INTERNAL STRUCTURE OF THE BUTTERFLY LARVA. 27 



intestine. Near the union of the stomach and the small intestine a 

 number of tubular glands, supposed to represent the liver of the higher 

 animals, open. The intestine ends in a chamber called the cloaca, in 

 which the indigestible and waste portions of the food are collected 

 before being expelled from the body as faeces. 



(3) The circulatory system: The blood circulates, not through actual 

 blood-vessels but, through lacunae or hollow channels in the tissues. 

 The blood is collected into a longitudinal membranous sac which is 

 placed just beneath the skin, in the middle of the dorsum or back, and 

 is known as the " dorsal vessel," and its rythmic contraction, when it 

 drives the blood into the tissues, can be detected in some of the more 

 thin-skinned lepidopterous larvae. In a larva of Brotolomia meticulosa it 

 was observed to beat 44 times in a minute. The functions of the dorsal 

 vessel are analogous with those of the heart in the higher animals, but it 

 consists of only one chamber, although the latter is divided into a num- 

 ber of sacs. The muscular tissue of which it is formed contracts from its 

 hinder part forwards, i.e., towards the head, and, by its contraction, forces 

 the fluid in it out in front into a number of little vessels which soon 

 come to an end in the little hollow passages or lacunae in the tissues. 

 These lacunae are very abundant around the tiny air-tubes which 

 branch off from the tracheae, and it is here, after the blood has been 

 over the system, that it is aerated. From here it is carried to the 

 dorsal vessel again to be once more distributed over the system to 

 be again aerated and to be returned again to the dorsal vessel or 

 heart. In vertebrates, the nervous system is placed dorsally, and the 

 circulatory and respiratory systems ventrally, in relation to the alimen- 

 tary canal. These positions are exactly reversed in insects, the nervous 

 system being placed ventrally, the circulatory and respiratory systems 

 dorsally, the alimentary canal being placed between them. It has, 

 however, been shown that this difference is more apparent than real, 

 the dorsum of the insect being really analogous with the venter of the 

 vertebrate, but with the position of the limbs reversed. The dorsal vessel, 

 although consisting of only one chamber, is divided into 8 or 9 sacs, 

 the latter with openings along the sides called ostia. It is composed 

 chiefly of muscular tissue, and is connected with the roof of the body 

 by short stout muscles, which keep it in position. In its passage 

 through the tissues, the nutritious parts of the food, which soak 

 through the walls of the stomach and intestine, enter the blood in the 

 lacunae found near these organs. The blood of insects is so different 

 from that of vertebrates, that one feels that it is a great mistake to call 

 them by the same name. Its function is to carry the nutritious 

 matters to the tissues, and to feed, as it were, the tissues it bathes. It 

 is frequently filled with somewhat crude fatty matters, and Graber 

 calls it a refined or distilled chyle. 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 peri- 

 cardial cavity or sinus. The membrane itself is incomplete, and, when 

 certain muscles contract so as to pull it down tightly upon the tissues 

 below, the movement 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 membrane, 



