several very lively individuals. I pierced the dorsal vessel in different parts at the time when the liquid had flown back, but I never perceived the least moisture issue out of it. As it is difficult to determine this point in a satisfactory manner, I repeated the experiments with all the precaution possible. The results were constantly the same : no liquid flowed out. If this were the only proof that could be given of the non-circulation of the humour contained in the dorsal vessel, it would not be of much importance; but, joined to those that we have already made known, it acquires a certain force, and even a considerable weight. But it may be said, that the reason why the humour does not exude when the dorsal vessel is punctured, is because it is too thick. I am very much inclined, I

, own, to that opinion ; for this humour has always appeared to me very little liquid, and in the voraceous larvæ it has a remarkable consistence. This consistence, joined to some other particularities, has put us on the way to determine with some precision the functions of the dorsal vessel in those animals that have no other circulation than that of air. The different movements of contraction and dilatation which we remark in the dorsal vessel, can never induce us to consider that organ as a heart; since, in the animals like the Naiades (Nereis, Lin.) in which there exists only a single organ, that of digestion, we observe pulsations as distinct as those exhibited by the heart of other animals. Therefore the pulsations of the dorsal vessel ought not to make us decide in any manner on the use of that organ, nor lead us to consider it as a heart.

In all animals which have a system of circulation and respiration, the influence of the one of these on the other has been perceived. * Thus often when the respiration is entire, the circulation is only half so, or when the circulation is complete, the respiration does not operate in a complete manner; so that a demicirculation, multiplied by a complete respiration, or vice versâ, gives always equal products; that is to say, a semioxygenation of the blood. But in mammiferous animals, in which the circulation and respiration are complete, the oxygenation of the blood is equally so ; and in birds which have a complete circulation with a double respiration, the oxygenation of the blood is doubled, in consequence of the quantity of air that combines continually with it. +

If the dorsal vessel of insects is an organ of circulation, it ought to experience the influence of the organs of respiration like the heart of animals with vertebra. This influence ought to be so much the greater, as insects, like birds, have a double respiration. The air penetrates into all the parts of their bodies, birds. It continually bathes their nutritive humour, or their blood, which has this particularity, that it is not confined in vessels; just as in birds it acts upon the blood in the great circulation, at the

a

a

as it does in

,

.

* See Cavier's Anatomie Comparée, t. iv. p. 567.

+ These speculations about the oxygenation of the blood are quite vague and. uncertain. T.

same time that the small circulation experiences the action of the air in the lungs. Some insects have even peculiar reservoirs of air in their bodies, reservoirs so numerous, that those which are provided with them ought to have a respiration more than double,

These reservoirs of air, called vesicular tracheæ; but which may likewise be called pneumatic pockets, exist only in those insects which have to exert a great muscular force, and which, having great spaces to traverse, ought to have their body specifically light. From what has been said, it is evident that the air ought to have a great influence on the general economy of insects. We shall see hereafter, that it is the only fluid which has a real circulation in them. Now the greater this influence is, the more ought it to exert itself on the organ of circulation, provided such an organ exist. As the dorsal vessel has been considered by different anatomists as coming in place of the heart in insects, let us see whether the influence of the respiratory organ is sensible on this vessel. But to lay open that influence in a more certain manner, let us examine in the first place the organs of respiration themselves, and see whether, when these organs undergo modifications, the dorsal vessel feels their effects.

All the organs of respiration in insects may be reduced to simple trached. These organs indeed have not all the same composition ; we must therefore divide them into two orders: the first, which we shall call tubular tracheæ, on account of their disposition, are composed of three membranes, one external, one internal, and one intermediate. The two first are formed of a cellular membrane, pretty thick, and very extensible; while the intermediate one consists of a cartilaginous string, rolled up into a spiral, which may be unrolled with great facility. It is the convolutions of this elastic string which forms those brilliant and silvery looking conduits, destined to hold air, and to transmit it to all parts of the body, These traches are always kept stretched by means of this cartilaginous string. They form tubes, and have a great elasticity, which enables them to dilate, when the air, continually circulating in them, expands. These tracheæ are likewise the only ones which are ramified, the multiplied branches of which go to all parts of the body, to communicate the impression of air, the aliment of life as well as of flame.

