There is yet another point to be considered, and that a very curious and at present unexplained one. Upon careful investigation we find that the tracheæ extend beyond the limits of the circulation, showing that they must serve some secondary purpose in addition to that generally attributed to them. For nature provides nothing in vain, and would not without good and sufficient reason have carried the breathing-tubes farther than necessary for their primary object of regenerating the blood. As to what this purpose may be, however, we have no certain knowledge, and can only conjecture that it is in some way connected with the olfactory system. It is well known that the sense of scent is in many insects very highly developed, enabling them to ascertain the position of their food while yet at a considerable distance. Burying-beetles and blowflies, for instance, will detect the faintest odor of putrid carrion, and will wing their way without hesitation to the spot whence it proceeds. Ivy-blossom, again, will attract almost every butterfly and moth in the neighborhood, and this clearly by reason of its peculiar fragrance. It may be, therefore, that the perfection of the organs of scent in insects is due to the fact that they are distributed throughout the body, instead of being localized as is the case with animals higher in the scale. That they must be connected with the respiratory apparatus would seem, judging by analogy, to be indisputable, for, so far as we know, an odor cannot be appreciated unless the air containing it be allowed to pass more or less rapidly over the olfactory nerves. And in no other part of an insect's structure could this requisite so well be observed as in the tracheæ themselves, through which a stream of air is continually passing, and which penetrate to the remotest parts of the body. With so wonderful a respiratory system, it naturally follows that an insect must be particularly susceptible to the effects of any poisonous vapor, which, being immediately carried to all parts of the body, must speedily be attended by fatal results. And this is the case in a very marked degree. A moth or beetle, which will live for hours, and even days, after receiving an injury which would cause instant death to a more highly organized being, will yet succumb in a few seconds to the fumes of ether or chloroform, owing to the fact that the deadly influence is simultaneously exerted upon all the nerve-centres of the body, instead of being confined to one or two alone.