There are cases where the insect reaches an unknown objective guided by a series of intermediary excitants. As Claparède observes, these cases approach the preceding ones, because each intermediary "determines the direction" and plays "in some sort the role of the unknown but appreciable end." The processionary caterpillars of the pine, studied by Fabre, offer us a perfect example of one of these cases. They are born upon the needles of the lower branches where the moth has deposited her batch of three hundred eggs. After having browsed upon the neighboring needles-first the two needles where they were born-they direct themselves through the foliage, pressed by the need of nourishment and surely also by tropisms. But it is by stages that they go, eating at each stop the needles which they meet. These needles are the intermediary excitants which lead them to an unknown objective, to the top of the tree. There they build a large common nest in which they hide during the winter. When the month of March arrives they finally leave this nest and descend to the ground to transform into chrysalids. Their objective is unknown to them, and no intermediary excitant directs them ; they are led by their geotropism, which has become positive after the physiological modifications which they have undergone. Indeed, whatever their geotropism at the time of their birth, all larvae of insects that transform in the soil present at the time of their maturity a more or less evident positive geotropism.
The history of the migratory grasshoppers gives rise to much more difficult problems. We can give a resumé of them from information which Kiinckel d'Herculais, the indefatigable student of these voracious creatures, has been good enough to furnish. The migratory locust of northern Africa (Schistocerca peregrina) is a native of the Sahara Desert region. It passes all of its life there ordinarily, but in certain years, when spring arrives, the adult grasshoppers assemble in swarms and fly by stages toward the north, using their stops to feed, to couple, and to lay eggs. They thus arrive at the Atlas frontier and invade the plain of Algeria quite to the Mediterranean littoral. ,The journey brings about changes of color, and, from the red which distinguished them at the start, they change little by little to chestnut and then lemon-yellow. The invasion stops in May to June, the period that ends their existence, which is rather long and punctuated by a dozen egg-layings. The young coming from these eggs have a series of molts and travel in considerable armies which devour everything on their way. When they have acquired the wings characteristic of the adult they group together in swarms, and by nocturnal flights direct themselves toward the south to gain the Sahara regions. Rose-colored at first, they pass to gray and then to yellow. Their sexual maturity is then perfect: they couple, lay, and thus produce the hibernating generation. The same species makes similar migrations in Arabia and in India. In the Argentine Republic, the migrations of Schisto-cerca americana are made in the opposite way.
Here we have migrations similar to those of birds, but more difficult to interpret because the grasshoppers never have as guides older individuals of the same species. Without doubt they start out to find food, and the unknown objective toward which they go is indicated by the plants which serve them as intermediary excitants. But why is the return always the opposite from the going? And why are the two journeys always in the meridian direction? Can it be said that the grasshoppers are guided by the sun? By the winds? By a magnetic sense? By an hereditary memory? These hypotheses will have to be supported by proof. Climatic conditions must certainly play a great part in these movements, but we are far from knowing in a satisfactory way what they are. The problem is still almost wholly a problem.
The objective is known: return to the nest. When the processionary caterpillars enter their nest after a meal they are guided toward this known end by a conducting thread of silk which they have spun out in going and which they use in returning. All the caterpillars of a colony behave the same, and return to their nest without appearing to manifest any psychic aptitude.
But how is the return of adult insects effected when biological exigencies lead them to a determined spot,-beetles of the genus Cicindela which have the habit of returning to the natal burrow, the scarabs which make provisionnants of dung, and the numerous nest-making Hymenoptera whether they are predators or honey-gatherers!
Before taking up this problem we must establish a distinction between aerial, or flying, insects and terrestrial species which move without leaving the ground. As Forel says, the former "have a knowledge of places very different from that of non-winged forms,-much more comprehensive and much broader. The terrestrial beings have their horizon continually obstructed, which renders their direction by means of sight much more difficult." The behavior cannot be the same in the two cases, and Ferton has given a striking proof. When the predatory wasp, Am~ mophila heydeni, carries to its burrow the numerous victims intended to nourish its larvae, it arrives by flight and alights exactly at the orifice of the nest when it has captured the little measuring-worms which ordinarily form its prey; but when necessity obliges it to take other and much larger caterpillars, it has to return on foot and finds its nest only after a rather long search.