January, 1908 



AMERICAN HOMES AND GARDENS 



29 



margin until every bristle coincides approx- 

 imately with a line drawn from its base to 

 the midrib, each bristle thus being nearly 

 or quite parallel with its fellow on the op- 

 posite margin of the trap. Moreover, the 

 amount of the outcurving is usually the 

 same throughout the length of each margin, 

 thus making these two rows of outward- 

 pointing bristles parallel. 



The second result of the outcurving of 

 the margins is evidently of more import to 

 the plant. As the line of contact of the 

 two halves recedes toward the midrib, the 

 double-convex portion of the trap becomes much reduced in 

 size, and the imprisoned insect is closely invested, wings, legs, 

 or other members being crowded, together with the body, 

 into small space. The trap is now held 

 shut with considerable force, so all chance 

 of escape of the prisoner is gone. As the 

 digestive juice is now poured about and 

 upon the insect in this cavity, the dimin- 

 ished size of the latter — requiring less di- 

 gestive juice to saturate and cover the prey 

 — is probably of importance to the plant 

 from an economical point of view. 



This position of the trap — margins uni- 

 formly recurved, causing each half to de- 

 scribe a compound curve from margin to 

 midrib, line of contact having receded 

 about one-third of distance toward mid- 

 rib, and bristles in two outward-pointing 

 parallel rows — is maintained during the 

 several days necessary to digest the prey. 

 We will term this the second position. 



The pressure exerted by the closed traps is surprising, 

 when we consider their small size. The two traps used in 

 the following experiments were each about 22 mm. — less 

 than an inch — in length. By means of gummed paper a loop 

 of thread was fastened to each 

 outer surface of a spring trap, 

 which was made to lie in a horizon- 

 tal position, midrib and margins 

 being in a horizontal plane. Se- 

 curely anchoring the thread from 

 the lower half of the trap to a firm 

 support directly below, I now at- 

 tached the thread from the upper 

 part of trap to one pan of a pair 

 of delicate balances suspended 

 above. A small weight (7 1-2 

 decigrams) was now placed in the 

 opposite pan of the scales to take 

 up the bit of slack in the thread, 

 and to correct the final result for 

 weight of the thread and of the 

 upper half of trap, together with 

 the bit of gummed paper. 



Any additional weight now 

 placed in the scales would tend to 

 pull the trap open; moreover, the 

 weight required to thus open the 

 trap would be the measure of the 

 force exerted by the trap in pre- 

 venting the escape of captured 

 prey. 



The first trap to be tested had 

 been closed by a touch, so it con- 

 tained no prey. It had been closed 

 about forty-five minutes, and was 

 in the first position. (The second 



^ This Trap Caught and Digested 

 a Daddy-Long Legs 



4| The Trap (A) a few hours after Catching 

 a Fly. The Trap in the Second Position 

 The Trap shown at B has been Closed by 

 a Touch to an Imperfect First Position 



<jj Manner of ascertaining pressure exerted by Trap at A, while 

 digesting a fly. Weight required to pull trap open as shown 

 was about 80 grams (the weight of three silver dollars). Trap 

 at B is open after several days' work in digesting a fly. 



position is assumed only when nitrogenous 

 substance, such as animal tissue, is enclosed 

 in trap.) Small weights were carefully 

 placed in the scales, small tacks being used 

 finally, one by one, until the edges of the 

 trap were pulled open 3 mm. — one-eighth 

 of an inch. The weight required was 39 

 grams, 8 decigrams, the weight, approxi- 

 mately, of a silver dollar, and a fifty-cent 

 piece. 



About three hours after this experiment 

 the weights were again placed in the scales, 

 and the force of the trap measured, as be- 

 fore. But the weight required to open the same trap this 

 second time was only 25 grams, 1 1-2 decigrams. Evidently 

 the trap was not exerting as much force as it did at finish. 

 This result agrees with the well-known fact 

 that the trap opens in a few hours if, in 

 closing, no prey be captured; the trap was 

 relaxing its pressure, preparatory to open- 

 ing. 



The following day, this same trap was 

 closed by contact with animal tissue, and 

 had assumed an imperfect second position, 

 when I again placed weights in the scales to 

 ascertain the amount of pressure exerted. 

 Results showed a surprising increase in the 

 pressure, a weight of 58 grams 61-2 deci- 

 grams being required to o'pen edges of 

 trap 11 mm., less than 1-2 inch. 



From this result it is evident that the 

 trap exerts more pressure when closed to 

 the second position than it does when in the 

 first. The second position is assumed only 

 when the trap has closed on animal tissue, or some nitrogen- 

 bearing material. Thus it exerts most pressure when closed 

 on animal tissue, and while digesting its prey. 



While this trap was being held open by the weight of 

 58 grams, 6 1-2 decigrams, I 

 touched one of the sensitive 

 hairs three or four times in quick 

 succession. In response the sides 

 of the trap drew together slightly, 

 (1 1-2 mm.) leaving the margins 

 separated by 9 1-2 mm. 



Another trap of about the same 

 size was tested in a simi- 

 lar manner. This trap, however, 

 was closed on a fly, which I held 

 by the leg, allowing him to clam- 

 ber over surface of the trap. He 

 touched a sensitive hair, and was 

 promptly captured. In about 

 forty-five minutes, the trap had 

 closed to the second position, and 

 I then measured its pressure. The 

 weight required to open the trap 

 only 3 mm. (one-eighth of an 

 inch) at middle of the margins 

 was 79 grams, 8 decigrams. This 

 is a trifle more than the combined 

 weight of three silver dollars. 



By a slight increase in the 

 weight used, this trap was pulled 

 open, several hours later, a dis- 

 tance of 6 1-2 mm. (one-fourth of 

 an inch). When this same trap 

 was in the natural position before 

 closure, the margins were separ- 

 ated by a distance of 14 mm. 



