108 THE LIFE-STORY OF INSECTS [CH. 



referable to existing families and often to existing 

 genera. We may conclude then, imperfect though 

 our knowledge of extinct insects is, that some of the 

 most complex of insect life-stories were being worked 

 out before the dawn of the Cainozoic era. Some in- 

 structive hints as to differences in the rate of change 

 among different insect groups may be drawn from the 

 study of parasites. For example, V. L. Kellogg (1913) 

 points out that an identical species of the Mallophaga 

 (Bird-lice) infests an Australian Cassowary and two 

 of the South American Rheas; while two species of 

 the same genus (Lipeurus) are common to the African 

 Ostrich and a third kind of South American Rhea. 

 These parasites must have been inherited unchanged 

 by the various members of these three families of 

 flightless birds from their common ancestors, that is 

 from early Cainozoic times at latest. On the other 

 hand, the various kinds of such highly specialised 

 parasites as the warble-flies of the oxen and deer, 

 must have become differentiated during those later 

 stages of the Cainozoic period which witnessed the 

 evolution of their respective mammalian hosts. 



The foregoing brief outline of our knowledge of 

 the geological succession of insects shows that the 

 exopterygote preceded, in time, the endopterygote 

 type of life-history. We have already seen that 

 those insects undergoing little change in the life- 

 cycle, and with visible, external wing-rudiments, 



