270 SIR dOHN LUBBOC‘K ON THE

On the Attraction of Flowers for Insects. By t,he Rt. €Ion. Sir JOHN LUBBOCK, Bart., F.R.S., M.P., F.L.S., D.C.L., LL.D.

[Read 4th November, 1897.1

I CONFESS I should have thought that the researches of Christian Sprengel, Darwin, H. Miiller of Lippstadt, Delpino, and others, had made it clear that while honey is the real object which attracts insects t o flowers, colour and scent guide them to the prize ; and that thus, as I have elsewhere said, it is to Insects “we owe the beauty of our gardens, the sweetness of our fields. To them, flowers are indebted for their scent and colour ; nay, for tbeir very existence in its present form. Not only have the present shape and outlines, the brilliant colours, the sweet scent, and the honey of (the) flowers been gradually developed through the unconscious selection exercised by insects j but the very arrangement of the colours, the circular bands and radiating lines, the form, size, and position of the petals, the relative situations of the stamens a n d pistil, are all arranged with relerence to the visits of insects, and in such a mauner as to eusure the grand object which these visits are destined to effect.””

Prof. Plateau, however, in three interesting memoirs t, has recently called this view in question, and his eminence as an observer of insect life justifies, and even demands, a reply.

He summarize$ his conclusions as follows :- “ (i.) Les Insectes visiteut activenient leu inflorescences qui

n’ont subi aucune mutilation, mais dont la forme et les couleurs sont masquCes pew des feuillelj vertes.

“ (ii.) Ni la forme ni les couleurs vives des capitules ne semblent avoir d’action attractive.

“ (iii.) Les fleurons phriph&ques color& des Dahlias simples, et, par conshquent, des capitules des autres Compos6es radikes, n’ont pas le r61e vexillaire ou de signal qui leur a 6th attribu6.

“(iv.) La forme et la couleur ne paraisseut pas avoir de r61e attractif; les Insectes sont hvidemment guides vers leu capitules de ComposCes par un autre sens que la vue, sens qui est probable- ment l’odorat.” $

* ‘Flowers and Insects,’ p. 50. t “ Comnient les Fleures attirent les Illmites ” : in Bull. Acad. Bruxelles,

S6r. 111. xxx. (1995) pp. 466-488 ; =ii. (1896) pp. 505-534; xxxiii. (1897) pp. 1741.

L. c. xgx. p. 487.

ATTRACTION O F FLOWERS FOR INSECTS. 271

Before, however, coming to Prof. Plateau’s new observations, it is necessary to have a clear idea of how Bees act.

If a Hive-bee is brought to, or finds, a small quantity of honey, she laps up all she can, carries it off to the neat, and returns for more, ofteh accompanied by one or more companions. It is, I believe, immaterial, or nearly so, to the Bee on what the honey rests, so long of course as it is not tainted. If during the Bee’s absence the honey is moved, the Bee returns to the same place, and then hawks about, looking for it all round. Even if it is covered o-rer, she will find it sooner or later.

I tried various experiments, placing the honey on slips of glass placed on coloured paper.

It must be remembered, as Aristotle was the first to mention, that the Hive-bee keeps to one kind of flower in each round of visits. I n nature flowers are more or less scattered ; they do not, at least not in all cases, grow in masses as in our gardens. It must be clear to everyone tha t when the Bee has finished the flowers on one plant, and has to find another of the same species, the coloured flowers would be a great help. Let anyone put a drop of honey on a leaf, and another on a piece of white, yellow, red, or blue paper on a grass-plot, and see whether t!le latter is not much the more easy to fiud.

Now, suppose a Bee to be visiting-say a daisy.

I now pass to Prof. Plateau’s experiments.

1. The first were made with single Dahlias. H e cut some squares

of coloured paper (red, violet, white, and black), and in the ceiitre of each he pierced a circular hole corresponding in size with the yellow heart of the flowers. He then pinned these papers on to some of the flowers.

He then says :-“ On aurait pu supposer que les Insectes se seraient port& exclusivernent sur lea autres capitules intacts voisins en grand nomtre et auraient nkglig6 complthement les inflorescences masqubes.”

The paper disks must have been very conspicuous : the insects were already accustomed to come to the bed of flowers, and, once there, would r e d l y find the groups of central florets. Moreover, Prof. Plateau does not give us the number of visits to other flowers, so that no comparison is possible.

I should not myself have drawn any such conclusion.

x s

272 SIR JOHN LUBBOCK ON THE

2 . He then cut some disks without central holes, and pinned

them over the flowers so as to hide, without touching, the tubular flowers. The visits of insects were about as numerous as before.

To this experiment, however, the same objections may be made as to the first.

