56 BRITISH LEPIDOPTEEA. 



The mouth opens into a long narrow tube (the oesophagus), into 

 which several long tubules pass. These represent the salivary glands 

 of the higher animals, and secrete a fluid, which is discharged into the 

 oesophagus, and which is swallowed with the food. It dissolves the 

 starch and cellulose of the food, and fits it to soak through the walls 

 of the alimentary canal, so that it can enter the system. The oeso- 

 phagus is composed essentially of muscular tissue, and expands into a 

 crop (or food receptacle), and then into a gizzard. This is provided 

 with hard plates, that help to grind up the food, which, after being so 

 ground up, is passed through another short tubular passage into the 

 stomach. The walls of the stomach secrete a fluid resembling the 

 gastric juice of the higher animals ; this changes the insoluble proteid 

 of the food into a soluble peptone, which is readily absorbed by the 

 walls of the stomach and intestine. The stomach opens into the 

 intestine, the upper end of which is connected with a number of 

 tubular glands. These are supposed to represent the liver of the 

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

 in which the waste matters are collected, and from which they are 

 expelled through the anus. 



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

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

 mentary canal. These positions are exactly reversed in insects, the 

 nervous system being placed ventrally, the circulatory and respiratory 

 systems dorsally, the alimentary canal being still 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 the position of the limbs is reversed. 



In the upper part of the body, and directly under the dorsal 

 integument, is a longitudinal organ, somewhat like a long tube, which 

 is known as the dorsal vessel. This corresponds with the heart of the 

 vertebrates, and it consists essentially of only one chamber, although 

 this 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. It opens towards the head into a kind of arterial 

 trunk. As the dorsal vessel contracts from behind forwards, the 

 blood, which consists of plasma, or fluid, and colourless corpuscles, is 

 driven forward into the trunk. The latter subdivides into smaller 

 vessels, which are soon lost, the walls gradually becoming inseparable 

 from those of the ordinary lacunse, or depressions found between the 

 tissues, and which are lined in many places with epithelium. As the 

 blood passes through these lacuna?, it is brought into contact with 

 the tracheal branches and aerated. At the same time the nutritious 

 parts of the food, which soak through the walls of the stomach and 

 intestine, enter the blood in the lacunse found near these organs. 



The great difference that exists between the blood of insects and 

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

 call two so dissimilar fluids, with different functions, by the same 

 name. The blood of insects varies with the species, sometimes even 

 with the various stages of the same insect. 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." 



