FACTS AND THEORIES CONCERNING 

 THE INSECT HEAD 



By R. E. SNODGRASS 



Honorary Research Associate 



Smithsonian Institution 



INTRODUCTION 



We can feel fairly confident that the insect head has not changed 

 since the first entomologist looked at it and described it. Yet a review 

 of what has subsequently been written about the insect head shows 

 that our ideas about its structure and segmental composition have 

 changed very much. Morphology is an attempt to understand the 

 significance of anatomical facts in their relation to one another, and 

 to reconstruct from the known facts the evolutionary development by 

 which the animal has come to be what it is today. Consequently as 

 new facts come to light our morphology has to be revised to fit them, 

 though it sometimes seems as if some morphologists find it easier to 

 make the facts fit their theories. Ontogeny and anatomy are visible 

 facts not always correctly observed ; morphology and phylogeny are 

 mental concepts that cannot be demonstrated. Hence, descriptions of 

 facts by different observers may be inconsistent, and theories about 

 them will vary according to our individual ways of thinking. The 

 present paper, therefore, is a version of the insect head structure ac- 

 cording to the facts now presumed to be known about it, and of mor- 

 phological ideas according to the writer's personal way of interpreting 

 the facts. 



No new theory is here introduced, but critical attention will be 

 given to some current theories about the segmentation of the insect 

 head. It does not seem that we really need a theory on the subject, 

 since the embryo gives us a very good idea about how the insect head 

 has been evolved. Nevertheless, some morphologists contend that the 

 embryo may be deceptive and itself needs to be interpreted, while 

 some would even discard embryonic evidence as having no evolu- 

 tionary value. 



Elongate animals that habitually move in one direction necessarily 

 have their principal sense organs at the forward end of the body. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 142, NO. 1 



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