THE QUESTION OF THE PHYLOGENETIC ORIGIN 

 OF TERMITE CASTES. 



CAROLINE BURLING THOMPSON 1 AND THOMAS ELLIOTT SNYDER.* 



4* 



It has recently been shown by one of the authors (C. B. T.) 



that the ontogenetic origin of the castes of the termite Reticuli- 

 lermes flavipes Kol., formerly Lencotermes flavipes, is due to 

 intrinsic causes, and not to the extrinsic stimuli which have long 

 been credited with formative, indeed almost creative powers. 

 Grassi (1893-94) an d his followers described the plastic, "un- 

 differentiated," newly hatched nymphs, upon which the action 

 of the stimuli of food, parasites, fraternal care, etc., wrought out 

 the highly differentiated adult castes. In favor of Grassi's 

 hypothesis is the fact, that, in most genera, the newly hatched 

 termite nymphs, about one millimeter long, are externally all 

 alike. On the other hand, Grassi's hypothesis is disproved by 

 the recent observations that termite nymphs are already dif- 

 ferentiated at the time of hatching (Bugnion, I9i2-'i3, Thomp- 

 son, 1917). 



Bugnion states that, at the time of hatching, the soldier 

 (nasutus) caste of the termite Eutermes lacustris Bugn. is sharply 

 differentiated from the other nymphs by external structural 

 characters. He has not worked out the differentiation of the 

 other castes, which is, of course, a simple matter of observation, 

 but his general conclusion, based upon this and other investiga- 

 tions, is that the castes of Eutermes lacustris originate in the 

 embryonic period, and that the cause of differentiation is deep 

 seated, and probably analogous with the cause of sex (Bugnion, 

 1912, '13). 



Thompson (1917) finds that in Reticulitermes flavipes the 

 newly hatched nymphs are externally all alike, but are differen- 

 tiated by internal structural characters into two clearly defined 



1 Department of Zoology, Wellesley College; Collaborator, Bureau of Ento- 

 mology, U. S. Department of Agriculture. 



2 Branch of Forest Entomology, Bureau of Entomology, U. S. Department of 

 Agriculture. 



