878 LIFE : OUTLINES OF GENERAL BIOLOGY 



Age there emerged primitive cockroaches and their relatives, primi- 

 tive dragonflies, and primitive Mayflies. As we have ahready said, 

 it was a time of luxuriant vegetation, with ferns and tree-ferns, 

 horsetails and club-mosses, and true seed-plants related to our 

 conifers and cycads and maidenhair- trees. It was an appropriate 

 environment for insects, and they seem to have made much of it. 

 Yet there were not in the Carboniferous Age any of the higher 

 insects with complete metamorphoses, such as Hymenoptera (ants, 

 bees, and wasps), Diptera (two-winged flies), Coleoptera (beetles), 

 or Lepidoptera (moths and butterflies). But our point is that from 

 the long-named primitive insects of the Upper Carboniferous all 

 the modern orders may be derived, and that without putting any 

 extreme strain on the scientific imagination. 



Our third question is: What was the pedigree of the primitive 

 winged insects ? And the answer is to be found in the minute Spring- 

 tails and Bristle-tails and their still simpler relatives. Everyone 

 understands that fleas and lice and the like are secondarily wingless 

 insects, which have degenerated from winged ancestors; but the 

 little "silver-fish" that run about in the pantry, and the spring-tails 

 that we sometimes see covering the surface of a shore pool, are 

 primitively wingless insects. They link the winged insects back to an 

 aboriginal wingless stock, the Protura, minute creatures that live 

 in damp places under stones and bark, or among moss. They are 

 exquisitely simple, and yet they are inseparable from insects. In 

 Eosentomon, which is very widely distributed, there are limbs on 

 the first three rings of the posterior body, and this takes our thoughts 

 back to certain very simple Centipede-like types. The Protura have 

 no antennae, and in Acerentomon there are no air-tubes or tracheae. 

 No doubt there may have been some secondary simplification in 

 retrogressive types, but the characters of Protura suggest that 

 insects were evolved from running creatures with posterior as well f 

 as anterior legs, and not far removed from the simplest Centipede- 

 types. These again may be linked back to the Ringed Worms or 

 Annelids. 



But this brings us to face our last question, the most difficult of 

 all: How did wings arise? We look for the answer in the "winglets" 

 of some of the oldest fossil insects, whose name we dare not repeat. 

 Insects' wings always develop as hoUow flattened pouches from the 

 upper part of the sides of the body in those rings that bear the 

 two posterior pairs of legs. The "winglets" of some Carboniferous 

 primitive insects grow out on the ring that bears the first pair of legs, 

 and the true wings are as usual. On the posterior body, however, as 

 we have said, there are numerous winglet-like paired outgrowths. 

 As there is no difficulty in assuming the appearance of side pouchings 

 of this sort, the problem narrows itself down to this : What was the 

 original use of these outgrowing flat pockets before they became big 



