a selection of twigs 
at all. It is well known that measurements made upon practically any plant organ 
vary around a modal condition, roughly speaking in accordance with some mathe- 
lyon Schrenk, H. Constriction of Twigs by the Bag Worm and Incident Evidences of Growth 
Pressure. Ann. Rept. Mo. Bot. Gard. 17, 155-181. 1906. 
