JOURXA.L 



JOftn igork 6lntoraoIogirflI %mM^. 



Vol. XXIX. June, 1921. No. 2. 



THE PHYLOGENETIC ORIGIN OF THE MANDIBLES 

 OF INSECTS AND THEIR ARTHROPODAN RELA- 

 TIVES—A CONTRIBUTION TO THE ST-yTJ^an inso, 

 OF THE EVOLUTION OF TIpT, 

 ARTHROPODA. 



By G. C. Cramptox, Ph.D.. 



Mass.xchcsetts Agricultural College 

 Amherst, Mass. 



The study of the mandil)les alone has furnished but a small portion 

 of the evidence upon which the conclusions concerning the evolu- 

 tion of insects and their arthropodan relatives, set forth in the 

 following series of papers, are based. It is obviously imiiracticahle. 

 however, to attempt to present all of the evidence available on the 

 subject, in a single paper, since it would require too great a number 

 of plates to illustrate the various features of comparative anatomy, 

 embryology, etc.. which must be considered in such a discussion. On 

 this account, it has seemed preferable to present the evidence gained 

 from a study of the remaining features of the comparative morphol- 

 ogy of the mandibles (the origin of only one type in insects has been 

 discussed in the present paper), as well as that gained from the 

 study of the head capsule with its appendages, the trunk segments 

 and their appendages, the terminal abdominal structures, the embryo- 

 logical develo|)ment of the arthropods in question, and all other 

 features having a bearing on the study of the i)hylogeny of the 

 Arthropoda. in a series of papers dealing with each phase of the 



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