POLLINATION MECHANISMS IN VIOLA.

The distribution of pollen upon the insect-visitors to three species of Viola is described and correlated with their feeding and cleaning behaviour. Visitors adopt one of two basic positions while feeding. In the prone position the insect alights on the anterior petal and probes for nectar so that its dorsal surfaces contact the floral genitalia. The supine position may be adopted after alighting on the anterior petal when the insect rotates so that it is virtually standing on its head. While probing for nectar from this position it is the ventral surfaces which contact the floral genitalia. Bumblebees adopt either feeding position; many large solitary bee species almost invariably adopt the supine one while large hoverflies always feed from the prone position. The behaviour of short-tongued visitors tends to be highly individual with respect to contact with the floral genitalia and many body surfaces may be involved. The varying activities of insect-visitors while feeding result in the deposition of pollen at different locations on the integument. Consequently insect-borne pollen is presented to the stigma in various ways and there is a spectrum of pollination effects: medium and long-tongued nectar-seeking insects tend to be systematic cross-pollinators while short-tongued or pollen-gathering species tend to be chance cross- or self-pollinators. The cleaning behaviour of insect-visitors strongly influences pollination as it removes much integumental pollen. Two requirements of the pollen deposition mechanism are postulated: that pollen should be placed (a) in a position not occupied by foreign pollen and (b) in a position irregularly or inefficiently cleaned. There were no positions which were free of foreign pollen throughout the visitor-species. On the other hand, violet pollen was found to be concentrated at locations inaccessible to the cleaning mechanism. [ABSTRACT FROM AUTHOR]