Page:Hardwicke's Science-Gossip - Volume 1.pdf/271

generally become smaller as they approach the extremity of the body, the last spiracle being almost a mere point, and without any complexity of structure. This order is reversed in the large water-beetle (Dytiscus), for in this case, the spiracles become larger as the segments of the abdomen become smaller, and the last segment contains the two largest spiracles.

The large terminal spiracle (fig. 2) of the Dytiscus is accurately figured in the "Micrographic Dictionary," but on too small a scale to show the beauty of the object. Another figure has been given in a publication which obtained a wide circulation, but in that figure I failed to recognise the spiracle I had so frequently mounted.

The upper and under plates of the abdomen are connected by a membrane and in this are the

spiracles; here, also, may the parasite (fig. 3), which is of a red colour, be frequently found and sometimes so deeply imbedded in the skin, and surrounded with a brown incrustation, that it is very difficult to take one out without tearing it to pieces.

Any one who has stood beside a pond, and observed the Dytiscus on the surface of the water, cannot fail to have remarked, that while the heavy portion of the body is immersed with the head inclined downwards, the two lighter and terminal segments are above the surface. In this natural and easy position, four of the largest spiracles are exposed for the reception of the supply of air, for which, no doubt, the insect rose to the surface.

Now, if the large spiracles were in the upper segments it would be necessary for the insect to expose its whole abdomen to the air, and this would require such a continual effort as would be inconsistent with the insect's ease.

In the case of the larva of the fly, when the head is buried in the tissue it is eagerly devouring, and the sides are bathed in the juices by which they are surrounded, the extremity containing the spiracles freely exposed to the air.

The above afford instances of the general conformity to system so prevalent in Nature, with those deviations in minor detail which, so far from being discrepancies, are among the harmonies of Creation.

Armagh.Lewis G. Mills, LL.B.

In a former number we gave a figure and description of a species of parasitic fungus commonly found in the autumn on the leaves of the Bramble. We now direct attention to a species of Brand, which is parasitic on the leaves of the Meadow-sweet (Spiræa ulmaria). This is not probably so common as the bramble brand, but can generally be found in wet solutions. The appearance which it presents is that of a number of small black spots or tufts on the under surface of the leaves, not larger than the head of a pin. These tufts are often scattered at some distance apart from each other, but are occasionally closely peppered over the whole under surface. When examined by a quarter of an inch objective, these pustules will be found to consist of an aggregation of three-celled spores, which are formed by one transverse and one longitudinal partition. The outer surface, or epispore, is studied with warts or protuberances. Each spore is furnished with a slender stipe or peduncle. It will be observed that this species differs considerably in microscopical characters from the bramble brand, hence botanists regard it as belonging to a different genus, and one which has only this single representative in Britain. To examine the present object it is only necessary to remove one of the pustules on the point of a sharp penknife, and transfer it to a glass slide, with a drop of water or spirit, and having covered it with thin glass, to submit it to a quarter of an inch power. A slight pressure and sidelong motion of the cover when laid on will serve to separate the spores from each other. Specimens may be mounted for the cabinet with balsam and chloroform.M. C. C.