I might also observe that to us Dahlias have no scent, and though it is possible that they may nevertheless emit an odour attractive to insects, there is no evidence of this. Surely it is somewhat illogical to assume that the Bees are guided by the sense of smell, when we have no evidence that, as amatter of fact, any scent is emitted.

3. I n his next series of experiments, Prof. Plateau used leaves

instead of coloured paper. The results were similar, but the experiments are open to the same objections.

Yet he observes that ‘‘ Apr& ces expchiences, il semble, au moins pour les Insectas obserrhs, que ni la forme ni la couleur des fleurs ne les attirent ; que c’est surtout et peut-Gtre exclu- Bivement l’odorat qui les guide.” Surely in any case, and even apart from the remarks already made, this is not a logical sequence. The theory which Prof. Plateau attacks is that insects are attracted to flowers by their smell and colour. It is part of the theory that the scent of flowers is important as attracting insects. Even if Prof. Plateau had proved, which I submit he has not, that in his experiments the iusects were attracted by smell, and that they did not see the coloured corollas, this would not prove that they are not guided by the coloured corollas when they can see them.

4. In his next series of experiments Prof. Plateau masked all the

flowers, aud yet insects came. “Dam ces conditions,” he says, “l’observateur place devant la rangee de Dahlias ne voit plus aucune fleur.” No j but let him move round, as Bees would, and he would see the backs and sides of the, flowers, which would be quite sufficient.

Moreover, it is evident that on previous days the Dahlias had been visited abundantly by insects. These would of course return, and though they might be a little disconcerted by Prof. Plateau’s disks, the flowers would have to be much better hidden if they were to remain undiscovered.

274 SIR JOHN LUBBOCK ON THE

by the colonred corolla, and surely the simple arid sitfficient reason why it did not light on them was that they contained no honey.

7. He treated in a similar way some flowers of an IpomEa purpurea.

Here, however, I should make the same objection as made to Seriek 4. I n this case also he gives no comparison with unmuti- lated flowers.

8. I pass over some similar observations on Delphinium Ajacis,

Centaurea Cyanus, D@ital is purpurea, Anfirrhinunz mujus, Heracleum Tischeri, &c., to which, I submit, the same criticism8

9. W ’ Y .

The next experiment as with the Cornflower (Centaurea Cyanus). Bonnier, in his ‘ MQmoire s u r les Nectaires,’ observes that in a field containing white as well a s blue Cornflowers (Centaurea, Cyanus), the Bees seemed (as is indeed probable) to visit them both nearly equally. Prof. Plateau says : “ Mes observations, tout en confirmant celle de Bonnier, sont plus complbtes.” The Cornflowers observed by Plateau were of four colours--’ulue, purple, rose, and white. He records 16 visits bp two Hive-bees, and 14 by four Megachiles, and observes that “l’indiff6rence pour la coloration est du reste & peu pr8s coinpli.te.” The number of visits seems to me quite insuficient to justify any conclusion, but so far as they go they tend to confirm 1 n ~ ~

experiments recqrded in this Journal, which shorn ed a preferellee for blue, since out of 30 visits recorded by Prof. Plateau, 16 were to blue flowers, 6 to purple, 6 to white, and 2 to rose. Prof. Plateau adds, however, that “ la pr6Mrence apparente pour Ies capitules bleus tient h cette particularith, indiqu6e plus haut, quo ceux-ci Qtaient plus nombreux que les autres vari6t6s.” He does not, however, tell us what the respective numbers of the different coloured flowers were, and under all the circumstances no con- clusion whatever can, I submit, be drawn from the observation.

10. He proceeds to dwell on the existence of certain flowers which

are iticonapicuous, and yet, in consequence of their strong scellt, much visited by insects. If anyone denied that scent serves to

ATTRACTION OF FLOWERS FOB INSECTS. 275

attract insects, this fact would no doubt be a complete reply. It is none, however, to those who fully admit t h e importance of scent in attracting, or, I should rather say, guiding insects to flowers.

11. Prof. Piateau then refers to the experiments of Perez, a110

placed drops of honey in certain flowers (Scarlet Pelargoniums) which contain no honey, and are therefore neglected by Bees ; when, however, drops of honey were put in them, insects are ere long attracted by the scent of the honey. Perez, honever, tells us that, after awhile, “ La couleur kcarlate s’Qtait si bien associbe dans leur souvenir B l’idee du miel, qu’elles se posaient & la fin sur des fleurs de cette couleur n’en ayant pas r e p , et ne les quittaient qu’aprh s’&tre assurkes, par un examen scrupuleux et persistant, qu’elles n’avaient rien B y recueillir.” Here, therefore, Perez clearly shows that the Bees were attracted by the red colour.

