-9.41029 し

3° 38′ 6 ι u

0.99776

The use of water instead of balsam, and perhaps the "setting' of the lens next behind the front, will slightly reduce the angle above given, but not by any material amount.

The objective, as will be seen in the figure, is composed of seven lenses. Lettering them in order. from the back towards the front, the elements, as given by Mr. Tolles, are as follows:

The distances between c and d and between ƒ and g are given by Mr. Tolles as very small, but not recorded in his memorandum; I have taken for the first 0.01 and for the second 0.006 of an inch. The adjustable distance between e and ƒ Mr. Tolles gives as 0.07 when adjusted for a cover 0·014 of an inch thick. I have used 0.065 of an inch, which slightly increases the aperture, and is theoretically correct for balsam contact, or a cover 0.016 of an inch thick.

NOTES AND MEMORANDA.

Insect Digestion and Development.-The Commission of the French Academy, reporting upon the researches of M. Jousset de Bellesme, to whom the Thore Prize was assigned, make the following remarks: Until lately the phenomena of digestion in insects were but slightly illustrated by experience, although some observations inclined to the belief that the stomach fluid of insects had the same properties as that of vertebrates. Researches on the Arachnids, animals closely allied to insects, seemed to confirm this opinion. A foreign savant, known for some good work, was however led to affirm that the digestion of insects, whether carnivorous or phytophagous, was effectuated under conditions different from those of vertebrates, and he regarded the secretions of their digestive tubes as neutral or alkaline. M. de Bellesme has made fresh observations, which appear quite decisive. Seeing that serious errors are easily made in endeavours to collect the fluid secreted by the stomach, he selected insects, such as Blattidæ, which have stomachs with sufficiently large cæcal prolongations to allow of the gastric juice being extracted without admixture of other matters, and by these means he has shown that the digestion of albuminoid substances takes place solely in the stomach, as in the case of higher animals. He has produced artificial digestions with the acid fluid taken from the cæcal tubes of the stomach.

By experiments of a similar kind he has demonstrated, as most naturalists supposed, that the sole agent for the digestion of amylaceous substances is supplied by the salivary glands.

Another question in insect life has also received from him a final solution. Observers have often been astonished to see a winged insect of considerable dimensions emerge from a comparatively small pupal envelope. The ant lion affords a striking example, and still more so the dragonflies and butterflies. At this moment the air filling the tracheal tubes, and a sudden activity of the circulation giving rise to pressure upon the still soft tegumentary parts, seemed to explain the increase of volume, but there was still a difficulty in understanding how the wings were unfolded, and on this point the observations of M. de Bellesme on dragonflies are conclusive.

When witnessing the development of flies, Réaumur admitted that air was introduced to expand the wings. He said, "the insect drinks in air to fill his body." Evidently he took no account of the way in which the air made its entrance into the insect's organism, and for this reason many authors imagined the remark of the great naturalist to be without scientific value. Some observers considered the expansion of the body and the unfolding of the wings to be caused by pressure of the blood, and the fine researches of Mr. J. Künckel on Volucella demonstrated that an afflux of blood had the result of enlarging the head and producing an extension of the wings. He attributed the movement of the blood to the contractions of the thoracic muscles. No doubt, when a fresh activity affects the whole organization, the muscles enter into play; but the animal is in repose-it has not yet taken flight-the muscular action seems too feeble to account for the powerful results that are seen.

M. Bellesme made his principal study on Libellula depressa, and watched its disengagement from the pupal envelope. He saw the body swell out, then the head enlarge, the eyes grow round, and the wings spread out in an uniform manner. At the moment of extreme distension, the volume of the body exceeded the dimensions it retained. While these phenomena were observed, little air penetrated the respiratory organs, but a slight prick of the abdomen produced instant collapse. In dissecting the swollen insects under water he always found the digestive tube filled with air and prodigiously extended. When the dragonfly disengaged his head from the pupal envelope, it took in air by its mouth as Réaumur supposed, and soon accumulated a considerable supply in the oesophagus, stomach, and intestine. In its dilatation the digestive tube drove the blood vigorously towards the sides of the body, towards the head, and the appendages, and by these means expanded the wings in a few minutes. Soon after the intestinal canal emptied itself, the body became flatter, acquired its true form and coloration, and respiratory movements were established. Το leave no doubt as to the part performed by the air that entered the mouth, a simple experiment suffices. At the right moment let the entry of the air to the digestive tube be arrested, or let it escape by a prick. In each case the wings do not unfold, or they stop expanding.*

Improvements in the Rivet-Leiser Microtome.-As this microtome has an increasing reputation and is becoming more widely known, it will not fail to prove interesting if some material improvements which have been made in it are published; they have all stood a thorough test, in every way, in trials made at the Zoological Institute. Everyone has experienced that when the object gets in a certain position the knife has to be drawn too far back, in which case it may easily happen that the knife-carrier slips out of its place and falls with the knife on to the table or into the lap of the operator. In the first case the knife only is damaged, but the second case is positively dangerous, as the carrier is of considerable weight, and it is a common though a bad habit to clutch instinctively with the hands anything falling off the table. This defect is remedied as follows:-In the perpendicular side of the rut for the knife a horizontal groove is made, stopping short of the farther end, and about 1.5 mm. deep. On the knife-carrier is a knob, which moves without friction in the groove during the process of cutting. It is evident that when the knife is drawn too far back the knob comes to the end of the groove, and the knife is prevented from falling out.

The object to be cut, as is well known, can only be raised 1 centimetre by the inclination of the sloping plane. When the object is larger it becomes necessary to loosen the screw and fix it higher. A number of suitable metallic plates have now been made which, when the cutting is commenced, are laid under the knife and screwed fast to it. When the object has been brought to the top, one or more of the plates are removed from beneath the knife, so that the latter sinks lower, according to the thickness of the plates; and the object requires, consequently, only to be drawn back again to enable the cutting to be

continued.

VOL. I.

'Comptes Rendus,' Jan. 7, 1878.

M