This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(843). The respiratory organs of the Insecta, as well as their circulatory apparatus, are constructed upon peculiar principles, and are evidently in relation with the capability of flying which distinguishes these minute yet exquisitely-constructed articulated animals. Any localized instruments for breathing, whether assuming the shape of branchiae or lungs, would materially have added to the weight of the body, and moreover have rendered necessary an elaborate apparatus of arteries and veins for conveying the blood to and fro for the purpose of purifying it by securing its exposure to the influence of air. By the plan adopted, however, all these organs are dispensed with; and the organs of respiration, so far from increasing the weight of the animal, actually diminish its specific gravity to the greatest possible extent. The blood, in fact, in insects is not brought to any given spot to be exposed to oxygen; but the air is conveyed through every part of the system by innumerable tubes provided for that purpose; and thus all the complicated parts usually required to form a vascular system are rendered unnecessary.
These observations, however, only apply to the insect in its perfect state; for in the larva and pupa conditions, where flight is not possible, various additional organs, frequently of considerable bulk, are provided, that we shall speak of in another place. If we examine the external skeleton of any large insect (abeetle, for example), we shall find, between the individual segments of the body, minute apertures or pores (spiracles) through which the air is freely admitted:these openings, ten in number on each side of the body, are situated in the soft membrane interposed between the different rings, and not in the rings themselves, - a provision for the purpose of allowing their orifices to be opened or closed at pleasure, instead of being rigid and motionless. The margin of the spiracle is frequently encompassed by thick horny lips, which may be approximated by muscles provided for the purpose, so that the opening can be shut at pleasure, in order to exclude any extraneous substances that might otherwise obtain admission.
In many insects, indeed, especially in beetles which crawl upon the dusty ground, an additional provision is necessary to prevent the entrance of foreign matter; and in such cases the spiracles are seen to be covered with a dense investment of minute and stiff hairs, so disposed as to form a sieve of exquisite fineness, - a beautiful contrivance by which the air is filtered, as it were, before it is allowed to pass into the breathing-tubes, and thus freed from all prejudicial particles. From every spiracle is derived a set of extremely delicate tubes (tracheae),that pass internally, and become divided and subdivided to an indefinite extent, penetrating to every part of the body, and ramifying through all the viscera; so that air is thus supplied to the entire system. Upon more minutely inspecting these air-tubes, they are found to assume various forms in different parts of the body, - being sometimes simple tubes of exquisite delicacy; in other cases they present a beaded or vesicular structure; and in many insects they are dilated at intervals into capacious cells or receptacles, wherein air is retained in great abundance.
The beautiful figure given in the preceding page (fig. 161), taken from Straus-Durckheim's elaborate work upon the anatomy of the Cockchafer, will illustrate this arrangement. The spiracles, situated at the points respectively marked by the letters a, b, c, d, e, f, g, h, i, open into two wide air-trunks, disposed longitudinally along the whole length of the body: from these, innumerable secondary branches are given off, many of them being seen to dilate into oval vesicles, from which smaller tracheae proceed; while others, without any vesicular enlargement, plunge at once into different textures, and supply the viscera and internal organs. The muscular system, the legs, the wings, the alimentary canal, and even the brain itself, are permeated in all directions by these air-conducting tubes; and thus the oxygen penetrates to every corner of the body.
* Physiologische Untersuchungen uber die thierische Haushaltung der Insekten. 8vo. 1817.
Fig. 161. Respiratory apparatus of Melolontha vulgaris.
(844). There is one circumstance connected with the tracheae which is specially deserving of admiration, whether we consider the obvious design of the contrivance, or the remarkable beauty of the structure employed. It is evident that the sides of canals so slender and delicate as the tracheae of insects would inevitably collapse and fall together, so as to obstruct the passage of the air they are destined to convey; and the only plan which would seem calculated to obviate this would appear to be, to make their walls stiff and inflexible. Inflexibility and stiffness, however, would never do in this case, where the vessels in question have to be distributed in countless ramifications through so many soft and distensible viscera; and the problem therefore is, how to maintain them permanently open, in spite of external pressure, and still preserve the perfect pliancy and softness of their walls. The mode in which this is effected is as follows: - Between the two thin layers of which each air-vessel consists, an elastic spiral thread is interposed (fig. 102, a), so as to form by its revolutions a firm cylinder of sufficient strength to ensure the calibre of the vessel from being diminished, but not at all interfering with its flexibility, or obstructing its movements; and this fibre, delicate as it is, may be traced, with the microscope, even through the utmost ramifications of the tracheae, - a character whereby these tubes may be readily distinguished.