814 Comparative Animal Physiology 



shortening the worm. Strychnine stimulates the motor system for both sorts 

 of muscle, so that under strychnine the worm shortens and also constricts;^*^ 

 normally one group of motoneurones is inhibited while the antagonists are 

 excited. Janzen-^^ described correlative reflexes controlling the setae. The lo- 

 comotor reflexes have both homolateral and crossed nervous components. 



The rhythmic contraction waves of peristaltic locomotion in annelids re- 

 quire chain reflexes involving tactile or muscle or sucker receptors and an- 

 tagonistic central arcs. What recovery mechanisms are so slow as to deter- 

 mine the frequency of contraction under continuous stimulation is not clear, 

 but they may well be in the muscle rather than in the central nervous sys- 

 tem. 



Arthropods. Arthropod locomotion depends on central conduction and on 

 segmental reflexes which show more complex connections than in annelids. 

 In insect larvae such as Lucanus, peristalsis depends on the integrity of the 

 nerve cord.-^^'^ When a segmental nerve is cut, its segment is paralyzed but 

 conducts a wave to succeeding segments if the cord remains intact. Tonus 

 is lost in skeletal muscles of a silkworm if ganglia are removed.^^^ In some 

 lepidopteran larvae (noctuids), each segmental ganglion serves its own seg- 

 ment and the one ahead.-*''^ 



The reflex control of insect walking and flight and of breathing is very pre- 

 cise. In 1899 Pompilian noted some automatism of thoracic ganglia for leg 

 movements in Dytiscus. Flight can be initiated reflexly by removing the legs 

 from support; i.e., tarsal contact inhibits flight.^"*'' Leg muscles of scorpions, 

 crayfish, and some beetles show tonic low potential discharges at 30-40/sec. 

 during rest, and muscle potentials of higher amplitude and frequency during 

 activity.'^-''^ When one or two segments— ganglion and limbs— are isolated, 

 reciprocal alternate contractions of flexors and extensors in successive seg- 

 ments occur. Pringle'^^^ made similar observations on the cockroach and no- 

 ted crossed inhibition. A variety of operations on the nervous system of the 

 walking stick Dixifpus show that each leg by its motion has a reflex stimu- 

 lating effect on neighboring legs.*''"' 



When the normal sequence of leg movement is upset by removing legs 

 from various spiders, crabs, beetles, etc., changes in coordination occur at 

 once."*^' -^'' Legs which previously alternated now work together, pedipalps 

 may function in locomotion, swimming is carried out by other legs than it is 

 normally. Holding a leg is not equivalent to removing it. The changes occur 

 too fast for learning and must represent pre-existing pathways and an adap- 

 tive property of the ganglia. Ten Cate"*"^ obtained alterations in coordination 

 in a cricket if he removed a foot only. He concluded that the normal se- 

 quence and rhythm is set up reflexly. However, the new response does occur 

 over central connections already established. Not only can changes in leg 

 rhythm be brought about by amputation, but they also occur with 

 normal changes in gait. The mantis leg order,^^^ for example, when the in- 

 sect is quietly walking, is L (left) 3, L 2, R (right) 3, R 2; when it is excited 

 and walking fast, the rhythm is L 2 -|- R 2, L 3 + I^ 3; when climbing, 

 L3-f-Rl.L2-(-R3, LI -fR2. In some insects the order of leg move- 

 ment depends on the kind of surface; i.e., the order is determined by pro- 

 prioceptive reflexes.-"^ 