The second order of trachea, or the vesicular trachee, do not form tubes like the preceding; they present, on the contrary, pouches more or less extended, which communicate with one another by means of ramifications, always single, and branched like those proceeding from the spiral tracheæ. These vesicular tracheæ are composed of two very white cellular membranes, very supple and very extensible. As these tracheæ have not the spiral string observable in the first order, they never communicate immediately with the air, but always by means of spiral trachese. Accordingly, in the species which have need of a great

quantity of air, and which have very extensive vesicular tracheæ, we observe a particular apparatus destined to supply the elasticity which they want.

This apparatus is composed of hemispherical cartilaginous hoops, furnished with particular muscles, and which, in consequence of this disposition, may be considered as a kind of ribs." In fact, these ribs elevate at every inspiration the vesicular tracheæ and increase their size, allowing them to receive a greater quantity of air; on expiration they depress the tracheæ, and thus serve to drive out the air. These ribs, fised by their base to the coriaceous envelope, are only moveable by their upper parts. They exist only in those insects that have vesicular tracheæ of a certain extent. We do not observe them in the lepidopteræ, the caleopteræ lainellicornes, and the dipteres, in which the vesicular tracheæ are scarcely a half millimetre (0 0196 inch) in extent. In certain orthopteres, on the contrary, as the gryllus, truxalis, acrydium, in which the tracheæ are large, several millimetres in extent, the cartilaginous hoops, of ribs, always exist, and are in these animals very necessary. Such are

the organs which serve in insects as reservoirs of air. Very different from the lungs and bronchiæ, they are not placed in any particular part; we see them, on the contrary, spread every where, with a sort of profusion : so that there is no part of these animals that does not breathe and receive the action of air.

The general disposition of the tracheæ, and the different particularities of the organization of insects, have occasioned the man. ner in which these organs communicate with the external air, à mode quite different from what we observe in other animals. The organ of respiration in insects being very much extended, and very much ramified, a single opening would not have been sufficient to distribute the air in all parts with that regularity and abundance that the circulation of that fluid required. Accordingly, the openings by which the tracheæ receive air are always more than one, having never fewer than two. Most commonly that number varies from eight to twelve, and sometimes amounts to 20. These openings have been in general called stigmata. But, as in certain species there exist some which open and shut, by means of an apparatus of which the true stigmata are destitute, we shall divide them into two different orders. We shall call the first simple stig. mata, and the second tremaer * stigmata.

The simple stigmata are most frequently placed on the sides of the body between the folds of the membrane of the back and abdomen. They are always disposed in pairs, presenting, in general

, a round opening like a button-hole, with a cartilaginous border. Sometimes, however, this border is totally wanting, and the stigmata are then surrounded by a cartilaginous scale of a different colour, from the coriacous envelope of the body. In the cater

The word is derived from pris, openings auding, nir.

.

pillars the stigmata are equally formed of small cavities, pretty deep, the edges of which are surrounded by a brown mark, and at the bottom of which we discover a stripe of the same colour.

