^^j;- THE WALK OF ARTHROPODS. 677 



the tripod has its special function, the foremost to draw, the middle 

 to support, the hindmost to push the body. He also describes in 

 detail the motions of the body, horizontal, vertical, and transverse, 

 which are such as to draw its centre of gravity towards the apex of 

 the triangle of support, and so facilitate the fall on to the other 

 tripod. He concludes from this that the insect progresses by a true 

 walk — " a series of successively arrested falls." His observations seem 

 all to have been made on beetles — Meloe, Ateuchus, and Orydes, and 

 he states that while the three legs of the set are placed on the ground 

 simultaneously, the hindmost is not lifted until after the other two. 



The photographic method adopted by Dixon enables the exact 

 order of the motions to be made out. He used a camera looking 

 vertically down on the animal, which ran on a white sheet of paper 

 floating on water. Sunlight was used so that the shadows cast 

 showed the exact position of the legs in relation to the ground. 

 He found that while in the cockroach and the house-fly the hindmost 

 leg is lifted last of the three, in the blow-fly it is the first to be raised. 

 A highly interesting result is the extension of the diagonal motion to 

 the anterior appendages, as in the cockroach and earwig each 

 antenna and maxillary palp moves with the front leg of its side : it 

 will be remembered that the second pair of maxillae (labium) comes 

 between the legs and the maxillae, and the mandibles between these 

 and the antennae. It is possible that some aid to the determination 

 of homologies may come from such observations, but it seems likely 

 that this rule of moving in diagonals would only be obeyed by those 

 appendages which remain functional. 



Dixon has shown that the motion of limbs in diagonals is not 

 universal in. insects. An aphis and some small flies sometimes moved 

 the legs of the same pair together, as did also the larva of a rove- 

 beetle. That the latter mode is general among caterpillars is already 

 well-known, and is recorded by Carlet (8) ; the claspers at the hinder 

 end of the animal are moved first, and the wave of motion travels 

 forwards. Dixon finds this motion of limbs in pairs also sometimes 

 takes place in a thysanure (Tomocerus), specially when the abdominal 

 sucker is in use. The fact that the primitive thysanure has a similar 

 walk to the larva of higher insects is interesting and suggestive, but 

 the median support aff'orded by the abdominal sucker makes such a 

 mode of progression very natural, and the similarity may be 

 homoplastic rather than homologous. 



With regard to myriapods, Gaubert, in the memoir already 

 referred to in Natural Science (p. 527), states that, while centipedes 

 move their legs alternately, millipedes move those of the same pair 

 together. 



The walk of arachnids is described by Gaubert as produced by 

 the motion of alternate limbs, the' first and third legs on one side 

 moving with the second and fourth on the other. This result agrees 

 with that obtained by Carlet and Wilkins ; but they seem to have 



