36 THE AGE OF THE EARTH 



of the Blattidae, Phasmidae, Acridiidae, and Locustidae 

 among the Orthoptera, the Perlidae among the Neuro- 

 ptera, and the Fulgoridae among the Homoptera. 

 The differences which separate these existing families 

 from their Carboniferous ancestors are most interesting 

 and instructive. Thus the Carboniferous cockroaches 

 possessed ovipositors, and probably laid their eggs one 

 at a time, while ours are either viviparous or lay their 

 eggs in a capsule. The Protophasmidae resemble living 

 species in the form of the head, antennae, legs, and 

 body ; but while our species are either wingless or, with 

 the exception of the female Phyllidae, have the anterior 

 pair reduced to tegmina, useless for flight, those of 

 Palaeozoic times possessed four well-developed wings. 

 The forms representing locusts and grasshoppers (Palae- 

 acridiidae) possessed long slender antennae like the green 

 grasshoppers (Locustidae), from which the Acridiidae 

 are now distinguished by their short antennae. The 

 divergence and specialization which is thus shown is 

 amazingly small in amount. In the vast period between 

 the Upper Carboniferous rocks and the present day the 

 cockroaches have gained a rather different wing venation, 

 and have succeeded in laying their eggs in a manner 

 rather more specialized than that of insects in general ; 

 the stick insects and leaf insects have lost or reduced 

 their wings, the grasshoppers have shortened their 

 antennae. These, however, are the insects which most 

 closely resemble the existing species ; let us turn to the 

 forms which exhibit the greatest differences. Many 

 species have retained in the adult state characters which 

 are now confined to the larval stage of existence, such as 

 the presence of tracheal gills on the sides of the abdomen. 

 In some, the two membranes of the wing were not 

 firmly fixed together, so that the blood could circulate 

 freely between them. On the other hand, they are not 

 very firmly fixed together in existing insects. Another 

 important point was the condition of the three thoracic 

 segments, which were quite distinct and separate, instead 

 of being fused as they are now in the imago stage. 

 This external difference probably also extended to the 



