Insect Pollination Entomophily

INSECT POLLINATION (ENTOMOPHILY) The special characteristics of entomophilous flowers are the at tractive colour of the floral envelope, the presence of scent and of nectar, and of pollen which is not powdery but sticky and is present in comparatively small quantities. The entomophilous is the most common type of pollination in flowering plants and spe cial floral conformations and irregularities adapted to insect visi tors are characteristic of the higher families of flowering plants, as will be seen below. The evolution of flowers and of insects must have gone hand in hand; such groups as Lepidoptera (butterflies and moths) and Hymenoptera (bees, wasps, etc.) could not have existed without the more elaborate and honey-bearing flowers and vice versa. Hermann Miiller (see Bibliography) has divided flowers into various classes according to their degree of specialisa tion for different insects, so that a brief survey of the types of insects concerned must be given.

Types of Insects.

There are five important classes of insects which visit flowers. The Hemiptera (bugs, etc.) have a few flower-visiting species but they show no special adaptation to flowers; the Coleoptera (beetles) have many species which visit flowers but they have only short tongues (only a few species with a length of 3 to 6 mm.) and so are able to reach only honey which is fully exposed. The Diptera (flies) include many species which visit flowers. The short-tongued ones (with tongues under 4 mm.) show no special adaptation to a diet of floral origin and are not usually clever enough to find any nectar which is not fully ex posed in the flower; many of these flies have also other sources of feeding. The long-tongued flies (such as hover flies or drone flies) have tongues from 4–I 2 mm. and confine themselves to a diet of nectar, and are clever in discovering it when concealed. The Hymenoptera (bees, wasps, sawflies, ants, etc.) include a very large number of flower-visiting forms. Bees are the only long tongued members of the group, and it is bees which have played the most important part in the evolution of the more complex flowers. The hive bee (Apis) and the humble-bee (Bombus) have long tongues (over 6 mm.) while most of the other bees have shorter tongues, i.e., less than 6 mm. The "cleverness" of bees, combined with the length of their proboscis, enables them to find and reach nectar which is deeply concealed in the flower. Bees do

not confine themselves to sucking nectar from the flower they also collect pollen (some flowers provide only pollen and no nectar), which is carried in small masses attached to the hairs of the hind legs. The Lepidoptera (butterflies and moths) are insects with tongues usually about as long as those of bees, but the hawk moths may have tongues, when unrolled, of enormous length. The British hawk moth, Sphinx convolvuli, has a tongue 8o mm. (31 in.) long and some tropical moths a tongue of 30o mm. (12 in.).

Classes of Flowers.

The entomophilous flowers have been divided by H. Muller into nine classes based on the structure of the flower and its relation to particular insects.

(i.) Class A. Flowers with Exposed Nectar.—In this class come most Umbelliferae, many Saxifragaceae, the bedstraws (Galium), ivy (Hedera) and such trees as maple, elder and lime. The flowers are wide open and usually small and the visitors are mostly short tongued ; they are rarely visited by bees and butterflies. Such flowers run the risk of the nectar being washed away by rain.

(ii.) Class AB. Flowers with Partially Concealed Nectar.—In this class fall the buttercups (Ranunculus), the Cruciferae, the strawberry (Fragaria) and the willows (Salix). The nectar is protected and concealed by the position of the stamens, by the development of hairs or scales, or by the flower being partially tubular, as in wallflower where the sepals stand erect and give a tubular form to the lower part of the flower.

(iii.) Class B. Flowers with Fully Concealed Honey.—In this class are the flowers of many Carophyllaceae (such as Gypsophila, Geranium), Polemonium, blackberry (Rebus), eyebright (Eu phrasia) mint (Mentha), heather (Calluna). In these the nectar may be concealed by the stamens, by the calyx, by the receptacle becoming hollowed, or by the petals being united to form a sym petalous corolla. The insect visitors are the smaller bees with a few of the longer tongued flies. This type of flower is clearly the most effective of the classes so far mentioned. The bees show a high degree of skill in reaching the concealed honey and mostly confine themselves during a given flight to one or a few species of flower, and thus avoid the great waste of pollen caused by shorter tongued insects, which are liable to carry it indiscriminately from the flower of one species to another.