EVOLUTION OF THE INSECT HEAD AND THE ORGANS 



OF FEEDING 



By R. E. Snodgrass 

 Bureau of Entomology, Umted Staltes Department of Agriculture 



The organs of feeding in nearly all animals are intimately asso- 

 ciated with the head, because it is the head end of the body that goes 

 forward in the usual modes of progression and is, therefore, the first 

 to come into contact with the food. The head is at the anterior end 

 of the body because the head was probably differentiated primarily 

 as the sensory pole of the animal. The primitive head did not neces- 

 sarily contain the mouth, and in some of the worms the mouth is 

 still located near the middle of the ventral side of the body. 



The head of an insect is a composite structure formed of several 

 of the j^rimitive anterior body segments. The mouth is located on 

 the head and the principal external organs of feeding are appendages 

 of some of the segments associated with the mouth. The head seg- 

 ments are so closely united in the cranium-like head capsule that the 

 primitive segmental areas are no longer discernible. The mouth 

 appendages were undoubtedly at one time legs, but they have become 

 so altered in adaptation to the feeding function that the primitive 

 leg structure is seldom apparent in them and is often entirely oblit- 

 erated. It is very difficult, therefore, to decipher the evolution of the 

 insect head and the organs of feeding from a study of adult insects, 

 and, though the development of the embryo throws some light on 

 the subject, embryological evidence is always subject to various 

 interpretations. Furthermore, the oldest known insects of the geo- 

 logical records are so much like modern insects that paleontology 

 gives little assistance in a study of the origin of insect structures. 

 Probably no other group of animals have so effectively covered their 

 evolutionary tracks as have the insects. 



It is an easy thing for anyone to becloud his intentions, or to 

 create uncertainty as to his future course of action, but to keep his 

 past a secret is quite a different matter. It often happens that in- 

 formation on an obscure subject is more easily acquired in a round- 

 about way than by going direct to the object of investigation. For 



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