We have limited ourselves so far to the study of the essential phenomena by which the activity of the entomological world manifests itself to the observer, and our study has brought us to rules which permit us to establish the variety of these phenomena, to speculate as to their origin, and to follow their connections. This will be the fundamental part of the present work, since it will put us-indeed, has put us, I think-in the possession of a working method.
But entomological activity in general presents the greatest complexity, and it is on account of the need of further study that we have been able simply to separate the principal phenomena. It remains now to envisage what we have seen manifest itself under natural conditions,-that is to say in its normal development,-with the multiplicity and the subordination of the phenomena which constitute it. This will be our object in the second part of this work.
Moreover, we cannot pretend to discuss in a small number of pages all the forms in which the activity of the entomological world is exhibited. A snecial volume will be necessarv to do this, and this we will present later. For the present it will suffice to study some of these forms, chosen from among the most suggestive and the most striking,-not only those which seem to defy all explanation, for it is necessary first to face the difficult problems, but even those which appear actually insolvable.
Therefore we are going to pass in review, in a series of chapters, the relations of insects to flowers, the mechanism of far-away orientation, the division of the sexes with the Hymenoptera, and the origin of the social fife of these insects.
Before beginning this study, it will be useful to note that insects and other arthropods are furnished with sensorial organs whose peripheral cells, impressed by excitants, are relayed to cerebral cells where these impressions are transformed into sensations. Most of the sensorial impressions of articulates are received by means of hairs connected with nerve cells. Tactile hairs may be distributed over the whole body, but are principally found on the antennae and the palpi of the mouth apparatus. These two organs, the first especially, carry also the olfactory nerve terminals, ordinarily situated at the bottom of little crypts. It is really with the hairs situated on the antennae that the articulates perceive sound vibrations, for they are deprived of any auditory vesieie, with the exception of certain Crustacea. As to taste, that has its seat in the mouth, where it operates without doubt also through nerve termini. The organs of sight are quite different and much more complicated. They are composed of a corneal lens, of a hyaline vitreous body, and of retinal cells which receive the images. With most adult insects and with the Crustaceans the two visual organs are composed of numerous elementary eyes juxtaposed, which function independently of one another and, by the juxtaposition of the luminous spots which they produce upon the retinas, give a direct image of the object. With the other articulates and with the larvss of insects the eyes are variable in number and are reduced to simple ocelli. The ocelli, which are simple eyes, coexist, with many insects, with composite eyes. They .seem especially to give a luminous impression, while the composite eyes take advantage of the: vision of forms, and better appreciate movements. The bees are blind for red, and its complimentary color, blue-green. Ants are sensitive to ultra-violet.