Fig. 48. Mouth-parts of the
Cockroach (Periplaneta orientalis),
after R. Hertwig. la, upper lip
or labrum. md, mandibles. mx1,
first maxillæ, with c, cardo, st,
stipes, li, internal lobe or lacinia,
le, external lobe or galea, and pm,
the maxillary palp. mx2, the
labium or second maxillæ, with
similar detailed parts.
Before I go on to explain the origin of the sucking proboscis of the bee from the biting mouth-parts of the primitive insects I must first briefly consider the latter.
The biting mouth-parts of beetles, Neuroptera, and Orthoptera (Fig. 48), consist of three pairs of jaws, of which the first, the mandibles (md), are simply powerful pincers for seizing and tearing or chewing the food. They have no part in the development of the suctorial apparatus either in bees or in butterflies, so they may be left out of account. The two other pairs of jaws, the first and second maxillæ (mx1 and mx2), are constructed exactly on the same type, having a jointed basal portion (st) bearing two lobes, an external (le) and an internal (li), and a feeler or palp, usually with several joints, directed outwards from the lobes (pm and pl). The second pair of maxillæ (mx2) differs from the first chiefly in this, that the components of the pair meet in the median line of the body, and fuse more or less to form the so-called 'under lip' or labium. In the example given, the cockroach (Periplaneta orientalis), this fusion is only partial, the lobes having remained separate (le and li); and the same is true of the bee, but in this case the inner lobes have grown into a long worm-like process which is thrust into the nectar in the act of sucking.
Fig. 49. Head of the Bee. Au, compound
eyes. au, ocelli. at, antennæ. la, upper lip.
md, mandibles. mx1, first maxillæ, with pm,
the rudimentary maxillary palp. mx2, second
maxillæ with the internal lobes (li) fused to
form the 'tongue.' le, the external lobes of
the second maxillæ, known as 'paraglossæ.'
pl, labial palp.
Even the burrowing-wasps exhibit the beginnings of variation in this direction, for the under lip is somewhat lengthened and modified into a licking organ. The adaptation has not gone much further than this, even in one of the true flower-bees, Prosopis, which feeds its larvæ with pollen and honey, and it is only in the true honey-bee that the adaptation is complete (Fig. 49). Here the so-called 'inner lobe' of the under lip (li) has elongated into the worm-shaped process already mentioned; it is thickly covered with short bristles, and is called the 'tongue' of the bee (li). The outer lobes of the under lip have degenerated into little leaf-like organs, the so-called accessory tongue or paraglossa (le), while the palps of the under lip (pl) have elongated to correspond with the tongue, and serve as a sensitive and probably also as a smelling organ, in contrast to the palps of the first maxillæ, which have shrunk to minute stumps (pm). The whole of the under lip, which has elongated even in its basal portions, forms, with the equally long first maxillæ, the proboscis of the bee. The first maxillæ are sheath-like half-tubes, closely apposed around the tongue, and form along with it the suctorial tube, through which the nectar is sucked up. Thus, of the three pairs of jaws in insects, only the first pair, the mandibles, have remained unaltered, obviously because the bee requires a biting-organ for eating pollen, for kneading wax, and for building cells.
But bees do not only feast on nectar and pollen themselves, they carry these home as food for their larvæ. The form already mentioned, Prosopis, takes up pollen and nectar in its mouth, and afterwards disgorges the pulp as food for its larvæ, but the rest of the true bees have special and much more effective collecting-organs, either a thick covering of hair on the abdomen, or along the whole length of the posterior legs, or finally, a highly developed collecting apparatus, such as that possessed by the honey-bee—the basket and brush on the hind leg. The former is a hollow on the outer surface of the tibia, the latter a considerable enlargement of the basal tarsal joint, which, at the same time, is covered on the inner surface with short bristles, arranged in transverse rows like a brush. The bee kneads the pollen into the basket, and one can often see bees flying back to the hive with a thick yellow ball of pollen on the hind leg. In those bees which collect on the abdomen, like Osmia and Megachile, the pollen mass forms a thick clump on the belly, and in the case of Andrena Sprengel observed long ago that it sometimes flew with a packet of pollen bigger than its own body on the hind leg.
All these are contrivances which have gradually originated through the habit of carrying home pollen for the helpless larvæ shut up in the cells. They have developed differently in the various groups of bees, probably because the primary variations with which the process of selection began were different in the various ancestral forms.
In the ancestors of those which carry pollen on the abdomen there was probably a thick covering of hair on the ventral surface of the body, which served as a starting-point for the selection, and, in consequence, the further course of the adaptation would be concerned solely with this hair-covered surface, while variations in other less hairy spots would remain un-utilized.
After all this it will no longer seem a paradoxical statement that the existence of gaily coloured, diversely formed, and fragrant flowers is due to the visits of insects, and that, on the other hand, many insects have undergone essential transformations in their mouth-parts and otherwise as an adaptation to a flower diet, and that an entire order of insects with thousands of species—the Lepidoptera—would not be in existence at all if there had been no flowers. We must now attempt to show, in a more detailed way, how, by what steps, and under what conditions, our modern flowers have arisen from the earlier flowering plants. In this I follow closely the classic exposition which we owe to Hermann Müller.