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.
This subclass contains those dicotyledons in which the petals are united together, at least at the base.
This order, which contains five British genera, Adoxa, Sambucus, Viburnum, Lonicera, and Linnsea, offers remarkable differences, especially in relation to the honey glands. Adoxa is a low, glabrous, light green herb: the flowers, which are coloured like the rest of the plant, secrete honey, which is exposed, and accessible to all insects. Sambucus nigra (The Common Elder), on the contrary, secretes no honey. It is nevertheless sweet-scented, and is visited by several insects, but often fertilises itself, as the stamens and pistil ripen simultaneously, Viburnum (the Guelder Rose) secretes honey, and the flowers are collected into a head as in the Elder, but the outer florets have the corolla considerably enlarged at the expense of the stamens and pistil. Although, therefore, they produce neither pollen nor seeds, they are useful to the plant, by rendering the other flowers more conspicuous, and thus attracting insects. In remarkable contrast to these species, with their exposed honey, is the genus Lonicera (the honeysuckle). Lonicera caprifolium has a honey tube no less than 30 mm. long, for the most part not above 1 - 2 mm. wide, and moreover a great part occupied by the style. It is often, however, half full of honey. As in the longest tongued bees (Bombus hortorum and Antho-phora pilipes), the proboscis only attains a length of 21 mm., those of Flies (Rhingia, Bombylius discolor) not more than 11 - 12 mm., they are none of them in a position to extract all the honey; and in fact Muller never found them attempting to do so, though they visit the flowers for the pollen. The honey of Lonicera caprifolium is therefore especially adapted for the larger moths. The flowers open in the evening, and are then specially fragrant. Muller found the following moths on this species: Sphinx convol-vuli; S. ligustri; S. pinastri; Deilephila elpenor; D. porcellus; Smerinthus tiliae; Dianthaecia capsincola, Cucullia umbratica, Plusia gamma, Dasyclura pudi-bunda.
L. periclymenum (the Common Honeysuckle) agrees in most respects with the preceding species, but the tube is rather shorter, and the honey in consequence more accessible to bees. In our third species again, L. xylosteum, the tube is still shorter, and the flowers are regularly visited by flies and humble-bees.
We have four British genera of this order, Rubia (the Madder), Galium, Sherardia (Woodruff), and Asperula.
The flowers are small, but in many cases rendered conspicuous by association. Several of the species are sweet-scented, and attract insects by means of honey, which is either exposed on a flat di-h (Rubia and Galium), or contained at the base of a short tube (Sherardia and Asperula). The stamens and pistil ripen simultaneously, and if not visited by insects, the flowers fertilise themselves. The florets of Rubia peregrina are greenish; those of Sherardia arvensis blue or pink; the others either white or yellow. Muller calls attention to the influence of colour in the case of Galium molliigo and G. verum, which agree closely in most points, but the former of which is white, while the latter is yellow, which he says renders it much more attractive to small beetles.
Fritz Muller has described (Bot. Zeit. 1866, p. 129) a very interesting South American species of this group, Martha (Prosoqueria) fragans, in which the stamens are irritable, and when touched by the proboscis of an insect, immediately explode, and throw the pollen on to the insect, at the same time closing the entrance to the tube of the flower, in which the pistil is situated, and thus preventing the possibility of self-fertilisation.
Of this family we have only one truly British genus, Valeriana, though Centranthus ruber, having been long cultivated in gardens, has become naturalised in some parts of England.
The flowers of the Allheal (Valeriana officinalis), though small, are rendered conspicuous by association. They are melliferous, and the honey is accessible even to short-tongued insects, by which they are much frequented. They are proterandrous.
Valeriana dioica, while agreeing with the preceding as regards the honey, is, on the contrary, generally dioecious, the male flowers being, as usual, larger than the female, and, consequently, in most cases visited first.