SURVEY OF INVERTEBRATES 



97 



nently in the Insects. It is common knowledge that caterpillars are 

 the larval, worm-like feeding forms of Butterflies and Moths. The 

 winged adult condition is attained by a final moult that takes place 

 while the larva is in a 'resting' condition, the pupa, made necessary 

 by the radical structural changes which are involved. Perhaps it 

 is not such common knowledge that even Locusts, or Grasshoppers, 

 undergo metamorphosis, because no one moult results in such 

 marked changes. The young Locust is similar in form to the adult, 

 but after each moult it is larger than before, and finally is a fully 

 winged adult. Such a transformation not involving a pupal 

 stage is frequently referred to as incomplete metamorphosis. 

 (Figs. 257-260.) 



So we conclude for the present our necessarily limited view of 

 the Arthropods, but as we proceed with our study we shall fre- 

 quently have occasion to discuss special representatives. Judged 

 by the stupendous number of species and individuals, or by variety 



Fig. 56. — Peripatus, Peripatus capensis. An interesting Arthropod that seems 

 to link the Arthropods with the Segmented Worms. (From Sedgwick.) 



of form, or by sheer success in competition with both lower and 

 higher animals in air, water, and on the ground, they are "Nature's 

 most successful Invertebrate experiment." The segmental plan 

 begun in a small way in some of the lower worms, is definitely 

 established in the Segmented Worms, and is made the most of 

 in the Arthropods. (Fig. 56.) 



However, Arthropods are hampered by inherent structural limi- 

 tations. The hard, dead, external skeleton imparts certain me- 

 chanical restrictions on size and freedom of action that are re- 

 moved in Vertebrates by an internal living skeleton. And small 

 size precludes a constant body temperature greater than the sur- 

 roundings, so they cannot achieve the constancy of living possible 

 to the warm-blooded Birds and Mammals. Nevertheless it has 

 been suggested as not beyond the range of possibility that Insects 

 may yet dominate the Earth! 



