MORPHOLOGY AND EVOLUTION OF THE INSECT HEAD 

 AND ITS APPENDAGES 



By R. E. SNODGRASS 

 Bureau of Entomology 



CONTEXTS PACK 



Introduction 2 



I. Evolution of tlie arthropod head 2 



Cephalization 3 



Development of the body iit segmented animals 12 



The protocephalon 19 



The definitive arthropod head 27 



II. General structure of the insect head 33 



The head capsule 34 



The labrum and epipharynx 41 



The stomodeum 42 



The hypopharynx 45 



The tentorium 50 



III. The head appendages 56 



The antennae 57 



The postantennal appendages 59 



The gnathal appendages 60 



The mandibles 62 



The first maxillae 74 



The second maxillae 77 



Morphology of the gnathal appendages 79 



IV. Summary of important points 9° 



V. The head of a grasshopper 94 



Structure of the cranium 95 



The antennae 99 



The mandibles ^0° 



The maxillae 102 



The labium 106 



The preoral cavity and the hypopharynx 107 



The stomodeum 112 



The mechanism for moving the head 118 



VI. Special modifications in the structure of the head 120 



Modifications in the fronto-clypeal region 120 



Modifications in the posterior ventral region of the head 125 



VII. The head of a caterpillar 131 



Structure of the head capsule • ^3~ 



The antennae ^37 



The mandibles ^38 



The maxillae and lal)ium I39 



The stomodeum ^45 



The musculature of back of head, and nature of insect neck 150 



Abbreviations used on the figures 153 



References ^55 



Smithsonian Miscellaneous Collections, Vol. 81, No. 3 



