CHAPTER XI. THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.
Insects visit the flowers of the same species as long as they can.
Cause of this habit.
Means by which bees recognise the flowers of the same species.
Sudden secretion of nectar.
Nectar of certain flowers unattractive to certain insects.
Industry of bees, and the number of flowers visited within a short time.
Perforation of the corolla by bees.
Skill shown in the operation.
Hive-bees profit by the holes made by humble-bees.
Effects of habit.
The motive for perforating flowers to save time.
Flowers growing in crowded masses chiefly perforated.
Bees and various other insects must be directed by instinct to search flowers for nectar and pollen, as they act in this manner without instruction as soon as they emerge from the pupa state. Their instincts, however, are not of a specialised nature, for they visit many exotic flowers as readily as the endemic kinds, and they often search for nectar in flowers which do not secrete any; and they may be seen attempting to suck it out of nectaries of such length that it cannot be reached by them. (11/1. See, on this subject Hermann Muller ‘Befruchtung’ etc. page 427; and Sir J. Lubbock’s ‘British Wild Flowers’ etc. page 20. Muller ‘Bienen Zeitung’ June 1876 page 119, assigns good reasons for his belief that bees and many other Hymenoptera have inherited from some early nectar-sucking progenitor greater skill in robbing flowers than that which is displayed by insects belonging to the other Orders.) All kinds of bees and certain other insects usually visit the flowers of the same species as long as they can, before going to another species. This fact was observed by Aristotle with respect to the hive-bee more than 2000 years ago, and was noticed by Dobbs in a paper published in 1736 in the Philosophical Transactions. It may be observed by any one, both with hive and humble-bees, in every flower-garden; not that the habit is invariably followed. Mr. Bennett watched for several hours many plants of Lamium album, L. purpureum, and another Labiate plant, Nepeta glechoma, all growing mingled together on a bank near some hives; and he found that each bee confined its visits to the same species. (11/2. ‘Nature’ 1874 June 4 page 92.) The pollen of these three plants differs in colour, so that he was able to test his observations by examining that which adhered to the bodies of the captured bees, and he found one kind on each bee.
Humble and hive-bees are good botanists, for they know that varieties may differ widely in the colour of their flowers and yet belong to the same species. I have repeatedly seen humble-bees flying straight from a plant of the ordinary red Dictamnus fraxinella to a white variety; from one to another very differently coloured variety of Delphinium consolida and of Primula veris; from a dark purple to a bright yellow variety of Viola tricolor; and with two species of Papaver, from one variety to another which differed much in colour; but in this latter case some of the bees flew indifferently to either species, although passing by other genera, and thus acted as if the two species were merely varieties. Hermann Muller also has seen hive-bees flying from flower to flower of Ranunculus bulbosus and arvensis, and of Trifolium fragiferum and repens; and even from blue hyacinths to blue violets. (11/3. ‘Bienen Zeitung’ July 1876 page 183.)
Some species of Diptera or flies keep to the flowers of the same species with almost as much regularity as do bees; and when captured they are found covered with pollen. I have seen Rhingia rostrata acting in this manner with the flowers of Lychnis dioica, Ajuga reptans, and Vici sepium. Volucella plumosa and Empis cheiroptera flew straight from flower to flower of Myosotis sylvatica. Dolichopus nigripennis behaved in the same manner with Potentilla tormentilla; and other Diptera with Stellaria holostea, Helianthemum vulgare, Bellis perennis, Veronica hederaefolia and chamoedrys; but some flies visited indifferently the flowers of these two latter species. I have seen more than once a minute Thrips, with pollen adhering to its body, fly from one flower to another of the same kind; and one was observed by me crawling about within a convolvulus with four grains of pollen adhering to its head, which were deposited on the stigma.
Fabricius and Sprengel state that when flies have once entered the flowers of Aristolochia they never escape,—a statement which I could not believe, as in this case the insects would not aid in the cross-fertilisation of the plant; and this statement has now been shown by Hildebrand to be erroneous. As the spathes of Arum maculatum are furnished with filaments apparently adapted to prevent the exit of insects, they resemble in this respect the flowers of Aristolochia; and on examining several spathes, from thirty to sixty minute Diptera belonging to three species were found in some of them; and many of these insects were lying dead at the bottom, as if they had been permanently entrapped. In order to discover whether the living ones could escape and carry pollen to another plant, I tied in the spring of 1842 a fine muslin bag tightly round a spathe; and on returning in an hour’s time several little flies were crawling about on the inner surface of the bag. I then gathered a spathe and breathed hard into it; several flies soon crawled out, and all without exception were dusted with arum pollen. These flies quickly flew away, and I distinctly saw three of them fly to another plant about a yard off; they alighted on the inner or concave surface of the spathe, and suddenly flew down into the flower. I then opened this flower, and although not a single anther had burst, several grains of pollen were lying at the bottom, which must have been brought from another plant by one of these flies or by some other insect. In another flower little flies were crawling about, and I saw them leave pollen on the stigmas.
I do not know whether Lepidoptera generally keep to the flowers of the same species; but I once observed many minute moths (I believe Lampronia (Tinea) calthella) apparently eating the pollen of Mercurialis annua, and they had the whole front of their bodies covered with pollen. I then went to a female plant some yards off, and saw in the course of fifteen minutes three of these moths alight on the stigmas. Lepidoptera are probably often induced to frequent the flowers of the same species, whenever these are provided with a long and narrow nectary, as in this case other insects cannot suck the nectar, which will thus be preserved for those having an elongated proboscis. No doubt the Yucca moth visits only the flowers whence its name is derived, for a most wonderful instinct guides this moth to place pollen on the stigma, so that the ovules may be developed on which the larvae feed. (11/4. Described by Mr. Riley in the ‘American Naturalist’ volume 7 October 1873.)With respect to Coleoptera, I have seen Meligethes covered with pollen flying from flower to flower of the same species; and this must often occur, as, according to M. Brisout, “many of the species affect only one kind of plant.” (11/5. As quoted in ‘American Nat.’ May 1873 page 270.)
It must not be supposed from these several statements that insects strictly confine their visits to the same species. They often visit other species when only a few plants of the same kind grow near together. In a flower-garden containing some plants of Œnothera, the pollen of which can easily be recognised, I found not only single grains but masses of it within many flowers of Mimulus, Digitalis, Antirrhinum, and Linaria. Other kinds of pollen were likewise detected in these same flowers. A large number of the stigmas of a plant of Thyme, in which the anthers were completely aborted, were examined; and these stigmas, though scarcely larger than a split needle, were covered not only with pollen of Thyme brought from other plants by the bees, but with several other kinds of pollen.