But it is not only the vanilla that requires the co-operation of living beings to produce its fruits. It is the case with many other orchids. In the flowers of these the pollen is massed together and cannot be transported by the air. It needs the aid of insects, as had already been pointed out by Sprengell in the eighteenth century, and above all by Darwin, whose splendid investigations are the basis of the following passages.[30] Insects, belonging to different groups, such as bees, wasps, flies, certain beetles, and many butterflies and moths, visit orchids to sip the nectar produced by the plants and stored in definite parts of the flowers. In order that their proboscis may reach the stores of sweet juice, the insects inevitably touch first the upper parts of the flowers, where the anthers are present. The pollen grains are clustered in masses, known as pollinia, and these adhere to the body of the visiting insect by means of an adhesive fluid which is secreted by an organ of the flower known as the _rostellum_. In this way the pollinia adhere firmly, it may be to the proboscis of butterflies, or to the head or any other part of the body of insects. They can leave the flower and fly away without losing the adhering pollinia, and in this manner they serve as the agents for sexual contact and for fertilisation of the orchids. Ménière relates that a person who kept bees near the garden of the Faculté de Toulouse complained that they returned from the garden with their heads covered with tiny yellow bodies which he was unable to clean off from them. It was easy to recognise in these bodies the pollinia of orchids very firmly attached to the bees’ heads.[31] When an insect, bearing these pollinia, introduces itself into another flower of the same species of orchid, it inevitably comes in contact with the female apparatus, more particularly with the viscous surface of the stigma. Some of the grains of pollen contained in the pollen-mass adhere to the stigma and are thus enabled to fertilise the ovule. This carriage of pollen from one flower to another brings about a crossing which is necessary for the production of good seed. On the other hand, the seed which is the result of self-fertilisation of a flower is inferior. An examination of the structure and form of the flowers of many orchids show that they are adapted in a truly marvellous way to the visits of insects that convey pollen. In each part of these flowers one can discern some useful arrangement to secure cross-fertilisation.