THE PROBLEMS OF TRANSFORMATION 269 



to find that in the latter order, much more highly specialized 

 in the structure of the imago, the abdominal larval legs, lost 

 in caddis-grubs, are partly preserved in a highly modified 

 form. Also the full set of wing-tracheae are preserved in the 

 lepidopteran pupa, while in the trichopteran there are only 

 two of the original series. It is safe to infer that the pupal 

 wing of the Paramecoptera had a full set of tracheae. Lepidop- 

 tera are very poorly represented among fossil insects, as 

 might be expected from their delicate structure and terrestrial 

 habit ; it is not until rocks of Tertiary age are reached 

 notably those at Florissant, Colorado, and Oeningen, Baden 

 that fossil butterflies and moths are found in any great variety, 

 and these are all referable to existing families. 



The Diptera are, as we have seen, the most highly specialized 

 of all insects as regards structure and life-history. But the 

 wing-nervuration of the more generalized Diptera shows points 

 of correspondence with that of scorpion-flies, and an extinct 

 family the Mesopsychidae, from the Upper Trias of Queensland, 

 serves to indicate a transition from the primitive Permian 

 Mecoptera to the earliest-known true Diptera of the European 

 Lias. These are referable to the Orthorrhapha whose larvae 

 are grubs with definite head, but without typical limbs ; the 

 presence of prolegs in some of them is probably merely analo- 

 gous to the condition in the mecopterous caterpillar, though 

 the dipteran grub is clearly derivable from such a caterpillar 

 type. The earliest Cyclorrhapha known are of Tertiary age ; 

 these are characterized by the maggot as the larval stage, in 

 which degradation is carried to its extreme degree, and their 

 late appearance in geological time confirms the conclusion 

 as to their high specialization which is reached by a study 

 of their form and life-history. 



There remain for consideration two large and important 

 orders of metamorphic insects, the Coleoptera (beetles) and 

 the Hymenoptera, as to whose origin there is little or no 

 direct evidence from fossils. The beetles must be an ancient 

 order, despite the specialization of their forewings, because 

 elytra and other remains have been found in the Triassic 

 rocks of Switzerland and also of Queensland (Australia), and 

 some of these are referable to the more specialized families 

 such as the leaf-beetles (Chrysomelidae) and the weevils 



