176 AN ESSAY ON THE DEVELOPMENT 
to exist is inaccurate. For the reasons just given no references to previous writers 
will be made, except incidentally, and as I have in some respects modified my views 
as to the homology of certain of the parts, I will go into the entire subject in such 
detail as is necessary to prove my point; but without reprinting my first paper, which 
should be herewith consulted. 
I do not expect denial at this day, when I claim that no explanation of the homol- 
ogies of the mouth parts of insects can be considered satisfactory which will not stand 
the test of criticism by the theory of evolution. If we assume the origin of all insects 
from one original type, we must, necessarily, assume that all the mouth structures are 
derivatives of one type, and we must so study them as to be able to explain, step by 
step, just what specializations have occurred. We may not be able to complete en- 
tirely each link in the chain of evidence, but we can, at any rate, reach a result con- 
sistent with all the facts known to us. Any explanation which satisfies all the require- 
ments of a regular and natural development is to be preferred to one which demands 
an unexplained specialization of any part, not in line with its function in other series. 
It is therefore necessary to study carefully the make-up of every separate mouth 
organ, and of every sclerite in each, to become thoroughly familiar with its uses and 
to ascertain the lines in which it varies or develops. 
It may be premised that the mouth parts of the Hemzptera in their present con- 
dition are not included in the range of these studies. I have examined numerous 
specimens and have devoted especial attention to Cicada and Thrips—the latter 
classed as hemipterous for present purposes only—and I believed at one time that I 
had made out the remnants of a mandibular sclerite, and so published it. Mr. C. L. 
Marlatt questioned my conclusions and asserted that the mandibles are represented by 
one pair of bristles. While I believe that I was wrong in my identification of the man- 
dibular sclerite, 1 am yet convinced that I am correct in claiming that beak and sete 
are all maxillary structures. I have concluded, however, after a careful review of all 
my preparations and of what has been written, that the Hemzptera in the mouth struc- 
ture are not descended from any well-developed mandibulate type, and that no trace of 
true mandibular structure occurs in any present form. 
In other words, the Hemiptera equal all the other orders combined in rank, for all 
others are mandibulate or derivatives from a mandibulate type. The archetypal Thy- 
sanuran with undeveloped mouth organs varied in two directions—toward the 
haustellate type now perfected in our present Hemiptera, and to the mandibulate type: 
and there has never since been any tendency toward a combination. The haustellate 
type proved ill adapted for variation and there is, in consequence, a remarkable same- 
ness throughout. This kind of stracture must be studied on an entirely new basis to 
