THE INTERNAL STRUCTURE OF THE LEPID0PTER0US LARVA. 57 



Beneath the dorsal vessel, a fine membrane is stretched in such a 

 manner as to separate the dorsal vessel from the surrounding organs, 

 and, at the same time, leave a cavity around the dorsal vessel itself. 

 This cavity is called the pericardial cavity or sinus. The membrane 

 itself is incomplete, and when certain delicate muscles connecting it 

 with the body-wall contract, they pull it down tightly upon the tissues 

 below, and this, of course, at once increases the size of the sinus. 

 The tissues thus pressed upon are full of chyle and blood, and the 

 fluid is squeezed from these structures through the incomplete mem- 

 brane, into the pericardial chamber, and from thence it re-enters the 

 dorsal vessel again. The number of contractions of the dorsal vessel 

 varies remarkably. They may amount to as many as a hundred per 

 minute ; they may cease altogether without death ensuing. It is 

 recorded as pulsating from 48 to 52 times per minute in the larva of 

 Triaena (Acronycta) psi, and 44 times per minute in the larva of 

 Brotolomia meticulosa. 



In spite of the fact that Swammerdam, Reaumur, Bonnet, De 

 Geer, and others, all speak of blood-currents, of fluids moving in the 

 body, of pulsations of the heart or dorsal vessel, and of circulation, 

 Kirby and Spence record their emphatic opinion that there is no circu- 

 lation in insects. The idea of circulation taking place in the lacunas 

 of the tissues does not appear to have suggested itself, and the early 

 authors appear to have thought that definite tubes with definable 

 parietes were necessary for circulation. Bowerbank, and others, placed 

 the matter beyond dispute, and it is only necessary to refer to it here, 

 because many entomologists still seem inclined to accept the state- 

 ment of Kirby and Spence. 



The fat-body is a very prominent part of the structure of the lepi- 

 dopterous larva. It consists of fat masses of various size, loosely 

 connected together, and enveloping most of the organs. It varies in 

 colour and appearance in almost every species of insect, and appears to 

 consist of a reservoir of reserve material, which increases in the larval 

 stage, when the insect is busily engaged in feeding, and upon which 

 the insect can draw in the future, when it is unable for along period to 

 take food, e.g., such periods as occur ateach exuviation of the larval skin, 

 and also at the more exhausting periods of metamorphosis. It must also 

 be looked upon as a storehouse on which the insect can draw when in 

 the more quiescent pupal stage. 



The respiration of the Lepidoptera has been partly dealt with in the 

 preceding chapter, and we have seen that air is conveyed into all parts 

 of the body by means of the tracheae. The tracheaa are elastic tubes, 

 held open by an inner chitinous layer, and they are all intimately 

 connected. Large tubes connect the spiracles longitudinally, others 

 pass from one side of the body to the other, whilst a set of tracheal in 

 the lower part of the body is connected with another set in the 

 upper part by ascending tubes. These main branches give out small 

 branches, which fork in all directions, and hence the body is 

 supplied most plenteously with air. The tubes have a white 

 glistening appearance, and hence can be detected in a freshly killed 

 insect without difficulty. [In insects of strong flight, there are air-sacs 

 connected with the tracheal, and capable of holding sufficient air to 

 decrease, when distended, the specific gravity of the insect.] The finest 

 tracheal tubes are supposed to penetrate cells, but it is not known 

 whether they terminate with open or closed extremities. 



