« AnteriorContinuar »
Some butterflies have, at the angles joining these hexagonal facets, a hair standing out, which, repeated at thousands of such angles, gives to the eye a rough or hairy look. Such hairy eyes are represented at No. 2. They are very dark. It is in these eyes difficult to see anything but the clothing of hairs.
But other butterflies, such as the white ones, have no hairs in the eye. The eye is then quite smooth and transparent, as at No. 1. Holding such a butterfly closely, with its head turned sideways to the sunlight, a very lovely picture is seen displayed, of an arrangement such as is figured at l, of dark hexagonal spots divided by a network of light-coloured lines. This is brilliantly lit up, and is exactly in appearance like the effect produced by those glass paper-weights which have a picture inside them, magnified by the round surface of the enclosing glass.
The most convenient fly to see this in is the small cabbage one, for it is more easily caught than the larger white. This the writer found from having told his little boy to catch some white butterflies in the kitchen-garden. He always brought in the smaller white, saying that the others were too quick for him. On seeking a reason for this slowness of sight, it is found that the smaller white has the upper part of the eyeball clouded with an opaque shading, as at PQ, which prevents it seeing anyone approach just above it, though leaving the sight clear at the sides.
Taking this conveniently blind little fellow, then, as the subject of examination, easily renewed, the theory of explanation of this appearance evidently is this :
The hexagonal network on the surface of the eye, as at No. 1, is reflected down on the interior retina in its very small natural size. Then the whole outer surface of the eye, considered as one surface, acts like a globular lens, magnifying the interior reflection till it is easily discerned with the observer's unaided eye, and assumes the very beautiful, picture-like appearance which lasts while the eyes are fresh and moist. Thus the butterfly's eye is a natural magnifying glass, exactly like what is known as a Stanhope lens. The observer uses it as at u, and sees the cells much enlarged.
That the hexagonal appearance is not exactly the same in outline as the actual network on the cornea of the eye is due to the fact of the retina upon which the reflection is cast not being exactly in the focus of the curved lens surface. This leads to an overlapping of the outlines of the facets, making them seem much thicker than the dividing lines between the facets of the surface really are, as at fig. O.
The proboscis or trunk of the butterfly next claims attention.
It is a tube of double structure, each half being easily separated or closed at pleasure by the fly; when not used it is nicely coiled up under the chin. When needed for sucking the juices of plants it is uncoiled, and, the two half tubes being kept together by neat little teeth interlocking, the tube draws up the juice into the mouth of the fly. To enable it to do this, each half of the tube is furnished with a multitude of fibres or nerves, which are reticulated across from side to side, making it easily compressible. The air is exhausted in the tube by this compression, and its place is at once filled by the juice which rises in the tube. Figs. R and s show the general appearance of the tube, fig. 1 the reticulated fibres for exhausting the air. Near the point of the trunk, an additional means of suction is probably furnished by a number of little protuberances, like nipples, which no doubt help to draw up, like a sponge, by capillary attraction, the juice when only in small quantities. These, however, do not seem to be perforated with suction holes : even when photographed with the microscope on a large scale, they show no sign of any channel through them.
This trunk is, in use, thrust into the nectary of flowers, and in its passage is often covered with pollen. Hence, those butterflies which flit from flower to flower indiscriminately, such as the Peacock or Admiral, would be likely to carry from one flower to another pollen grains, of a useless or different kind, which might interfere with the proper fructification of the flowers they are brought to. Now here comes in one of those wise adaptations of the insect to the vegetable world, which proves the superintendence of one Master Mind. Such butterflies are furnished with a long pair of soft brushes, instead of their forelegs; with which, while using the two other pair of legs to support themselves, they wipe and clean their trunks after each time of using. So that all adhering pollen is wiped off, and not carried to other plants where it is not needed.
In other insects, such as the bee, the provision for preventing pollen mixing is accomplished by a beautiful instinct, which leads the bee to keep only to one class of flower in each trip it takes for honey.
But the butterfly having no settled home, spends its little life in roaming anywhere; and hence has this modified change of legs into brushes, as seen at fig. B.
Its roaming seldom lasts much longer than necessary to meet some mate, and deposit the eggs, like carved ivory balls, as at fig. C, on the plant which will be best fitted for nourishing its young when hatched.
The life of one that has not yet met its mate may be prolonged artificially for weeks. Nature's great object being the continuation of the species, life is prolonged in the hope of so doing. Then if allowed its liberty it still seeks its mate; and perhaps this accounts for the hibernation of such specimens as are the earliest every year; creeping out with faded glories, to try if they can accomplish their destiny, which was not fulfilled before the winter.
