GREAT STEPS IN ORGANIC EVOLUTION 877 



insects of to-day, and the generally accepted answer is that the 

 simplest forms are to be found in the order Orthoptera, the cock- 

 roaches and earwigs, the locusts and grasshoppers. Thus their wings 

 have the simplest patterns, their mouth-parts are the most general- 

 ised, and their life-history is not complicated by any true meta- 

 morphosis. All zoologists agree that those insects (exopterj^gotes) in 

 which external wing-buds appear early in the life-history are 

 antecedent to those (endopterygotes) in which hidden wing-buds 

 appear late and lie apparently within the body. The members of 

 the cockroach-locust order are the most primitive living exoptery- 

 gotes, and represent a type from which it is possible to derive in a 

 tentative way all the other extant orders of winged insects. 



Our second question is: What does the rock-record say ? And the 

 answer is that the oldest known fossil insects belong to the Upper 



Fig. 151. 



One of the Extinct Primitive Insects or Palaeodictyoptera. Note a pair of 

 plate-like processes in front of the anterior wings. After Tillyard. 



Carboniferous, and that the most primitive of them form an order 

 with a very long name — the Palaeodictyoptera. The name may be 

 tedious, but the insects themselves are fascinating in their primitive- 

 ness. Thus the rings or segments of the body are all very like one 

 another; the wings stand straight out and do not seem to have been 

 foldable. In front of the wings there are two strange laterally 

 projecting lobes, sometimes with a few veins, extraordinarily like 

 wings in the making. Similarly on each side of the posterior body 

 there is a row of winglike platelets, recalling those seen on the 

 aquatic larvse of Mayflies. The body always ends in a pair of slender 

 processes, often like the cigar-shaped structures projecting at the 

 tail end of a cockroach. Remarkable in many cases is the large size, 

 for some had a wing-span of 20 inches, which almost justifies the 

 long title Palaeodictyoptera ! 



Now, it is from an extinct order like this that the extant orders 

 of flying insects can be reasonably derived ; and in the Carboniferous 



