136 EVOLUTION AND GENETICS 



Terrestrial Arthropoda. The source of the Chilopoda and 

 Diplopoda (centipedes and milUpedes) is obscure, but they seem 

 to have come from trilobites. 



Handlirsch interprets the insects as descendants of a trilobite 

 whose second and third thoracic segments bore extensions of the 

 lateral lobes which later developed into wings. 



Several more obscure classes have also been worked out by 

 Handlirsch from his studies of fossils. Of these the Onychophora 

 are significant as the most primitive air-breathing or tracheate 

 Arthropoda. They are worm-like animals which show definite 

 evidences of segmentation in the paired appendages and nephridia. 

 They show evident annelid affinities. 



Handlirsch incorporates his analysis of arthropod phylogeny 

 in a diagram which is here reproduced in part, with the group 

 names translated into familiar terms where possible (Fig. 75) . 



Insects. The two insect groups indicated are looked upon 

 as subclasses. The Apterygota include such famiUar forms as the 

 fish moths or silver fishes and the spring-tails. They are primitive 

 wingless forms which apparently have an origin associated with 

 that of the more abundant Pterygota. While many of the latter 

 are wingless, most of them are winged, and all show evidences of 

 relationship with winged ancestors. It is in this subclass that 

 most well-known insects belong. 



Metamorphosis. The two subclasses differ further in that the 

 Apterygota are without metamorphosis. When they hatch from 

 the egg they resemble the adult, and the only conspicuous change 

 is growth. The Pterygota when hatched may resemble the adult 

 to some degree but important changes always take place before 

 maturity is reached. Three types of metamorphosis are recog- 

 nized in the Pterygota. The Paurometabola are not unlike the 

 adult when hatched although they lack wings. They live like the 

 adults and develop gradually into mature insects, the chief trans- 

 formation being in the development of wings. The Hemimetabola 

 are wingless naiads when hatched and are adapted for a mode of 

 life very different from that of the adults. After a period of growth 

 their last transformation brings them suddenly into the adult 

 stage which is normally winged (Fig. 76). The Holometabola 

 emerge from the egg as larvae, very different in appearance from 

 the adults (Fig. 6). They are wingless and often have differently 

 formed mouths. Some are caterpillars, some grubs, some maggots; 



