The insect world : being a popular account of the orders of insects, together with a description of the habits and economy of some of the most interesting species / by Louis Figuier.

Louis Figuier

Date:: [1872]

Credit: The insect world : being a popular account of the orders of insects, together with a description of the habits and economy of some of the most interesting species / by Louis Figuier. Source: Wellcome Collection.

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$elliptical or conical eyes, which one generally finds among insects, are less perfect than those referred to above. “The différences which exist in the organisation of the eye among insects are explicable, to a certain point, on the theory which we are about to explain in a few words. Those species which live in the same substances on which thev feed, and those which are parasitical, hâve small and flatteneu eyes ; those, on the contrary, which hâve to seek their food, and which need to see objects at a distance, hâve large or very convex eyes. For the sanie reason the males, which hâve to seek their females, hâve larger eyes than the latter. The position of the eyes dépends also on their size and shape; those which are flat, and hâve consequently a short fïeld of vision, are placed close together, and rather in front than at the sides of the head, and often adjoining. Spherical and conveX eyes, on the contrary, are placed on the sides, and their axes are opposite. But the greater fïeld of vision which they are able to take in makes up for this position.” Almost ail insects are provided with a pair of compound eyes, which are placed on the sides of the head. The size and form of these organs are very variable, as we shall presently see. They are generally placed behind the antennæ. Although simple eyes (ocelli or stemmata) are common, they do not exist in ail the orders of insects. They are generally round, and more or less convex and black, and there are three in the majority of cases. When there is this number they are most frequently placed in a triangle behind, and at a greater or less distance from the antennæ. Under the cornea, which varies in convexity, is found a transparent, rather hard, and nearly globular body, which is the true crystalline resting on a mass, which represents the vitreous body. This vitreous body is enclosed in an expansion of the optic nerve. Besides these, there is a pigment, most frequently red-brown, sometimes black, or blood-red. The organisation of these eyes is analogous to the eyes of fishes, and their refractive power is very great. With these eyes insects can only see such objects as are at a short distance. Of what use then can stemmata be to insects also provided with compound eyes ? It has been remarked that most insects having this arrangement of eyes feed on the pollen of plants, and it has been surmised that the stemmata enable them to distinguish the parts of the flowers. The antennæ, commonly called horns, are two flexible appendages, of very variable form, which are joined to different parts of the head, and are always two in number. The joints of which they are made$

The insect world : being a popular account of the orders of insects, together with a description of the habits and economy of some of the most interesting species / by Louis Figuier.

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