There cannot be a better illustration of this latter effect which I have mentioned the study of natural history produces on the mind, than that afforded us by the history of the birth and after life of the insect I have headed this article with--“the Gadfly.” Strange and wonderful though the transformations be to which the butterfly and many other individuals of the insect world are subject, those of this little creature far surpass them all. Many of my readers are well acquainted with that fly which in the latter part of summer is seen to be so annoying to the horse, buzzing about him, and every now and then dashing itself with some degree of violence against his sides and legs. This motion, to all appearance, is without design; but a closer study of the habits of the insect will show that, far from being the effect of chance, it is one of paramount importance to the existence of the fly, as on it depends the continuation of its species. If attentively observed, it will be found that it is the female of this fly alone who resorts to this peculiar motion; this she does to deposit her eggs in the hair of the horse, to which they at once become attached by a gelatinous fluid surrounding them; by this mucus they are enabled to retain their hold for a few days, during which time they are fitted to be hatched, and the slightest touch will liberate a little worm they contain. The horse, in resorting to the common practice of licking himself, breaks the egg, and the small worm contained in it adhering to the tongue of the animal, is conveyed with the food into the stomach; there it clings by means of hooks placed at either side of its mouth, and its hold is so tenacious that it will be broken before it can be detached. Here, in this strange abode, changing as it were its nature in becoming a parasite, it remains for the whole of the winter, feeding on the mucus of the stomach. At the end of the ensuing spring, having reached its full perfection in this secondary state, led by that instinct which regulates all the animated creation, from man to a monad, it detaches itself from the cuticular coat, and is carried into the vilous portion of the stomach with the food, passes out of it with the chyme, and is at length evacuated with the feces. The larva or maggot, now a second time changing its nature, seeks shelter in the ground, and after some time becomes a chrysalis; in that helpless state it lies for some weeks, when, bursting from its deathlike sleep, it wakes into life and activity in the form of a perfect fly. There is hardly a parallel to this wonderful chain of causes and effects, and effects and causes, to be met with in all the varied and mysterious workings of nature; scarcely one which exhibits so many acts apparently so unconnected with the ultimate results. V. IRISH ODDITIES--No. I. SNAP RIVERS.