THE CHROMATROPISM OF MERMIS 

 SUBNIGRESCENS 



A Nemic Parasite of Grasshoppers 



CONTRIBUTIONS TO A SCIENCE OF NEMATOLOQY XX l 



BY N. A. COBB 



The adult female Mermis subnigrescens, when ripe for ovijection, 

 has away of moving her head in more or less horizontal curves; her 

 head, directed skyward, is waved in "circles," now clockwise, now the 

 reverse. This seemingly purposeful behavior occurs when she emerges 

 from the soil and while she is ascending the herbage to deposit her eggs. 

 Inasmuch as the head of the egg-laying female, unlike that of the 

 young female as well as that of the male (neither of which ever quits 

 the subterranean darkness), contains reddish transparent pigment 

 rather definitely distributed with reference to certain cephalic nerves, 

 the question arose whether we do not have here a phototrope 2 and an 



1 Reprinted from the JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES, Vol. 19. 

 No. 8, April 19, 1929. 



2 It is suggested that the mechanisms through whose activation the responses of 

 organisms termed tropisms find expression be called "tropes;" "tropism" to be taken 

 in almost any of its more or less well accepted meanings. 



These meanings (interpretations of various investigators, see Mast, 1915) vary all 

 the way from (1) "an inherent tendency to respond" (Standard Dictionary), to (2) an 

 "irresistible" or "predictable" orientation as definite and mechanical as that of a magnetic 

 needle; but whatever the accepted interpretation, the reaction-mechanism must always be 

 present, and be a system of intimately connected elements or organs, as is the digestive 

 system, for instance, or the excretory system. Since we have for this system of inti- 

 mately connected elements no inclusive single descriptive term, and since it is found 

 highly convenient, or even necessary, for purposes of thought and discussion mentally 

 to "isolate," and separately to denominate, the digestive system or enteron, and other 

 systems, it is suggested that in behavior studies a like situation be met by a similar, 



Waverly Press, Inc., Baltimore, Maryland, June, 19SO 



