jNovember J 7, 1892] 



NATURE 



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A midge and some arachnids very frequently use the 

 front pair of walking legs as antenna. The midge 

 which I observed probably belonged to the Cheironomids ; 

 it often, when at rest, stood on the two posterior pairs of 

 legs with the anterior pair aloft in the air ; when walking 

 it moved them much as a beetle moves its antennae, 

 gently tapping the ground in front of it with them, their 

 inotions being always subject to the diagonal rule ; in 

 flight the midges often hold the anterior pair of legs 

 straight out in front, while the last pair are held out in a 

 similar manner behind, and probably have the effect of 

 balancing the insect. 



The spiders photographed {Tegenaria Derhatnii and 

 Tarantula pulverulenta) also exhibited the diagonal 

 motion and sometimes the use of the anterior pair of 

 legs as antennae. When, in order to photograph them, 

 these animals were put on a piece of paper floating in a 

 shallow dish of water, so as to confine them without cast- 

 ing a shadow on the space in which they walked, they 

 used frequently to stand on the three posterior pairs of 

 legs at the edge of the paper, while they moved their 

 anterior pair of legs through the air, or touched the water 

 lightly with them. Several spiders— for instance, Theridion 

 Sisyphum — have the anterior pair of legs longer than the 

 others, and very frequently seem to use them as tactile 

 organs. Specialization in this direction is carried very 

 far in the Pedipalpi, in which group the anterior pair of 

 legs are very long, thin, and flagelliform. 



The wave of motion in one set of diagonals (i.e. the 1st 

 and 3rd of one side, and the 2nd and 4th of the other) in 

 the Tarantula sometimes travelled from before back- 

 wards and sometimes in the opposite direction ; while in 

 Tegenaria it passed on the whole forwards, but some- 

 times commenced by the raising of one of the middle 

 legs, or by the raising of the two extreme legs of a set. 



When confined on the floating island of paper, the 

 Tarantula sometimes, after a good deal of hesitation, 

 took to the water. When on the surface of the water, its 

 legs, and sometimes the under surface of its abdomen, 

 made conical capillary depressions in the surface, so that 

 the water acted as a diffusing lens to the sunlight, and 

 a dark circular shadow surrounded with a bright line 

 appeared on the bottom of the dish corresponding to the 

 depression at the tip of each leg. This suggested a 

 method of determining the weight supported by each 

 leg, for the diameter of the depressions, and conse- 

 quently that of the shadows, bears some ratio to the 

 weight on the point which causes the depressions. By 

 fixing the leg of a spider on the end of a straw, hung 

 delicately as a balance-beam, and by measuring the 

 diameters of the shadows caused by the depressions in 

 the surface of the water formed by this leg for various 

 positions of a rider on the straw, I find that these 

 diameters are approximately proportional to the weight 

 on the point causing the depressions. Thus, by dividing 

 the total weight of the spider proportionally to the 

 diameters of the shadows, we get approximately the 

 weight on each leg. 



Fig. I is from a photograph of the Tarantula standing 



m 



on water ; above the spider in the picture is its shadow 

 on the bottom of the vessel, and at the ends of the three 

 posterior pairs of legs in the shadow appear circular 

 shadows corresponding to the depressions made by the 

 legs ; and there is also a shadow thrown by the depres- 

 sion caused by the abdomen. The weight of this spider 

 was 30 mgrs. Thus we find that approximately the 

 NO. 1203, VOL. 47I 



weights on the legs are the following :— On the right side, 

 2nd supports 1-875 mgrs. ; 3rd, 7-125 mgrs. ; 4th, 3-375. 

 Cm the left side, 2nd, 4 875 mgrs. ; 3rd, 5-250 ; 4th, 3000 ; 

 and the abdomen supports 4500 mgrs. When walkings 

 the Tarantula usually supported all its weight on a tripod 

 formed by the 2nd and 4th legs on one side, together with 

 the 3rd leg on the other side. The weights on the tips of 

 the legs when one photograph was taken were found to 

 be :— On the 2nd right leg, 950 mgrs. ; on the 4th right, 

 1025 ; and on the 3rd left, 10 25. 



Profile photographs also seem to show that the ist pair 

 of legs are not generally used to support much weight. 

 Fig. 2 is a diagram of the positions of the ist pair of legs 



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drawn from a number of profile photographs of Tegenaria, 

 The first position, a, is that of the leg which has been 

 thrown forward, and is just about to come to the ground ; 

 d shows the position of the ist leg when the body has come 

 forward, owing probably to the traction of this leg as well 

 as to. the pushing of some of the other legs, and so the leg 

 is bent ; b and c are intermediate positions. The next 



/'/V 



Fig. 3. 



figure is a somewhat similar diagram of the 4th pair of 

 legs made from profile photographs. At a the leg has 

 just been moved forward, and is on the ground, and is in 

 a good position both for bearing the weight of the bo ly 

 and shoving it forward. At e it is stretched to its full 

 length, and so is not of any use in driving the spider for- 

 wards, while, owing to its almost horizontal position, it is 

 almost useless in supporting the weight of the body. 

 Accordingly the spider has commenced to raise the 

 extremity of the leg prior to lifting the leg completely off 

 the ground. 



Last autumn I had the opportunity of observing two 

 scorpions which Mr. R. J. Moss brought from North 

 Africa and exhibited at the Royal Dublin Society. These 

 also appear to proceed according to the diagonal rule ; 

 but I do not know what is the order of succession in one 

 set of diagonals, as I have not yet photographed any of 

 these animals. 



The hermit crab uses three pairs of legs in walking — 

 the chelcC, and two pairs of thoracic walking legs ; these 

 it uses according to the diagonal rule, whether it walks 

 sideways or forwards. Sometimes it simply shoves the 

 chelae along the ground without lifting them, while it 

 moves the two pairs of legs in a diagonal manner. One 

 of the Asellidae I found often used the opposite legs in 

 pairs simultaneously when walking. 



The centipede does not either raise its opposite legs in 

 pairs together, nor does it move its legs according to the 

 diagonal rule. In a number of photographs taken with 

 an exposure of about the -\yth of a second the legs appear to 

 move in threes diagonally, for instance, the 3rd, 4th, and 

 5th, and the 9th, loth, and nth on one side move simul- 

 taneously with the 6th, 7th, and 8th of the other side, 

 while on the first side mentioned the 6th, 7th, and 8th, 

 with the 12th, I3ih, and 14th are on the ground, and on 

 the other side the 3rd, 4th, and 5th, and 9th, loth, and 

 nth are also on the ground. At either end of the 

 body this order is usually more or less disturbed ; thus 

 on the right side the Hih" leg might be on the 

 ground, while on the left the 13th, 14th, and 

 15th would be also in contact with the ground; but 



