As is well known, several orders of insects possess mouth-parts which are suited for sucking up fluids, and these have evolved, through adaptation to a fluid diet, from the biting mouth-parts of the primitive insects which we see still surviving in several orders. Thus the Diptera may have gradually acquired the sucking proboscis which occurs in many of them by licking up decaying vegetable and animal matter, and by piercing into and sucking living animals. But even among the Diptera several families have more recently adapted themselves quite specially to a flower diet, to honey-sucking, like the hover-flies, the Syrphidæ, and the Bombyliidæ, whose long thin proboscis penetrates deep into narrow corolla-tubes, and is able to suck up the nectar from the very bottom. The transformation was not so important in this case, since the already existing sucking apparatus only required to be a little altered. Again, in the order Hemiptera (Bugs) the suctorial proboscis does not owe its origin to a diet of flowers, for no member of the group is now adapted to that mode of obtaining food. [Illustration: FIG. 47. Head of a Butterfly. _A_, seen from in front. _au_, eyes. _la_, upper lip. _md_, rudiments of the mandibles. _pm_, rudimentary maxillary palps. _mx´_, the first maxillæ modified into the suctorial proboscis. _pl_, palps of labium or second maxillæ, cut off at the root, remaining in _B_--which is a side view. _at_, antennæ. Adapted from Savigny.] The proboscis of the Lepidoptera, on the other hand, depends entirely on adaptation to honey-sucking, and we may go the length of saying that the order of Lepidoptera would not exist if there were no flowers. This large and diverse insect-group is probably descended from the ancestors of the modern caddis-flies or Phryganidæ, whose weakly developed jaws were chiefly used for licking up the sugary juices of plants. But as flowering plants evolved the licking apparatus of the primitive butterflies developed more and more into a sucking organ, and was ultimately transformed into the long, spirally coiled suctorial proboscis as we see it in the modern butterflies (Fig. 47). It has taken some pains to trace this organ back to the biting mouth-parts of the primitive insects, for nearly everything about it has degenerated and become stunted except the maxillæ (_mx´_). Even the palps (_pm_) of these have become so small and inconspicuous in most of the Lepidoptera that it is only quite recently that remains of them have been recognized in a minute protuberance among the hairs. The mandibles (_md_) have quite degenerated, and even the under lip has disappeared, and only its palps are well developed (_B_, _pl_). But the first maxillæ (_mx´_), although very strong and long, are so extraordinarily altered in shape and structure that they diverge from the maxillæ of all other insects. They have become hollow, probe-like half-tubes, which fit together exactly, and thus form a closed sucking-tube of most