58

COMPLEX STRUCTURE OF THE ANTENNÆ.

are many modifications; for instance, complex pits often arise from the confluence of several small ones. The structure of the antennæ is then very complex, and increases with the importance of the antennæ in the life of the insect. Among the Hymenoptera, Lyda has about 600 pits; Tenthredo, 1200; Sirex, 2000; Pompilus, 3000; Paniscus, 4000; Ichneumon, 5000; Hylous, 6000; the wasp (Vespa), about 13,000 pits and 700 cones; the blowfly, 17,000; the hive bee, according to Hicks, about 20,000 pits and 200 cones. Among beetles (Coleoptera) the numbers are generally small, but the cockchafer (Melolontha) possesses, according to Hauser, on each antenna as many as 35,000 in the female, and 39,000 in the male. Moreover, it is significant that in those species where the females are quiescent and are actively sought out by the males, the antennæ are much less highly developed in the female sex than in the male.

As already mentioned, the antennæ probably serve partly as organs of touch, and in some cases for smell.

On the other hand, I do not believe that touch and smell are the only two senses possessed by the antennæ. Forel and I have shown that in the bee the sense of smell is by no means very highly developed. Yet their antenna is one of those most highly organized. It possesses, as I have just mentioned, besides 200 cones, which may probably serve for smell, as many as 20,000 pits; and it would certainly seem unlikely that an organization so exceptionally rich should solely serve for a sense so slightly developed.

Much as these antennal structures differ from one another in form, arrangement, and structure, they are all reducible to one type-to a hair-more or less de

VARIOUS USES OF ANTENNÆ.

59

veloped, more or less deeply seated, standing in connection with the ganglionic cells, and so with the cerebral ganglia. Even the long-necked "bottles" (Fig. 43, k) may be regarded as an extreme form of this type, especially if the inversion at the end can be, as seems probable, regarded as a hair.

All entomologists are agreed that some of the antennal hairs serve as organs of protection, and others as organs of touch. The evidence is, as we have seen, very strong, that some of them serve as organs of smell. They fulfil, therefore, at least three different functions, and when we consider their manifold variety, there is not only no à priori improbability, but, on the contrary it seems very probable that some of them, at least, perform some other function in the animal economy.

There is, indeed, strong reason, as we shall see in the next chapter, to believe that, in some cases at any rate, the antennæ act also as ears; while some of these peculiar antennal organs, though obviously organs of sense, seem to have no special adaptation to any sense of which we are cognisant.

CHAPTER IV.

THE SENSE OF HEARING.

THE sensation of sound is due to vibrations of the air striking on the drum of our ear. The intensity of the sound depends on the extent or amplitude of the soundwave; while the pitch of the tone depends on the frequence of vibration, and consequently on the number of waves which strike the ear during a given interval. The fewer the number of vibrations in a second, the deeper the sound; the more numerous, the shriller it becomes. Our pianos generally begin with the C of 32 vibrations in a second, and extend to A""" of 3520 vibrations. The number of vibrations for the tone A', which is that of the hum of a bee, is about 440 in a second. If the vibrations are fewer than 30 in a second, they produce only a buzzing and groaning sound, while the shrillest sound we can hear is produced by about 35,000 vibrations in a second.

It may seem curious that there should be any dif ficulty in ascertaining whether an animal can hear. But, in the first place, in order to experiment on them, we are often obliged to place them in situations very unlike those to which they are accustomed ; and, secondly, it is by no means always easy to say

ORGANS OF SOUND-MOLLUSCA-CRUSTACEA. 61

whether they are affected by a real noise, or whether they are merely conscious of a concussion or vibration.

As regards the lower animals, it appears to me, I confess, that many organs have been described as auditory, on grounds which are anything but satisfactory. At the same time, it cannot be doubted that many of the lower animals do possess the power of hearing, especially as some have elaborate organs for the production of sound.

Among the lowest groups, none of the Protozoa or Colenterata are known to produce sounds, and in the Mollusca, also, the power is very rare. The Pectens, which are the most lively of bivalves, moving actively by the sudden opening and closing of their valves—as Pliny says, "Saliunt Pectines et extra volitant seque ipsi carinant "-also produce in the same way a certain sound, which Aristotle* gives as an exceptional case among the Mollusca.

Nor is the production of sound much more frequent among the Crustacea. In one genus of crabs (Ocypoda), the claw bears a rasp, or file, which can be rubbed against a ridge on the basal segment of the limb, and thus produces a harsh, jarring sound. Some of the lobsters also (Palinurus) make a noise by rubbing one segment of the antennæ against another; but, considering that the ear is well developed in this group, it is rather remarkable how few of them are known to possess the power of producing sounds.

Passing on to the insects, the song of the Cicada has been celebrated from time immemorial; the chirping of the crickets and grasshoppers is also familiar to us all. For the reasons, however, already alluded to in the

"Historia Animalium."

62

INSECTS-LOCUSTS.

preceding chapter, no insect possesses a true voice. The sounds they make are produced in various ways--for instance, by the wings or the spiracles, by rubbing one part of the body against another, etc.

The power of producing sounds audible to us is possessed by many insects scattered sporadically through all the great groups.

In many of these cases, the power of producing sound is confined to the males. Their sounds are really lovesongs.

In Locusts, as Westwood says, "The stridulating powers of these insects must have attracted the notice of every one who has walked through the fields in the autumn. Unlike the insects of the two preceding families, it is owing to the motion of the hind femora, either conjointly or alternately rubbed against the sides of the wingcovers, that the sound is produced, the insects resting on their four anterior legs during the operation; the veins of the wingcovers being considerably elevated, so as to be easily acted upon by the rugose inner edge of the thigh. Some species, according to Goreau, may be observed to execute this movement without producing any sound perceptible to our ears, but which he thinks may be perceived by their companions."

Fig. 44.-Leg of Stenobothrus pratorum (after Landois).

* The females are not, however, invariably dumb. In Ephippigera both sexes are able to produce a sound, which, however, is not very loud.

t Westwood, "Modern Classification of Insects."