MOTION. 



443 



the coxae to the trunk is by cotyloid joints, 

 as in the Rhimophorae, or by ginglymoid 



joints, as in the Lamellicornes ; and between 

 the trochanter and femur, the coxa and tro- 

 chanter, the femur and tibia, the joints are 

 usually ginglymoid : the axis of each of these 

 joints is turned at right angles to the next, 

 so that, as Dr. Iloget remarks,* " there results 

 from the combination of both, a capability in 

 the thigh of executing a circular motion, in a 

 manner almost as perfect as if it had revolved 

 in a spherical socket. The principle of this 

 compound motion is the same as that employed 

 on ship-board for the mariner's compass and 

 other instruments which require to be kept 

 steady during the motion of the ship. For this 

 purpose, what are called gimbals are used, the 

 parts of which have two axes of rotation at 

 right angles to each other, so as to enable the 

 compass to take its proper horizontal position, 

 whatever may be the inclination of the ship." 

 The remaining joints of the legs of insects are 

 also ginglymoid. The tarsi, which vary in 

 number from two to six, terminate by a double 

 hook ; those on the anterior pair of legs are 

 directed backwards; those on the middle pair 

 inwards ; and those on the tail-piece, forwards ; 

 by which disposition the insect is enabled to 

 lay hold of rough surfaces, and to walk up in- 

 clined or vertical planes with security. 



In the progression of insects, the fore and 

 middle legs are extended, and the hind legs 

 flexed previously to urging the body forwards ; 

 in doing which, the actions of these legs are 

 reversed. The simple hook terminating the 

 locomotive organs of most insects will not en- 

 able them to walk on water, to climb vertically 

 on glass, or stand inverted on ceilings, actions 

 which many can perform, and for this purpose 

 an additional apparatus is therefore provided. 

 The common Gnat and some Coleopterse 

 which walk on the surface of water, have 

 the tarsi furnished with a brush of fine hairs, 

 which appear, when the surfaces are free from 

 moisture, to repel the fluid with sufficient 

 force to sustain the weight of the animal, and 

 in confirmation of this theory, it is found that 

 if the legs are moistened with spirit of wine, 

 the animal immediately sinks and is drowned. 

 Those insects which ascend vertically on the 

 surface of glass, or remain suspended in an 

 inverted position from the ceiling, are furnished 

 with an additional apparatus. We have a 

 familiar example in the House-fly, which has 

 the extremities of its feet furnished with two 

 funnel-shaped membranous stickers, moveable 

 by muscles in every direction, by which they 

 are capable of exhausting the air on very 

 smooth surfaces, thus causing the pressure of 

 the atmosphere to sustain the weight of the 

 body : the area of these suckers is so beautifully 

 adjusted to the weight of the insect, that the 

 pressure of the air alone is more than sufficient 

 to sustain the weight of the insect without ex- 

 ertion, and to suspend its body to a ceiling in 

 an inverted position. The centre of gravity is 

 is thus suspended, instead of being supported, 



* Uridgcwatcr Treatise, i. 294. 



the legs having merely to resist the force of 

 gravity upon the body. In the Bluebottle- fly 

 ( jMusca Vomitoriu) these suckers are conspi- 

 cuous, and the edges being serrated enable 

 them to apply the disc of this pneumatic 

 apparatus to any kind of surface. In the 



Fig. 236. 



Fig. 237. 



Bibio febrilis (fg. 236), the foot is furnished 

 with three suckers, in the Musca domestica 

 with two (Jig. 237), and in the Cymbex 

 Lutea with five. Numerous other species, 

 amongst which is the common Wasp, are fur- 

 nished with cushions and analogous suckers, 

 which enable them to ascend vertically on 

 glass. 



The predaceous insects run with great velo- 

 city in proportion to their height. Those which 

 are furnished with very short legs must ad- 

 vance them at intervals of time corresponding 

 to the square roots of their length, on the 

 supposition that their legs are subject to the 

 same physical laws as those of the human 

 race. Mr. Delisle observed a minute fly run 

 three inches in half a second, making 540 



steps in the same time ; each of these steps 



3 

 must have been consequently -- = 0.0056 



of an inch in length. The great number of 

 steps taken by these minute animals conveys 

 to the mind of the observer an impression 

 that the animal is running, whereas it is merely 

 walking, the body not swinging freely in the 

 air, as is necessary, according to the definition 

 of Weber, to constitute the act of running. 



Myriapoda. In the Myriapods, the great 

 number of legs and the celerity of their move- 

 ments, as for example, in the Scolopendra, 

 render it difficult to detect the order of their 

 motions. The numerous segments entering into 

 the lengthened form of the trunk, each of 

 which is furnished with a pair of legs, give to 

 the body great flexibility, and enable the 

 Myriapods to turn from a right line to any 

 curved or angular path, or to pass over rough 

 surfaces with facility. The legs, in number from 

 fourteen to forty-two, are short, and directed 

 laterally ; they are composed of four segments ; 

 all the joints, except that by which they are 

 attached to the trunk, arc ginglymoid, and 

 terminate in a sharp conical claw, which gives 

 precision and security in climbing. The legs 

 appear to move in a determinate order; every 



