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 i)ai-ts, I will go into the entire subject in such 

 detail as is necessary to prove my [)oint ; but without reprinting my first paper, which 

 should be herewith consulted. 



T 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 ci'iticism by the theory of evolution. Tf we assume the origin of all insects 

 from one original type, we must, necessaiily, assume that all the mouth structui'es 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 prefei'rcd 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 Hemiptera 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 1 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 seta" 

 are all maxillary structures. I have concluded, however, after a careful review of all 

 my preparations and of what has been written, that the Hemiptera 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 oi'ders combined in rank, for all 

 others are mandibulate or derivatives from a mandibulate type. The archetypal Thy- 

 sauuran 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 structure must be studied on an entirely new basis to 



