Page:Hardwicke's Science-Gossip - Volume 1.pdf/216

live for hours without its head. No doubt many a young reader, and perhaps some old ones too, will wonder at this fact, and look for an explanation.

The fact becomes interpreted when it is known that flies, in common with other insects, do not breathe through their mouths and heads, but have a special and peculiar organization for carrying on this important and natural operation of life.

The head of the fly contains the compound eyes with which it sees; the mouth and tongue, with which it feeds, but cannot make sounds; and the antennæ, which are variously set down as organs of hearing, smelling, and feeling; whilst some say that they may be organs of a sense of which we may have no idea.

The body it is which contains the breathing apertures; these form a series of pores along each margin of the under surface, and are called spiracles, or stigmata.

The spiracles of insects form beautiful microscopic objects, and the amateur frequently makes the attainment of a number of them from different insects, one of his first designs in his earliest efforts at the preparation and mounting of objects. This, no doubt, arises from the great beauty of the preparations, and the ease with which subjects are obtained for dissection.

One of the best microscopic handbooks says that the "spiracles are generally visible on the exterior of the body of the insect, especially on the abdominal segments," and that "they are easily dissected by means of a knife or a pair of scissors."

This statement, generally correct, and sufficient in the case of the Water-beetle (Dytiscus), the Cock-chafer, and other insects, fails in the case of the Blow-fly; and the amateur will have much trouble and frequent disappointments in his endeavours, under this guidance, to discover the spiracle of the fly, so beautifully figured in the same handbook.

I make these observations, not to find fault with my handbook, which value as one of my best books, but to show the necessity and utility of such practical lints as can be best given through the medium of such a publication as Science Gossip.

The spiracles usually mounted are situated in the thorax, near to the head.

There are four spiracles in the thorax, two near the head, and two near the wings; they are not easily seen, and may be mistaken and not found.

To obtain them, first steep the fly in liquor potassæ for a week, then remove the head and abdomen, and divide the thorax into two parts, cutting it down along the centre of the back. The muscles, &c., can now be easily removed, leaving the two portions of skin tolerably clean; these must be well washed in water, on a glass slide, with a camel-hair pencil, until every remaining portion of muscle and trachea he completely removed. The spiracles will now appear under the microscope, or even to the naked eye, if they be held up against the light, as small oval openings in the skin.

The cleaned skin should now be placed on a glass slide and covered with a piece of strong glass, and subjected to gentle and constant pressure until it becomes nearly quite dry. It should then be steeped in clean turpentine for a few days (prolonged steeping bleaches too much), when it may be taken out, trimmed round the edges, and mounted in balsam.

The spiracle near the wing I have never seen figured in the handbooks, or mounted by the professional artist, yet it is interesting from its peculiarity. In it the branching proceeds from one side only and its position in the body of the insect is not symmetrical, being placed perpendicular rather than parallel to the direction of the spiracle near the head. There are spiracles in the abdomen also, but they are very small, and not worth mounting.

I would fain give some directions to the inexperienced amateur in microscopy as to the spiracle of the larva of the fly, but I find I have done enough for one occasion. The subject, though humble, will