THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 263 



it may next be asked, which affords us any clue as to the original 

 stock from which the insect type, in all its fulness and variety of form, 

 has been derived? Assuming it to be a probable and consistent 

 view of the case that the mere metamorphosis of insects is a matter 

 which, as already explained, is adaptive and secondary rather than 

 original, what of the parent stock ? and what does the study of meta- 

 morphosis, closely viewed, teach us concerning that root form ? How 

 is it, we may lawfully inquire, that such characteristic features of an 

 insect as its wings and its mouth-parts were evolved ? 



It may facilitate our comprehension of these matters if we firstly 

 begin with wings and mouth, and finally direct attention to the probable 

 origin of insects as a whole. There are two main types of mouth in 

 insects one illustrated by the butterflies and moths, in which all the 

 organs are modified to serve as a suctorial apparatus for drinking up 

 the nectar of flowers; and the other, typically represented in the 

 beetles, where we find a high development of jaws adapted for masti- 

 cation and prehension. Intermediate between the suctorial and the 

 biting mouth, we find that of the bees and wasps, where jaws coexist 

 with a tongue or proboscis. It may be said, however, that there is but 

 one type of insect mouth, all the forms of this apparatus being merely 

 modifications of the one type form. Very curious, however, are some 

 of the changes which the mouth-parts undergo in the course of their 

 development. For example, a caterpillar begins life as a biting 

 insect, and is provided with powerful jaws a fact which its ravages 

 on leaves fully endorse. Ultimately, as the butterfly, its mouth is 

 wholly suctorial ; its chief organ being the long antlia, or proboscis^ 

 used for drinking up the flower juices, and in reality corresponding 

 with the second pair of jaws in a beetle. There is a clear aid to our 

 thoughts on this matter, when we discover that the varied mouths of 

 insects are thus all really built up on one type. Our difficulty, there- 

 fore, is not that of accounting for the origin of new structures, so much 

 as that of saying how one phase of an organ becomes modelled to form 

 another phase of the same type. 



An acquaintance with the broad facts of natural-history study 

 reveals modifications quite as wonderful in other groups of living 

 beings. It is even more curious to find the arm of man, the wing 

 of the bird, the fore leg of the horse, and the wing of the bat built 

 up on the same type, than to discover a change of type in one and 

 the same insect's mouth in the course of development. If we go 

 back to insects in which the mouth-parts are simple and possess jaws 

 of elementary pattern, we may as readily conceive of these jaws be- 

 coming altered to form suctorial organs, as of the same type of 

 limb being modified in one case to walk and in another to fly. 

 The alteration of ways of life and living, and changes in food, 

 would be sufficient causes for the modification, which, proceeding 



