No attempts appear to have been made to ﬁnd whether such a trail is also unidirectional. The odour of trail pheromone deposited on inert material such as glass or nylon mesh induces temporarily disorientated bees to expose their Nasonov glands (page 114) at the hive entrance (Ferguson and Free, 1981).
The ﬂoor and the inside walls of the hive or nest and the combs themselves are also probably marked with trail pheromone. The accumulation of trail pheromone on comb may partially explain why old comb is more attractive than new (page 81). Butler (1967) found that bees he had trained to forage in a darkened arena produced an odour trail between their hive and the dish of sucrose syrup.
Trail pheromone while foraging
It is well established that a glass dish on which bees have been foraging for sucrose syrup is more attractive to potential foragers than a clean dish, probably because of an attractive trail pheromone the foraging bees have left behind.
Experiments that have demonstrated this involved training bees to collect sucrose syrup from tubes or dishes placed on a circular table. The tubes or dishes with syrup were replaced by empty ones, provided with different odours and placed equidistant from the table centre; the number of bees that landed on or touched each tube or dish was compared. The table was rotated continuously so the bees did not become conditioned to any particular position (see Ribbands, 1954; Butler et al., 1969; Ferguson and Free, 1979).
Bees visiting a site mark it with an attractive pheromone irrespective of whether they have foraged successfully there. Ribbands (1954) showed that it was only necessary for a bee to land brieﬂy on a particular empty tube for it to prefer that tube subsequently, Free (1970b) found that would-be foragers were attracted to the odour bees had left on a glass sheet covering model ﬂowers from which they could not obtain food, and Ferguson and Free (1979) demonstrated that dishes on which bees had landed and had not foraged became attractive to others. It has been shown that small blocks of plaster of Paris, small sheets of glass or squares of wire gauze that had been kept on the ﬂoor of a hive just inside the entrance for a few hours and on which numerous bees had walked, became attractive to foraging bees and retained this attractiveness for many hours (Butler et al., 1969, Ferguson arid Free, 1979). Thus it appears unlikely that the attractive pheromone involun- tarily deposited by foragers is exclusive to foraging, but is probably the same general trail pheromone responded to at the hive entrance. It is certainly P more primitive form of communication than the Nasonov pheromone.
It seems that foraging bees may also have a preference for trail odour deposited by bees of their own colony. Bees from two colonies were trained to two separate but adjacent dishes of dilute sucrose syrup; the dilute syrup in each dish was then replaced by concentrated syrup so that dancing and recruiting were encouraged. Newcomers that arrived were preferentially attracted to the dish visited by their nestmates (von Frisch and Rosch, 1926; Kalmus and Ribbands, 1952), and so deposition of trail pheromone at a source of forage favours survival of their own colony. Undoubtedly the bees were exposing their Nasonov glands as they arrived at the dishes, but there is no evidence that Nasonov pheromone is colony speciﬁc (page 103).