428 



would be difficult to observe, anywhere, a more beautiful 

 instance of a histogenetic convergence — of the development of 

 similar structures from embryologically quite distinct elements. 



THE INTESTINE AND RELATED STRUCTURES. 



It will be most convenient to consider these organs under 

 the following headings: — (1) The anatomy and structure of 

 the adult intestine ; (2) the ana^tomy of the larval intestine ; 

 (3) the changes which go in the intestine during larval and 

 pupal life, and which convert it into that of the imago. This 

 will be described under several headings, viz.: — (a) The 

 foregut; (bj the midgut; (c) the hindgut. It will then be 

 necessary to consider certain closely related structures, viz. : 

 — (4) The salivary glands, and (5) the malpighian tubules. 



(1) The Anatomy and Structure of the Intestine of the Adult. 



The mouth faces downwards and backwards in the posi- 

 tion in which the head is usually held. This opens into a 

 narrow high buccal cavity continuous above with a dilated 

 pharynx, which opens into the oesophagus, a long very narrow 

 tube, which passes forwards, then backwards through the 

 circumoesophageal nerve ring, and enters the thorax. Here 

 it becomes even narrower, and passing through the thorax 

 and propodeum as a very fine tube, enters the abdomen, 

 where it forms a great dilatation, with fine papery, usually 

 collapsed walls — the crop. 



The crop occupies only a small anterior part of the 

 abdomen. Behind, it partly envelops and communicates with 

 a very short gizzard, which in turn opens into a small drum- 

 shaped chamber, considerably shorter than the gizzard. 

 Behind this chamber lies the great s^tomach, occupying about 

 one-third the volume of the abdomen, and between the two is 

 a structure, formed by the slight projection of the chamber 

 into the stomach, which evidently acts as a valve, to prevent 

 any forward move of the contents of the stomach. The stomach 

 is the true midgut, being endodermal in origin. All the 

 structures preceding it constitute the foregut, and are, as 

 will be seen, ectodermal in origin. Behind, the stomach is 

 continuous with the hindgut, also ectodermal in origin. 



The hindgut is composed of two parts, an anterior por- 

 tion, the small intestine, and a terminal portion, the rectum, 

 which opens in the last segment by a small anus (fig. 22). 

 The small intestine does not, however, open into the termina- 

 tion of the stomach ; it communicates with that organ by a 

 small aperture situated oh its ventral side about one-quarter 

 the length of the stomach from its posterior extremity. The 

 small intestine then passes forwards half-way along the 



