442 



MOTION. 



perfect insect, which support the three 

 first rings of the trunk. In many larvae 

 there are no organs of locomotion, whilst 

 others are furnished with a variable num- 

 berof rudi mental legs, presenting differently 

 constituted organs for progression. Most 

 of the larvae of the Lepidoptera have ten 

 pair of these pro-legs, respectively arti- 

 culated to the sixth, seventh, eighth, ninth, a 

 and anal segments of the body. One 

 family, the Lophyrus, has sixteen pro-legs. 

 Others, as the Stylotoma, have fourteen, 

 and the Tenthredo twelve. The perfect legs 

 move (according to Kirby and Spence) in the 

 same order as in the imago state ; the pro- 

 legs serve not only to raise and support the 

 abdominal and caudal segments of the trunk, 

 but also to assist in grasping objects in the 

 plane of motion, and in urging the centre of 

 gravity forwards. 



When the head and thoracic segments are 

 fixed, the body and tail are drawn forwards ; 

 the trunk is arched in the vertical plane ; the 

 tail being fixed, the pro-legs are advanced 

 successively in pairs, beginning from the anal 

 segment; the body is then extended, and 

 the head advanced to take a new position ; a 

 conspicuous undulation of the body is produced, 

 proceeding from the caudal to the cephalic seg- 

 ments. The larva of the Ant-lion (Myrmeleon) 

 moves in a backward direction, even after the 

 removal of its legs. Many larvae, such as the 

 Caterpillar of the Hawk-moth, move with ex- 

 treme slowness, whilst others possess consi- 

 derable powers of locomotion, as the Apotela 

 Leporina, which has received its appellation 

 from the rapidity of its movements. But of all 

 terrestrial larvae, the most remarkable for their 

 attitudes and motions are the Geometrae. The 

 true Geometrae have only two anal, and two in- 

 termediate pro-legs ; with these they grasp any 

 object so as to fix the anal extremity : the trunk, 

 with the head, is then extended, elevated, and 

 inclined from the horizontal towards the vertical 

 position, and the animal appears to be in the 

 act of surveying surrounding objects as repre- 

 sented in Jig. 233. In progression, the head being 



Fig. 233. 



Fig. 234. 



fixed on the surface of motion at c (fig. 234) ; 

 the anal extremity is drawn forwards to the 

 thoracic segments, from a to b ; the trunk is 

 then again extended to d, and a series of the 

 same alternate flexions and extensions is em- 

 ployed to carry the larva onwards. During 

 progression, the Geometrae spin a silken cord, 

 which they fix by the head on the plane of 

 position at each step, thus measuring the dis- 

 tance over which they pass. The use of this 

 n-<\ is in enable them to descend from the 



d 



trees, however lofty, on which they feed, and 

 to reascend by the same means, without the 

 necessity of taking a circuitous route, and 

 encountering the inequalities of the trunk and 

 branches. In like manner the Caterpillars of 

 the Cabbage-butterfly weave a ladder of silk 

 on the plane of a glass-window, which serves 

 as a fulcrum for its legs, and thus enables the 

 animal to ascend. 



Perfect Insects. The order in which the legs 

 of the Hexapods move in walking or running has 

 been accurately explained by Professor Muller. 

 Whilst watching insects which move slowly, 

 he observed that three of their legs were always 

 moving at the same time ; these were advanced 

 and put to the ground, whilst the other three 

 propelled the body of the insect forwards. 

 The feet, which moved simultaneously, were 

 the fore and hindmost foot on one side, and 

 the middle foot of the opposite side ; then 

 the fore and hind foot on this side, and the 

 middle one of the other side, so that in two steps 

 all the six feet are set in motion.* In the first 

 movement, whilst the legs, 1, 2', 3, (Jig. 235) 



Fig. 235. 



remain on some solid to support the body, and 

 project it forwards, the other three legs, 

 1,' 2, 3', are raised and advanced ; then, whilst 

 the legs, ]', 2, 3' are, in their turn, supporting 

 the body, 1, 2', 3 are raised and advanced, and 

 so on alternately. It will be observed, that 

 the base of support in these movements is a 

 triangular plane, with the three feet placed on 

 the three angles ; the base and apex of the 

 triangle alternating at each alternate movement 

 of each set of legs ; so that in the first move- 

 ment, the apex, which is at 2, takes the oppo- 

 site side at 2' in the second step. The Hexa- 

 pods are supported by their three pairs of legs, 

 and the stability of the animal is increased by 

 the horizontal direction of the legs outward, 

 this arrangement affording a larger base for the 

 support of the centre of gravity. The first 

 pair of legs being articulated to the prothorax, 

 the second pair to the mesothorax, and the 

 third to the metathorax, also gives to the longest 

 axis an increased stability. The articulation of 



' Muller, by Dr. Baly, p. 970. 



