The circulation of the nutritive fluids is, in fact, limited to their free diffusion amongst all the internal viscera, and is effected in the following manner: - If we examine the back of a silkworm, or of any transparent larva, a long pulsating tube is seen running beneath the skin of the back, from one end of the body to the other. Its contractions may readily be watched: they are found to begin at the posterior extremity, and are gradually continued forwards; so that the vessel presents a continual undulatory movement, by which the fluid contained in its interior is pushed from the tail towards the head. This dorsal vessel, which may be so well observed in the thin-skinned larva, exists likewise in the perfect insect, although, from the opacity of the integument, its movement is no longer apparent except by the vivisection of the animal.

* Sorg, Disquisitiones Physiologicae circa Respirationem Insectorum et Vermiumactivity.

(850). This dorsal vessel, or heart as we shall call it for the sake of brevity, is organized in a very singular manner; for, instead of being a closed viscus, it communicates most freely, through several wide lateral apertures, with the cavity of the abdomen, and from thence derives the blood with which it is filled. The dorsal vessel is widest in the abdominal region; but is continued, nevertheless, through the thorax into the head, where it terminates as a simple or furcate tube - that is, not closed, but open at the extremity.

(851). The structure of this remarkable heart has been fully investigated by Straus-Durckheim*, and is extremely curious: it consists, in the Cockchafer, of eight distinct compartments, separated from each other by as many valves formed by productions from the lining membrane, and so disposed that the blood passes freely from the hinder chambers into those which are placed more anteriorly, but is prevented from returning in the opposite direction.

(852). Each compartment of the dorsal vessel communicates by two wide slits, likewise guarded by valves, with the cavity of the belly; so that fluids derived from thence will readily pass into the different chambers, but cannot again escape through the same channel. The arrangement of these valves will, however, be best understood by reference to the accompanying figure (fig. 163), representing a magnified view of the interior of a portion of the heart of the Cockchafer, as depicted by the celebrated entomotomist before alluded to. The organ has been divided longitudinally, so that one-half only is represented in the figure, upon a very large scale. The compartments (a a a) are distinctly composed of circular muscular fibres; the large valves (d d) separate the individual chambers, allowing the blood to pass in one direction only, viz. towards the head; while the openings (c), likewise closed by semilunar membranous valves, admit blood from the cavity of the abdomen, but effectually prevent its return.

* Op. cit.

(853). Let us now consider the movements of the circulating fluids produced by the contractions of this apparatus. The chyle or nutritive material extracted by the food exudes, as we have already seen, by a species of percolation, through the walls of the intestine, and escapes into the cavity of the abdomen, where it is mixed up with the mass of the blood, which is not contained in any system of vessels, but bathes the surface of the viscera immersed in it. When any compartment of the heart relaxes, the blood rushes into it from the abdomen through the lateral valvular apertures; and as it cannot return through that opening on account of the valves (c) that guard the entrance, nor escape into the posterior divisions of the heart by reason of the valves (d), the contraction of the dorsal vessel necessarily forces it on towards the head. "When it arrives there, it of course issues from the perforated termination of the heart, but does not appear to be received by any vessels, and therefore becomes again diffused through the body. The difrused character of the circulation met with in insects may easily be made a matter of observation in many of the transparent aquatic larvae that are readily to be met with.

When any of the limbs of these larvae are examined under a powerful microscope, continual currents of minute oat-shaped globules are everywhere distinguishable, moving slowly in little streams - some passing in one direction, others in the opposite: but that these streams are not contained in vascular canals is quite obvious from the continual changes which occur in the course of the globules; their movements, indeed, rather resemble those of the sap in Chara, and other transparent vegetables, in which the circulation of that fluid is visible under a microscope.

(854). The organs appropriated to furnish the different secretions met with in the economy of insects are modified in their structure to correspond with the character of the circulation, and are invariably simple tubes or vesicles of various forms immersed in the fluids of the body, from which they separate their peculiar products. The poisonous saliva of bugs, and the innoxious salivary fluid of other insects - the bile and auxiliary secretions subservient to digestion - the venom which arms the sting of the wasp, and the silky envelope of the caterpillar, - are all derived from the same source, and in some mysterious manner elaborated from the blood by variously-formed vessels: but of this we have already given many examples, and others will present themselves in the following pages.

Internal view of a portion of the dorsal vessel of a Cockchafer.

Fig. 163. Internal view of a portion of the dorsal vessel of a Cockchafer: aaa,bb, muscular walls of the compartments; d d, in-tercompartmental valves; c, valve defending one of the orifices communicating with the general cavity of the abdomen.