Origin and Age of Insects 375 



be a very divergent shoot ; more probably they had 

 a separate origin. The air-breathing ancestors of 

 Centipedes and Insects agreed with the Crustacean 

 and differed from the Arachnid stock in having the 

 fifth pair of appendages, like the fourth, used as 

 maxilla ; the first five segments forming a well- 

 marked head. The Centipedes diverged from these 

 ancestors by a multiplication of segments, retaining 

 the paired legs on each. The Insects, like the 

 Arachnids, underwent a reduction in the size of the 

 limbs on the segments behind the eighth ; those on 

 the seventeenth and nineteenth remained as stylets 

 and cercopods, and a marked differentiation between 

 the fore-body with its six legs and the hind-body 

 resulted. The fusion (probably in an early ancestral 

 form) of the eleventh with the tenth abdominal 

 segment began that compression of the hind-body 

 which is so marked a feature in the higher insect 

 orders. The acquisition of wings (whose origin 

 can only be guessed at) made Insects the leading 

 class of land-invertebrates, and ultimately brought 

 about the wonderful transformation of the higher 

 insects, as we have already seen (pp. 356-8). The 

 accompanying table (fig. 184) in which the probable 

 history of Insects and the other arthropod Classes is 

 shown in graphic form, may serve to summarise the 

 conclusions of this chapter. 



Age of Insects. — The most striking thought sug- 

 gested by the history of Insects is their vast geological 

 age. In the Secondary Epoch, when the mammals, 

 now the dominant class of land-vertebrates, were 

 small and struggling, and orders of huge reptiles 

 now quite extinct, lorded it on the continents, 

 insects of the highest orders living to-day — Lepid- 

 optera, Diptera and Hymenoptera — were already flying 

 through the air. We must go back into the Primary 



