306 ''Wf ANIMAL KINGDOM 



Antenna.- 



Coinpound eye 



'Thoraci J /^^V ' 



legs f=^lr- 



AbdoTninal linits — , 



Figure 16.16. Primitive wingless insects (Apterygota), showing a silverfish (left) and 

 a springtail (right). (.After Lubbock (left) and Carpenter and Folsom (right).) 



Centipedes can run rapidly, the legs moving in waves from rear to front. 

 Coordination follows the annelid pattern with reflex pathways between 

 adjacent segments. 



Millipedes, class Diplopoda (Fig. 16.15 B), are herbivorous scaven- 

 gers, feeding primarily on decayed and living plant material. The first 

 maxillae appear in the embryo but later disappear. The labium is 

 well developed, and its segment is fused ventrally with the first body 

 segment. The next three body segments remain single, but beginning 

 with the fifth and sixth every two segments fuse together during de- 

 velopment. Since each embryonic segment has a pair of legs, most of 

 the apparent segments of the adult body bear two pairs of legs, giving 

 the order its name. Millipedes may have from 13 to nearly 200 pairs of 

 legs, manipulated like those of the centipedes. The legs are short, and 

 millipedes cannot move fast. 



The superclass Hexapoda includes only the class Insecta, although 

 there is a growing tendency to separate the primitive wingless insects 

 such as the silverfish and springtails (Fig. 16.16) from the winged 

 groups. These wingless forms have small appendages on the abdominal 

 segments, suggesting a relationship with the myriapods. Silverfish do, 

 however, have compound eyes like the winged insects. 



The insects proper are the winged forms, including all hexapods 

 lacking abdominal appendages except those at the posterior end used 

 in reproduction. Typically they have two pairs of membranous wings, 

 on the second and third thoracic segments, ft is beyond the scope of 

 this book to represent adequately an invertebrate class that is divided 

 into 25 orders. The cockroach will be presented as a generalized insect 