A good deal might be said about the number and situation of these stigmata, if we considered these organs in the different orders of insects. But as these details would carry us too far, we shall satisfy ourselves with observing, that the number of these stigmata is so much the more considerable the more need the animal has of air. Accordingly, in caterpillars we reckon 18 or 20 stige mata, and in a great number of the orthopteres there are 12 or 16, without reckoning the two tremaers. In certain species, which only require a small quantity of air, the number of stigmata does not exceed two; among the apteres the faucheurs exhibit this disposition. Besides these stigmata, we see in several orthopteres two openings situated at the bottom of the abdomen; but as their use is not the same as that of the stigmata, we think it unnecessary to describe them here. But we shall not pass over the large oval opening which exists in the breast of the locust above the first pair of paws. It communicates with a large trachea, which extends the whole length of that paw. This opening serves too evidently to introduce air into the superior part of the body, not to be considered as a true stigma; especially as it is formed like the other stigmata of the extremities of the tracheæ. As to the situation of the stigmata, it is subject to numerous variations, always depending on the quantity of air which the insect requires. The more air is required the more is the situation of the stigmata such as to admit an easy introduction of it. The orthopteres, most of the larvæ, the lepidopteres, the hymenopteres, and the dipteres, seem the most favoured in this respect. In the caleopteres the tracheæ are generally placed not advantageously for the introduction of air. Some of those that live in water are obliged when they wish to respire to elevate a little their elytres, that the air may enter more easily into their trachea.

The second kind of stigma, which we shall call tremaer, is composed of an oval opening,

and shuts by means of two moveable horny pieces, set in motion by a peculiar apparatus of muscles. The general form of that opening is that of an elongated oval, the greatest diameter of which is from below upwards. In expiration the moveable pieces separate from one another; they shut on the contrary when the inspiration is terminated. These moveable pieces open from within outwards, and do not separate from each other more than half a millimetre, (0.01-96 inch.) The movements of the tremaers agree sufficiently with those of the stigmata, and like them they communicate with the tracheæ. It is even very easy in dissecting these parts to recognize the tracheze which

are composed of fibres sufficiently distinct, which are attached in the second cavity of the thorax in the erismes. Hitherto we have observed the tremaers only in a certain number of orthopteres, where they present very various forms. In general, however, they exhibit the arrangement of which we have spoken;, and it is only in the mantes that we see them situated on the lateral and external side of the thorax, between the superior and inferior portion of that part. The tremaers have then a triangular form. Instead of presenting two moveable pieces, they have only a single piece moved by a particular înuscle. It is always by means of this muscle that the moveable part is raised up; and as soon as the muscle ceases to act, the moveable part falls down and shuts. The membrane situated below the tremaer is so thin, that we can see in the expirations and inspirations the elevation and depression of the trachea, in proportion as the air enters or goes out.

Though the position of the tremaers experiences some variations, being sometimes situated in the neck, sometimes in the breast, this is not the case with the number of these parts. We never see more than two, the size of which is always proportional to the quantity of air which the insects respire. Besides that manner of receiving air the insects swallow it likewise by the mouth. What is obtained in that way indeed can only make its way to the organs of nutrition; and as we have elsewhere explained its influence on digestion,* I shall not resume the subject at present. The last mode of respiration which certain insects present, is to have their stigmata placed at the anus. Those exhibiting that organization are chiefly the insects that decompose water, as the larvæ of libellulas and dytics. These stigmata, or the openings which allow the water to escape, are surrounded by small triangular and moveable pieces, the principal use of which is probably to keep away those bodies that might hinder the introduction of water into these stigmata, and at the same time to shut the opening exactly when the insects suspend this introduction. Accordingly, when these insects choose to introduce water into their respiratory organs, they separate the moveable pieces of which we have spoken, and they shut them in the contrary case. But when these pieces are separated we easily distinguish the round opening by which water is introduced into the respiratory apparatus, This opening presents a diameter of about a millimetre, 0.03937 inch,) and it is easy by means of it to let out the water which may exist in the interior of the body.

Considered with respect to their respiratory organs, insects form three distinct classes ; namely, those that breathe air immediately; those that live in water and are obliged to come to the surface of that liquid in order to receive the impression of the air, though they likewise lay hold of that contained in the water; and those

* See my Memoir on the Intestinal Tube of Insects, iqscrted in the Annales du Museum.d'Histoire Naturelle, tom. 19.