Gentry, in speaking of the introduced Wistaria sinensis, says "that nearly every flower had been perforated." (11/12. Dr. Ogle 'Pop. Science Review' July 1869 page 267. Bailey 'American Naturalist' November 1873 page 690. Gentry ibid May 1875 page 264.)
As far as I have seen, it is always humble-bees which first bite the holes, and they are well fitted for the work by possessing powerful mandibles; but hive-bees afterwards profit by the holes thus made. Dr. Hermann Muller, however, writes to me that hive-bees sometimes bite holes through the flowers of Erica tetralix. No insects except bees, with the single exception of wasps in the case of Tritoma, have sense enough, as far as I have observed, to profit by the holes already made. Even humble-bees do not always discover that it would be advantageous to them to perforate certain flowers. There is an abundant supply of nectar in the nectary of Tropaeolum tricolor, yet I have found this plant untouched in more than one garden, while the flowers of other plants had been extensively perforated; but a few years ago Sir J. Lubbock's gardener assured me that he had seen humble-bees boring through the nectary of this Tropaeolum. Muller has observed humble-bees trying to suck at the mouths of the flowers of Primula elatior and of an Aquilegia, and, failing in their attempts, they made holes through the corolla; but they often bite holes, although they could with very little more trouble obtain the nectar in a legitimate manner by the mouth of the corolla.
Dr. W. Ogle has communicated to me a curious case. He gathered in Switzerland 100 flower-stems of the common blue variety of the monkshood (Aconitum napellus), and not a single flower was perforated; he then gathered 100 stems of a white variety growing close by, and every one of the open flowers had been perforated. (11/13. Dr. Ogle 'Popular Science Review' July 1869 page 267. Bailey 'American Naturalist' November 1873 page 690. Gentry ibid May 1875 page 264.) This surprising difference in the state of the flowers may be attributed with much probability to the blue variety being distasteful to bees, from the presence of the acrid matter which is so general in the Ranunculaceae, and to its absence in the white variety in correlation with the loss of the blue tint. According to Sprengel, this plant is strongly proterandrous (11/14. 'Das Entdeckte' etc. page 278.); it would therefore be more or less sterile unless bees carried pollen from the younger to the older flowers. Consequently the white variety, the flowers of which were always bitten instead of being properly entered by the bees, would fail to yield the full number of seeds and would be a comparatively rare plant, as Dr. Ogle informs me was the case.
Bees show much skill in their manner of working, for they always make their holes from the outside close to the spot where the nectar lies hidden within the corolla. All the flowers in a large bed of Stachys coccinea had either one or two slits made on the upper side of the corolla near the base. The flowers of a Mirabilis and of Salvia coccinea were perforated in the same manner; whilst those of Salvia grahami, in which the calyx is much elongated, had both the calyx and the corolla invariably perforated. The flowers of Pentstemon argutus are broader than those of the plants just named, and two holes alongside each other had here always been made just above the calyx. In these several cases the perforations were on the upper side, but in Antirrhinum majus one or two holes had been made on the lower side, close to the little protuberance which represents the nectary, and therefore directly in front of and close to the spot where the nectar is secreted.
But the most remarkable case of skill and judgment known to me, is that of the perforation of the flowers of Lathyrus sylvestris, as described by my son Francis. (11/15. 'Nature' January 8, 1874 page 189.) The nectar in this plant is enclosed within a tube, formed by the united stamens, which surround the pistil so closely that a bee is forced to insert its proboscis outside the tube; but two natural rounded passages or orifices are left in the tube near the base, in order that the nectar may be reached by the bees.