Having settled their eggs on the chosen plant, the parent butterfly never sees them come to maturity : a fair lesson for faith to rejoice in. Trust may never be felt by the insect, but is evidently taught us through it. And how wonderful is the destined change of the eggs it trusts, while itself will not be there to watch! At all times the glorious change from a first stage of grub or caterpillar, through the trance-like sleep of the chrysalis, into the joyous creature that might well be called a "flutter-by," has been a theme for poets and philosophers.
BY DR. RICHARDSON, F.R.S.
THE PHENOMENON OF SLEEP. 6 MHE twinkling of oblivion,” as Wordsworth exquisitely
I defines the phenomenon of sleep, has, from the time of Hippocrates to the present hour, engaged the attention of thoughtful minds. Poets have found in the phenomenon subject matter for some of the most perfect of their works. Menander exalts sleep as the remedy for every disease that admits of cure; Shakespeare defines it, “The birth of each day's life, sore labour's bath ; " Sir Philip Sydney designates it, “ The poor man's wealth, the prisoner's release ;” and wearied Dryden sings of it
“Of all the powers the best. Ob ! peace of mind, repairer of decay,
Whose balıns renew the limbs to labours of the day." As to the philosophers and the physicians who have said and written on sleep, I dare hardly think of them, lest I should commit myself to an historical volume instead of a short physiological essay; so I leave them, except such as are simply physiological, and proceed on my way.
Perfect sleep is the possession, as a rule, of childhood only. The healthy child, worn out with its day of active life, suddenly sinks to rest, sleeps its ten or twelve hours, and wakes, believing, feeling, that it has merely closed its eyes and opened them again; so deep is its twinkle of oblivion. The sleep in this case is the nearest of approaches to actual death, and at the same time presents a natural paradox, for it is the evidence of strongest life.
During this condition of perfect sleep, what are the physiological conditions of the sleeper? Firstly, all the senses are shut up, yet are they so lightly sealed that the communication of motion by sound, by mechanical vibration, by communication of painful impression, is sufficient to unseal the senses, to arouse the body, to renew all the proofs of existing active life. Secondly, during this period of natural sleep the most impor
s alsoto work it mand influence direct, control there
tant changes of nutrition are in progress; the body is renovating, and if young is actually growing ; if the body be properly covered, the animal heat is being conserved and laid up for expenditure during the waking hours that are to follow; the respiration is reduced, the inspirations being lessened in the proportion of six to seven as compared with the number made when the body is awake; the action of the heart is reduced ; the voluntary muscles, relieved of all fatigue and with the extensors more relaxed than the flexors, are undergoing repair of structure and recruiting their excitability; and the voluntary nervous system, dead for the time to the external vibration, or as the older men called it “ stimulus” from without, is also undergoing rest and repair, so that when it comes again into work it may receive better the impressions it may have to gather up, and influence more effectively the muscles it may be called upon to animate, direct, control.
Thirdly, although in the organism during sleep there is suspension of muscular and nervous power, there is not universal suspension; a narrow, but at the same time safe, line of distinction separates the sleep of life from the sleep of death. The heart is a muscle, but it does not sleep, and the lungs are worked by muscles, and these do not sleep; and the viscera which triturate and digest food are moved by muscles, and these do not sleep; and the glands have an arrangement for the constant separation of fluids, and the glands do not sleep ; and all these parts have certain nerves which do not sleep. These all rest, but they do not cease their functions. Why is it so ?
The reason is that the body is divided into two systems as regards motion. For every act of the body we have a system of organs under the influence of the will, the voluntary, and another system independent of the will, the involuntary. The muscles which propel the body and are concerned in all acts we essay to perform, are voluntary; the muscles, such as the heart and the stomach, which we cannot control, are involuntary. Added to these are muscles which, though commonly acting involuntarily, are capable of being moved by the will : the muscles which move the lungs are of this order, for we can if we wish suspend their action for a short time or quicken it; these muscles we call semi-voluntary. In sleep, then, the voluntary muscles sleep, and the nervous organs which stimulate the voluntary muscles sleep; but the involuntary and the semi-voluntary muscles and their nerves merely rest : they do not veritably sleep.
This arrangement will be seen, at once, to be a necessity, for upon the involuntary acts the body relies for the continuance of life. In disease the voluntary muscles may be paralysed, the
w suspend the call semind the nervene