The antennal senses (tactile, and especially olfactory) play a predominant part in guiding ants on their roads. We already know that the Amazons are incapable of following, when the antennae have been cut off. Forel has shown that Formica pratensis wanders in all directions and finishes by remaining motionless, when it has been submitted to the same operation ; while it makes turns and detours but nevertheless arrives at the correct objective when the eyes have been varnished. Piéron reached the same result with Messor. These ants depend greatly on tactile impressions, for they are disquieted and tap their antennae in all directions when some familiar object is removed or when one places an obstacle on their route. Lasius fuligi-nosus, whose sense of smell is much more acute, does not seem to hesitate when traps of this kind are prepared. With Formica cinerea the eyes and the antennae seem to play an equal part.

An old experiment of Bonnet (1745), often repeated since, emphasizes the role of smell. It consists in passing a finger across the road used by ants. There is then a great disturbance. The travelers pause, hesitate, disoriented by the disappearance of the familiar scent and by the sensation of a new odor. Sometimes a simple sweeping suffices, as is the case with the Tapi-nomas. With others, like the Messors studied by Piéron, vegetable odors, washing with water, or the displacing of dust do not interrupt the march. Like the Tapinomas, the Amazon ants, stopped by this means, end nevertheless by again taking up the march. One of them experiments in crossing the barrage, others follow, and soon the entire phalanx emerges on the refound scent. This shows, says Forel, that the sense of smell rather than sight guides the insect on its route. As we shall see later, "the ant on her return knows and feels by her scent from which side she went, and necessarily deduces from this that to return she ought to direct herself in the opposite way. Should a barrier appear (nature produces one sometimes) -she hesitates, then learning what the matter is, since the scent which she has behind her she brought from the pillaged nest and not from her own, and since there are no others, she does not hesitate any longer to cross the barrier."

To conclude this section I will cite some experiments of Santschi, who shows very well the role of the different senses in the orientation of ants by scent. A stick is laid across a road followed by Messor; this is a normal obstacle which the passengers cross going and coming, but when it is raised and placed out of the ro-ad, ants remain attached to it or go away a little to return to it, for it is their only point of guidance, a point especially odorous. Light rays reflected by a mirror simply produce an eddy in a band of Messor which is following a scent ; they do not prevent these insects from continuing their journey. If the scent is interrupted, on the contrary, the ants are disoriented and direct themselves in all sorts of ways. They were guided by the odorous trace which has disappeared, and the light rays, far from serving them as a guide, have only increased their perplexity.