This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
Belt and Delpino have, I think, suggested the true function of these extra floral nectaries. The former of these excellent observers describes a South American species of acacia: this tree, if unprotected, is apt to be stripped of the leaves by a leaf-cutting ant, which uses them, not directly for food, but, according to Mr. Belt, to grow mushrooms on. The acacia, however, bears hollow thorns, while each leaflet produces honey in a crater-formed gland at the base, and a small, sweet, pear-shaped body at the tip. In consequence, it is inhabited by myriads of a small ant, which nests in the hollow thorns, and thus finds meat, drink, and lodging all provided for it. These ants are continually roaming over the plant and constitute a most efficient bodyguard, not only driving off the leaf-cutting ants, but, in Belt's opinion, rendering the leaves less liable to be eaten by herbivorous mammalia. Delpino mentions that on one occasion he was gathering a flower of Clerodendrons fragrans, when he was suddenly attacked by a whole army of small ants. M. Boissier also makes the interesting observation that many plants produce honey in some countries and not in others. Thus Potentilla tormentilla and Geum urbanum give honey in Norway, and none, or hardly any, near Paris. Indeed a careful comparison showed that most plants gave more honey in Norway than at Paris. No doubt, in consequence of this, some plants which are visited by insects in the north are neglected in the south. Thus he observed five species of Hier-acum to be frequented both by bees and humble-bees in Denmark, while near Paris they are never visited by those insects.
M. Boissier found that by watering a plant copiously he could increase the supply of honey; nay, more, that he could even cause some species to give honey which do not generally do so.
The pollen of course, though very useful to insects, is also essential to the flower itself; but the scent and the honey, at least in their present development, are mainly useful in securing the visits of insects, though the honey is also sometimes of service in causing the pollen to adhere to the proboscis of the insect.
Bees rarely visit flowers unless for some real advantage. The Diptera (flies) appear to be less intelligent, and among fly-flowers we find not only those which attract the insects by honey or pollen, but also trap-flowers, as, for instance, the Arum; and deceptive flowers, such as Parnassia, where five of the stamens terminate in a number of beautiful yellow glands which look like drops of honey, or Stapelia, in which the flowers both in colour and smell resemble decaying meat.
That bees are attracted by, and can distinguish, colours, was no doubt a just inference from the observations on their relation to flowers, but I am not cognisant of any direct evidence on the subject. I thought it therefore worth while to make some experiments; and a selection from them will be recorded in the forthcoming volume of the Journal of the Linnean Society. I placed slips of glass with honey, on papers of various colours, accustoming different bees to visit special colours, and when they had made a few visits to honey on paper of a particular colour, I found that if the papers were transposed the bees followed the colour.
But if flowers have been modified with reference to the visits of insects, insects also have in some cases been gradually modified, so as to profit by their visits to flowers. This is specially the case with reference to two groups of insects, namely, Bees and Butterflies, which have been specially studied by H. Muller with reference to this point; and from his works the following facts are mainly taken. Although the whole organisation of the insect might be said to have reference to these relations, still the parts which have been the most profoundly altered are the mouth and the legs. If we are asked why we assume that in these cases the mouth and legs have been modified, the answer is, that they depart greatly from the type found in allied insects, and that between this original type and the most modified examples, various gradations are to be found.
The mouth of an insect is composed of (1) an upper lip (Fig. 11 a), (2) an under lip (Fig. 11d) (3) a pair of anterior jaws or mandibles (Fig. 11 b), and (4) a pair of posterior jaws or maxillae (Fig. 11 c). These two pairs of jaws work laterally, that is to say, from side to side, and not, as in man and other mammalia, upwards and downwards. The lower lip and maxillae are each provided with a pair of feelers or palpi (Fig. 11, ,c and d, x). The above figures represent the mouth parts of a wasp, in which, as is very usually the case, the mandibles are hard and horny, while the maxillae are delicate and membranous. In the different groups of insects, these organs present almost infinite variations.
Fig. 11. - Mouth-parts of a Wasp a, labrum or upper lip; b, mandibles; c, maxillae; d, labium or lower lip; x.x, palpi.
Fig. 12 represents the mouth parts of a bee, Prosopis (Fig. 13), seen from below; md being the mandibles; pm, the palpi of the maxillae /a, pl, those of the lower lip.
The bees belonging to this genus construct their cells in sand, or in the centre of dry bramble sticks, lining them with atransparent mucus,which they smooth down with their trowellike lower lip (Fig. 12 li), and which hardens into a thin membrane (Smith "Catalogue of Brit. Hymenoptera," p. 7).
That the mouth of Prosopis probably represents the condition of that of the ancestors of the Hive-bees, before the mouth-parts underwent special modifications, may be inferred from the fact that the same type occurs in allied groups, as is shown in Fig. 14, which represents the mouth of a wasp (Polistes) also seen from below. We may therefore consider that Prosopis shows in this respect no special adaptation for the acquirement of honey, and in fact, though the bees belonging to this genus feed their young on honey and pollen, they can only get the former from those flowers in which it is on the surface. In Andrena (Fig 15), Halictus (Fig. 16),