628 



SCIENCE. 



[N. S. Vol. XIV. No. 356. 



Despite the continued attention of ento- 

 mologists through nearly one hundred years 

 to the problem of the homologies of the 

 insect mouth parts, an agreement has not 

 yet been reached as to the interpretation 

 of these homologies in the Diptera and 

 perhaps in other holometabolic groups. 

 The extreme modification of certain parts 

 and the reduction to mere unintelligible 

 vestiges, or, indeed, to total disappearance, 

 in the imaginal condition of the more spe- 

 cialized flies and the fact that even in the 

 generalized flies the fully developed mouth- 

 parts are so modified that a comparison 

 with the typical biting or orthopterous 

 mouth is difficult and hazardous, has deter- 

 mined this long-continued uncertainty in 

 the determination of the mouth-part 

 homologies. For the problem has been 

 heretofore attacked exclusively, or nearly 

 so, by the method of the comparative study 

 of the fully- developed mouth structures. 

 It is certain that no absolute determination 

 of the homologies can be reached by this 

 method alone. I have therefore made an 

 attempt to trace the development of the 

 imaginal mouth parts in the Diptera, 

 Lepidoptera, Coleoptera and Neuroptera, 

 while a student of mine, Mr. A. B. Spaul- 

 ding, has undertaken similar work on the 

 Hymenorptera. The special difficulty of the 

 work lies of course in the remarkable his- 

 tolytic and histogenetic conditions attend- 

 ing the post-embryonic development of the 

 holometabolic insects. 



It is perfectly feasible to trace continu- 

 ously the development of mouth parts of 

 the dipterous larvae, from the first budding 

 of the appendages on the successive head 

 segments to fully developed larval condi- 

 tion. But the utter casting aside at pupa- 

 tion of these larval mouth structures and 

 their supplanting by the radically difi'erent 

 imaginal parts, which have developed from 

 imaginal histoblasts (derived from the lar- 

 val hypoderm) make it impossible to trace 



a perfect continuity from embryonic anla- 

 gen to the definitive imaginal structures. 



But we find in the advanced larva that 

 the developing imaginal structures push 

 out into and perfectly correspond with the 

 larval parts, and that an interpretation of the 

 homologies of the adult mouth parts in any 

 of the holometabolic insects can be got at on 

 the basis of this correspondence in position. 



The value of this evidence* from position 

 is made apparent when the development of 

 the imaginal mouth structures in holometa- 

 bolic insects with biting mouths, as the 

 Neuroptera and Coleoptera, are studied. 

 There is no question in the minds of ento- 

 mologists regarding the certainty of the 

 homologies of imaginal mandibles, maxillae 

 and labium of these insects with the same 

 parts of the imaginal orthopterous mouth. 

 But a study of the development of the im- 

 aginal mouth parts in the Neuroptera and 

 Coleoptera has revealed ,that perfect corre- 

 spondence in position between the develop- 

 ing imaginal parts and the larval parts, as 

 is apparent in the Diptera, Hymenoptera 

 and Lepidoptera. 



I have, therefore, been able to show that 

 the interpretation of the homologies of the 

 imaginal dipterous mouth parts formerly 

 advanced by me (see papers on Psyche, 

 1900) on the basis of a comparative study 

 of the fully-developed imaginal mouth 

 structures, in all the generalized families 

 and several of the specialized families is 

 confirmed by the evidence derived from 

 a study of the post-embryonic development 

 of the imaginal parts. And further, this 

 study shows that the ordinarily accepted 

 interpretation of the homologies of the im- 

 aginal mouth parts of the other holometa- 

 bolic orders of insects is confirmed by the 

 study of their development. 



7. ' On Taxonomic Eelations between 

 Scolytids and their Host Plants ' : A. D. 

 Hopkins, Entomologist, W. Ya. Agric. Exp. 

 Sta. 



