ORIGIN OF THE ERUCIFORM TYPE. 179 



the young Caddis fly. In like manner the caterpillar-form is 

 probably the result of the leaf-eating life of a primitive Lepti- 

 form larva. In like manner the soft-bodied maggot of the 

 weevil is evidently the result of its living habitually in cavities 

 in nuts and fruits. Did the soft, baggy female Stylops live 

 exposed, like its allies in other families, to an out-of-doors life, 

 its skin would inevitably become hard and chitinous. In these 

 and multitudes of other cases the adaptation of the form of the 

 insect to its mode of life is one of cause and effect, and not a 

 bit less wonderful after we know what induced the change of 

 form. 



Having endeavored to show that the caterpillar is a later 

 production than the young, wingless cockroach, with which 

 geological facts harmonize, we have next to account for the 

 origin of a metamorphosis in insects. Here it is necessary to 

 disabuse the reader s mind of the prevalent belief that the 

 terms larva, pupa and imago are fixed and absolute. If we 

 examine at a certain season the nest of a humble bee, w r e shall 

 find the occupants in every stage of growth from the egg to the 

 pupa, and even to the perfectly formed bee ready to break out of 

 its larval cell. So slight are the differences between the differ 

 ent stages that it is difficult to say where the larval stage ends 

 and the pupa begins, so also where the pupal state ends and the 

 imago begins. The following figures (205-208) will show four 

 of the most characteristic stages of growth, but it should be 

 remembered that there are intermediate stages between. Now 

 we have noticed similar stages in the growth of a moth, though 

 a portion of them are concealed beneath the hard, dense chrys 

 alis skin. The external differences between the larval and pupal 

 states are fixed for a large part of the year in most butterflies 

 and moths, though even in this respect there is every possible 

 variation, some moths or butterflies passing through their trans 

 formations in a few weeks, others requiring several months, 

 while still others take a year, the majority of the moths living 

 under ground in the pupa state for eight or nine months. The 

 stages of metamorphosis in the Diptera are no more suddenly 

 acquired than in the bee or butterfly. In all these insects the 

 rudiments of the wings, legs, and even of the ovipositor of the 

 adult exist in the young larva. We have found somewhat simi 

 lar intermediate stages in the metamorphoses of the beetles. 

 The insects we have mentioned are those with a &quot;complete 