Prof. Plateau, indeed, states that his experiments did not in this respect confirm those of Perez. Yet he, himself, subsequently says, “Lorsque l’insecte avait ainsi absorb6 le liquide d’un certain nombre de fleurs miell8es. il lui arrivait de se diriger vers les Pelargoniums non munis de miel.” This statement confirms that of Perez, and shows that the Bees, having fouud honey in some scarlet Pelargonium flowers, went and inspected others. I n this case they were evidently guided by the colour, for, as already mentioned, the scent of scarlet Pelargoniums does not a t t rx t them. This seems a crucial case, fatal to Prof. Plateau’s contention.

12. Lastly, Prof. Plateau made some final observations on single

Dahlias. Selecting a plant in the middle of a group, he care- fully removed the central flowers and consequently the honey, leaving, however, the conspicuous ray-florets. Having done SO,

he says * :-“ Durant trois quarts d‘heure d’observation attentive, on ne voit aucun insecte se poser sur les in0orescences trans- f0rmQes.I’ But why should they do so when the honey had been removed? H e adds, however, that “Leg Bourdons ou leu NQgachileu qui quittent les capitules de Dah!ias intacts se portent naturellement assez souvent vers les Dahlias mutiles in6langQs aux prkcbdents, mais ces insectes Be bornent A d6crire devant ces inflorescences quelques courbes prouvant inconteut-

* Bull. Acad. Briixelles, SBlr. 111. xxxiii. (1897) p. 39.

276 SIR JOHN LUBBOCK ON THE

ablement un examen rapide, puis fondent tout droit sur un Dahlia intact.” This behaviour is natural from our poiut of view, because they were attracted by the coloured ray-flowers ; but seems quite inconsistent with Prof. Plateau’s contention that scent is the only attraction, because the honey-bearing florets had been removed. So far from supporting his view, therefore, these last observations weaken, if they do not disprove it.

The experiments which the Society has already done me the honour of publishing, and which I subsequeutly brought together in my two books ‘ Ants, Bees, and Wasps,’ and ‘ The Senses and Intelligence of Animals,’ seem to me conclusively to have proved that Bees and other insects can distinguish, and are attracted by, the colours of flowers.

I haye, however, made some more experiments, following to some extent the idea of Prof. Plateau, and endeavouring to get a crucial test between the respective attractions of scent and colour. I brought a Hive-bee up to my room, and, when she had got ac- customed to come to some honey on B particular part of the table, I put out, a foot on one side of the place where the honey had been, the flower-head of an Eryn.yi2cm amethyhystinum after removing the blue bracts, with a drop of honey on an ordinary glass microscopic slide ; and, a foot on the other side of the place where the honey had been, a similar slip of glass with a drop of honey, and placed close t o it the blue bracts. These, I need not sap, are brilliantly blue, and measure about 4 inches across. The flower-head, on the contrary, though an inch in length, is not conspicuously coloured. NOW, if insects were attracted by the scent of the honey alone, they would in such circumstances go to the drop of honey near the flower-head, or to the flower-head itself, and not to the bracts. While if they were attracted both by colour and scent, they would go Rometimes to the one and sometimes to the other, -probably, as the bracts are so conspicuous, more often to the honey close to them. For shortness, I will indicate the floaer- head and the drop of honey near it as F, the drop close to the bracts as B.

A fes minutes after the original drop of honey was removed, the bee returned at 3.59, and, after buzzing about a little, settled on B .4fter each visit I transposed the flower-head and bracts, leaving, however, the two drops of honey. This, of course, eliminated any possible difference as regards the honey. The Bee returned as follows :-

b ' U " l h C T I O S 01.' FLOWERS FOR IXSECTS. 27;

Tlic followi,ig day I repeiitetl tlic same experiment. Tlw 13ee ratiimcd as f'ollows :-

278 ON THE ATTRACTION OF FLOWERS FOR INSECTS.

I n these 93 visits, therefore, the Bee came 60 times to the honey near the bract, and 33 times to that uear the flower-head itself.

I then applied the same test, placing, however, the drops of honey, with the flower-head and the bracts respectively, near the bed containing the growing plants and moving them about after each visit. Sixteen Bees came to the honey near the bracts, seven to that near the flower-head.

These observations go far, I think, to disprove Prof. Plateau's conclusion.

Such observations need to be made with much care, and it is iiot safe t o assume that the senses of animals are exactly similar t o ours. For instance, Saccharine and sugar have to UB the Pame sweet taste. I, myself, am quite unable to distinguish them. I, however, placed side by side two saucers containing water sweetened (1) by sugar and (2) by saccharine, and found that while Bees and Ants thoroughly enjoyed the firat, they would not touch the second.

I n co~~clusioi~, then, I submit that the observations made by Prof. Plateau do not at all weaken the view that the colours as ae l l as the scent of flowers serve to attract Insects and guide them to the honey, in securing which they confer upon the plants the great advantage of cross-fertilization.