CHEMICAL SENSITIVITY OF TARSI OF PYRAMEIS 59 



remaining at or near the place where they were released upon 

 completion of a trial. This inactivity was doubtless due in a 

 large measure to the effect of the spring clothes-pin holder on 

 the wings in repeated trials, to continuous confinement, and to 

 the constancy of the environmental conditions, viz., temperature, 

 humidity, and light. 



As in my previous work, the results of the present experiments 

 have been obtained entirely through a study of the conditions 

 which effect an extension of the proboscis. I have described this 

 response in considerable detail (Minnich, '21, p. 178), so that it 

 will be sufficient merely to mention the essential features here. 

 In the unstimulated animal, the proboscis remains compactly 

 coiled against the head, but upon appropriate stimulation it is 

 extended and begins to probe the substrate. Not infrequently, 

 however, a given stimulus fails to elicit a complete extension, 

 producing only a partial extension followed by a subsequent 

 recoil. Indeed, the partial extension may be so slight that the 

 compact coil of the proboscis merely exhibits a jerk or two with 

 no further sign of activity. Between such slight reactions and 

 complete extension, all gradations may be observed. 



It is clear that all extensions of the proboscis, whatever their 

 degree, represent responses. But it is equally clear that these 

 responses differ in intensity. To measure such differences is 

 difficult. Nevertheless, I have endeavored to approximate a 

 measurement by weighting all responses in which the proboscis 

 was uncoiled less than one-half at 0.5, and all in which it was 

 uncoiled one-half or over at 1. In figure 1, no. 1, the proboscis 

 is shown as it normally appears in the unstimulated animal. In 

 the same figure, nos. 2 and 3 illustrate responses which would be 

 weighted at 0.5, while nos. 4 and 5 show responses which would 

 be weighted at 1. As a matter of fact, the case illustrated in no. 

 4 is virtuallj^ never encountered, for the proboscis is rarely 

 extended one-half or more of its length without being completely 

 extended. This scheme for measuring the intensity of the pro- 

 boscis response, therefore, virtually amounts to weighting small 

 partial extensions at 0.5 and complete extensions at 1. I shall 

 employ this scheme in all comparative statements, in order that 



