In the snapdragon the two lips of the flower are tightly closed, the lower one decidedly projecting. Any small insect reaching this lower lip as a natural landing place finds no entrance. When the bumble-bee alights upon the lower lip, however, his weight depresses it and he forces his way in, and in passing to another flower effects pollination. It is interesting to note that after this first and important visit the lips remain open and other insects pass in freely, "being invited," as some one has said, "to eat at the second table." In most of the orchids there is a very complete adaptation of the flower to its insect, by which almost every insect excepting one special kind obtains nothing for its visit. The nectar is usually in the end of a long spur, and to obtain it the head of the insect must just fit between two sticky buttons to which the pollen-masses are attached. The length of the spur is nicely adjusted to the length of the proboscis of the visiting insect, but his head must also be of a certain breadth. If an insect visits the flower with a proboscis too short or too long, or a head too broad or too narrow, its visit is unavailing. The danger of such narrow specialization is apparent in the case of the orchids, for each plant is so dependent upon a special insect that the disappearance of the latter seriously endangers the continued existence of the former. Protective closure.--It has long been noticed that certain flowers open only in the evening, the evening primrose being a conspicuous example. These flowers are adapted to the visits of the night-fliers, the moths, and about clusters of evening primroses numerous large hawk-moths may be seen after sunset. During the day the flowers are closed and safe from the visits of any insect, but by opening in the evening they are not only ready for the visits of the night-fliers, but they avoid the visits of most creeping insects, notably the ants, who are not abroad after "the dew falls."