he had covered with opaque varnish, might almost, for instance, be quoted to prove the same with reference to the compound eyes. "Mes Camponotus aux yeux vernis," he says, "attaquaient et tuaient aussitôt une Formica fusca mise au milieu d'eux, la saisissaient presque aussi adroitement que ceux qui avaient leurs yeux. Ils déménageaient un tas de larves d'un coin de leur récipient à l'autre avec autant de précision qu' avec leurs yeux." *

On the other hand, Forel goes so far as to say that if the compound eyes are covered with black varnish, insects cannot even perceive light ("Cela prouve qu'elles ne voyaient plus même la lueur"). In fact, the use of the ocelli seems a great enigma, at least when the compound eyes are present.

We must remember that some other Articulata― spiders, for instance-possess ocelli only, and they certainly see, though not probably very well.

Plateau has made some ingenious observations, from which it appears that spiders are very short-sighted, and have little power of appreciating form. He found they were easily deceived by artificial flies of most inartistic construction; and he concludes that even hunting spiders do not perceive their prey at a greater distance than ten centimetres (about four inches), and in most cases even less. Scorpions appeared scarcely to see beyond their own pincers.

I have also made some experiments on this point with spiders (Lycosa saccata). In this species, which is very common, the female, after laying her eggs, collects them into a ball, which she surrounds with a silken envelope and carries about with her. I captured a

Recueil Zool. Suisse, 1887.

SHORT SIGHT OF OCELLI.

179

female, and, after taking the bag of eggs from her, put her on a table. She ran about awhile, looking for her eggs. When she became still, I placed the ball of eggs gently about two inches in front of her. She evidently did not see it. I pushed it gradually towards her, but she took no notice till it nearly touched her, when she eagerly seized it.

I then took it away a second time, and put it in the middle of the table, which was two feet four inches by one foot four, and had nothing else on it. The spider wandered about, and sometimes passed close to the bag of eggs, but took no notice of it. She wandered about for an hour and fifty minutes before she found it-apparently by accident. I then took it away again, and put it down as before, when she wandered about for an hour without finding it.

The same experiment was tried with other individuals, and with the same results. It certainly appeared as if they could not see more than half an inch before them -in fact, scarcely further than the tips of their feet.

I may also mention that they did not appear to recognize their own bags of eggs, but were equally happy if they were interchanged.

On the other hand, it must be remembered that the sac is spun from the spinnerets, and the Lycosa had perhaps actually never seen the bag of eggs. Hunting spiders certainly appear to perceive their prey at a distance of at least several inches.

Plateau has shown, in a recent memoir, that caterpillars, which possess ocelli, but no compound eyes, are very short-sighted, not seeing above one to two centimetres.*

"Rech. Exp. sur la Vision chez les Arthropodes." Bull. de 'Acad. Roy. de Belgique, 1888.

180

OCELLI OF CAVE-DWELLING SPIDERS.

Lebert has expressed the opinion* "that in spiders some of their eight eyes-those which are most convex and brightly coloured-serve to see during daylight; the others, flatter and colorless, during the dusk." Pavesi has observed † that in a cave-dwelling species (Nesticus speluncarum), which belongs to a genus in which the other species have eight eyes, the four middle eyes are atrophied. This suggests that they serve specially in daylight.

Returning for a moment to the ocelli of true insects, it seems almost incredible that such complex organs should be rudimentary or useless. Moreover, the evidence afforded by the genus Eciton seems difficult to reconcile with this theory. The species of this genus are hunting ants, which move about in large armies and attack almost all sorts of insects, whence they are known as driver ants, or army ants. They have no compound eyes, but in the place of them most species have a single large ocellus on each side of the head, while others, on the contrary, are blind. Now, while the former hunt in the open, and have all the appearance of seeing fairly well, the latter construct covered galleries, and seek their prey in hollow trees and other dark localities.

Insects with good sight generally have the crystalline lenses narrow and long, which involves a great loss of light. The ocelli are specially developed in insects, such as ants, bees, and wasps, which live partly in the open light and partly in the dark recesses of nests. Again, the night-flying moths all possess ocelli; while they are entirely absent in butterflies, with, accord"Die Spinnen der Schweiz."

"Sopra una nuova Specie di Ragui.”

PROBABLE FUNCTION OF OCELLI.

181

ing to Scudder, one exception, namely, the genus Pamphila.

On the whole, then, perhaps the most probable view is that, as regards insects, the ocelli are useful in dark places and for near vision.*

Whatever the special function of ocelli may be, it seems clear that they must see in the same manner as our eyes do that is to say, the image must be reversed. On the other hand, in the case of compound eyes, it seems probable that the vision is direct, and the difficulty of accounting for the existence in the same animal of two such different kinds of eyes is certainly enhanced by the fact that, as it would seem, the image given by the medial eyes is reversed, while that of the lateral ones is direct.

Forel, in his last memoir, inclines to this opinion.

( 182 )

CHAPTER VIII.

ON PROBLEMATICAL ORGANS OF SENSE.

In addition to the organs of which I have attempted in the preceding chapters to give some idea, and to those which from their structure we may suppose to perform analogous functions, there are others of considerable importance and complexity, which are evidently organs of some sense, but the use and purpose of which are still unknown.

*

"It is almost impossible," says Gegenbaur, "to say what is the physiological duty of a number of organs, which are clearly sensory, and are connected with the integument. These enlargements are generally formed by ciliated regions to which a nerve passes, and at which it often forms enlargements. It is doubtful what part of the surrounding medium acts on these organs, and we have to make a somewhat farfetched analogy to be able to regard them as olfactory organs."

Among the structures of which the use is still quite uncertain are the muciferous canals of fishes. The skin of fishes, indeed, contains a whole series of organs of whose functions we know little. As regards the

* "Elements of Comparative Anatomy."