The Food Relations of the Carabidte and Coccinellid<e. 
35 
sionally, at least, by the principles of natural selection, especially 
as applied to the machinery of food prehension. Whatever 
departure from the primitive vegetarian habit of animals any 
group has acquired, was of course initiated to enable it to draw 
on other food resources than those previously open to it. But as 
animal food is usually less abundant and less generally distributed 
than vegetable, it w T ould not, at first, be to the advantage of any 
that they should become exclusively dependent upon the former; 
their interests would be best served by such modifications of 
structure and habit as would enable them to draw upon one or the 
other store, according to circumstances. Acquiring some power 
to capture and masticate animal food, they would not wholly lose 
that of appropriating vegetable food also; and however well fitted 
their prehensile and digestive organs might become for the former 
function, we should expect that they would not altogether lose 
their fitness for the latter. It would be only as competition on 
this higher plane increased to the pressure point, that a few mem- 
bers of the dilferentiating group would be forced to the highest 
plane of complete dependence on animal food alone. 
The first results of an attempt at a more exact and exhaustive 
investigation of this subject, were given by the writer in a brief 
paper published in Bulletin 3 of this series, in November, 1880.* 
In another paper by Mr. F. M. Webster in the same Bulletin, a 
summary of previously recorded observations was given, together 
with many additional and original field notes. A few other items 
have since been published by others, but confined, as far as known 
to me, to chance observations on single insects. 
The method here followed, as in the paper above mentioned, 
has been that of dissection. The alimentary canals of beetles 
taken in a great variety of situations, at various seasons and at 
different times of day, have been removed, placed in glycerine on 
microscope slides, and opened with small knives and mounted 
needles, so as to display the contents completely. These have 
then been studied with whatever power of the microscope was 
ne.cessary, and mounted as microscope slides for permanent pres- 
ervation and repeated examination. The amount of information 
* Notes on Insectivorous Coleoptera. By S. A. Forbes. Illinois State 
Laboratory of Natural History, Bulletin No. 3, pp. 153-160. 
IPp. 149-152. 
5 
