248 



TAKING UP OF POLLEN BY INSECTS. 



examples will be described, and as they are to a certain extent identical with those 

 already referred to in connection with the subject of the protection of honey, we 

 need not enter into them at so great length as would otherwise be necessary. In 

 the first place, there are the flowers which are furnished inside with prickles or 

 sharp, stiff bristles. It is well known that honey-sucking insects, such as humble- 

 bees, are very careful of their probosces, keeping them in special grooves in their 

 bodies whenever they are not in use, and taking pains when they do use them not 

 to thrust them against rigid points on account of their liability to injury. Thus 

 a flower, furnished with sharp prickles or bristles, only admits an insect's proboscis 

 by a well-defined path. The intruder avoids the points, and is thereby prevented 

 from entering by a route which would not involve its rubbing against the anthers 

 — and is induced to take a course inevitably accompanied by the deposition of 

 pollen on its back, head, or proboscis. This occurs, for instance, in some Cruciferse 

 (Braya alpina, Malcolmia Africana; see fig. 267 % where the insects are guided 

 to the honey between two series of rigid upriglit bristles borne by the ovary, and 

 are obliged to brush their heads and probosces against the pollen-laden anthers. 

 The same thing happens in Leonurus heterojohyllus (see fig. 267 "), a Labiate, which 

 has a patch of sharp prickles in the throat just behind the under-lip. Insects 

 desirous of possessing the honey secreted at the base of the flower, and at the same 

 time of avoiding the prickles, are obliged to insert their probosces close under the 

 upper-lip, and are thus brought into contact with the pollen-covered antliers there 

 situated. In several small alpine Gentians, such as Gentiana glacialis and G. nana, 

 the entrance to the interior of the flower is covered by four valves with lacerated 

 margins which are so pliable as to form no barrier to the entrance of the stronger 

 kinds of insect. But no anthers would be brushed by their probosces if they were 

 to enter by that way, and the possibility of such an occurrence must, therefore, be 

 obviated. The fringed edges of the valves closing the throat are for this purpose 

 thickly studded with minute prickles. Insects reject the route as too risky and 

 prefer to enter between the points of insertion of the valves whence passages of 

 adequate size and quite free from danger lead to the honey. In passing along them 

 the insects brush the anthers which are situated close by. The compulsory condition 

 imposed on insect-visitors that they should rub the pollen off" with their probosces, 

 and occasionally with the tops of their heads and front parts of their thoraces, 

 depends in many cases on the fact that there is only one approach leading to the 

 honey, and the external orifice of this passage is straitened by a ring-like callo- 

 sity, or by the presence of flaps or scales, whilst the anthers are situated round the 

 orifice, or just underneath it. This arrangement is found, for example, in many 

 Boragineffi, Oleaceaj, Primulaceae, and PolemoniaceiB. The hawk-moth, which sucks 

 honey in the autumn from the flowers of the Phlox, a plant belonging to the Pole- 

 moniacese, and butterflies, which feast in the spring on the sweet juices of Lilac- 

 flowers load only their probosces with pollen, foi-, in consequence of the form and 

 disposition of the various parts of the flowers, this jjart alone comes into contact 

 with the anthers. 



