46 CENTIPEDES 



In contrast, the evolution of the Scolopendromorpha appears to 

 have been associated with acquiring increased speed. Fast gaits 

 are possible as the relative duration of the backstroke of the limbs 

 is shorter than in most other Arthropoda. The body may momen- 

 tarily be supported by only tw^o or three legs on each side, but 

 excessive sagging of the body between these widely separated sup- 

 ports is countered by the body musculature. At the same time 

 alternate-sized tergites appear to contribute an anti-undulation 

 mechanism which becomes necessary as the legs increase in 

 length and the gait becomes faster. These achievements have been 

 made at the expense of the primitive flexibility of locomotory 

 movements as are found in the Onychophora and which have been 

 exploited in relation to the burrowing habit of the Geophilomorpha. 

 Thus the gaits of the Scolopendromorpha are performed with 

 greater regularity than those of the Geophilomorpha, but little 

 choice of footholds is possible. 



The legs of the Lithobiomorpha and Scutigeromorpha are 

 longer than those of the orders so far considered. The gaits in 

 these two groups are essentially similar. Only a small range is pos- 

 sible, and movements must be executed with great precision 

 because the fields of up to four successive legs may overlap. Un- 

 dulations of the body at faster speeds are partially controlled in 

 Lithohius by the alternate-sized tergites, and more completely by a 

 reduction in tergite number in Scutigera in which the legs are of 

 differential length, the fourteenth pair being almost double the 

 length of the first. With the great fieetness of the Scutigeromorpha 

 are associated several structural advances beyond the conditions 

 seen in other Chilopoda. The long legs require a firm grip on the 

 ground to prevent them from slipping, and instead of the single or 

 double digitigrade spine present in other centipedes, the leg of 

 Scutigera ends in a multi-articulate plantigrade foot, each joint pos- 

 sessing numerous gripping hairs. Fast running indicates a high 

 metabolic rate correlated with greater complexity of the maxillary 

 excretory gland, a unique respiratory system and the presence of 

 an efficient respiratory carrier in the blood. 



The mechanism of breathing and blood circulation is similar to 

 that of spiders and scorpions since in the Arachnida the blood 

 must traverse the interlamellar spaces before returning to the 



