2 SMITHSOMAX MISCKLLANEOLS CULLECTlUXS VOL. 8l 



INTRODUCTION 



Tt is regrettable that we must arrive at an understanding of things 

 by way of the human mind. Lacking organs of visual retrospection, 

 for example, we can only hold opinions or build theories as to the 

 course of events that have preceded us upon the earth. Knowledge 

 advances by what biologists call the method of trial and error, but the 

 mind can not rest without conclusions. Most conclusions, therefore, 

 are premature and consequently either wrong or partly wrong, and, 

 once in every generation, or sometimes twice, reason back tracks and 

 takes a new start at a different angle, which eventually leads to a new 

 error. By a zigzag course, however, progress is slowly achieved. Error, 

 then, is a byproduct of mental growth. It is not a misdemeanor in 

 scientific research unless the erring one clings to his position when he 

 should see its weakness. It is better to write beneath our most positive 

 contentions that we reserve the right to change of opinion without 

 notice. The reader, therefore, should not take it amiss if he finds 

 certain conclusions drawn in this paper that do not fit with former 

 statements by the writer, for no apology will be ofifered. 



I. EVOLUTION OF THE ARTHROPOD HEAD 



The head, as a dififerentiated region of an animal, is a more ancient 

 structure than is any other specialized part of the body, and a proper 

 understanding of the head structure involves an examination of the 

 evidence of cephalic evolution from the very earliest period when 

 evidence of head development can be found. Most of the Arthropoda 

 have well developed heads, and that the arthropod head is a specialized 

 body region, just as is the thorax or the abdomen in forms where these 

 body regions are difi:'erentiated, is shown by the fact that in the embryo 

 it consists of a series of body segments. In most cases, and particularly 

 in insects, however, the head differs from the other body regions in 

 that its component segments become so thoroughly consolidated in 

 the adult as to leave little evidence of the primitive elements in the 

 head structure. Even in the ontogenetic record the true history of the 

 head development is so oljscure in many respects, and so much deleted 

 in the early passages, that, though all the facts of embryology were 

 known, it is probable that the assembled information would still give 

 but an incomplete account of the phylogenetic evolution of the head. 

 It is only by a comparative study of the head structure and its develop- 

 ment in the various arthropod groups, and by an effort to correlate 

 the known facts of arthropod organization with what is known in 

 other animals successively lower in the scale of evolution, that we 



