433 



secretion evidently enters the duct by percolating through this 

 spongy tissue. 



In its other regions the duct is lined by flattened epithelial 

 cells, which thicken near its opening. Here a number of 

 muscles are attached, which are inserted at the other ends 

 upon the walls of the head. 



The duct is lined by a chitin sheath, which presents 

 spiral ridges similar to those seen in tracheae. 



The malpighian tubes (fig. 150) are eight thread-like 

 cylindrical structures, each with a narrow flattened duct; the 

 duct is formed essentially by the incomplete junction of 

 embryonic cells in irregular pairs. Sometimes adjacent cells 

 do not fuse completely and the canal in consequence extends 

 between these also. 



The cytoplasm of the cells is very clear and homogeneous 

 but often exhibits very large vacuoles. The nuclei are large 

 and granular and have one or two medium-sized nucleoli. 



(2) The Intestine of the first Larval Instar. 



In the larva there is a small mouth (fig. 1), the openings 

 of the conical buccal cavity, which contains the minute sharp 

 jaws (fig. 47). Tliis leads behind into a long oesophagus 

 (figs. 1, 140) which passes horizontally backwards through 

 the circumoesophageal nerve ring, and opens in the third seg- 

 ment into a great sac-like dilatation, the midgut (figs. 2, 140). 

 Tlie latter is endodermal, the oesophagus ectodermal in origin. 

 The midgut occupies the greater part of the body extending 

 backwards to the third last segment. Here it lies in close 

 connection with the hindgut, but the two do not communicate 

 till the time of defaecation, i.e., till about one day before 

 pupation. The midgut, indeed, is simply a blindly ending 

 sac (fig. 143), and it is not till the last day of larval life that 

 the unabsorbed food is discharged. 



From a portion of the oesophagus (foregut) the whole of 

 the intestine of the imago as far back as the beginning of the 

 stomach will develop during metamorphosis. From the great 

 larval midgut the stomach of the imago is formed, while the 

 small intestine and rectum are developed from the hindgut. 

 The endodermal portion of the intestine is therefore much 

 smaller in the adult than in the larval insect, and it is entirely 

 from the ectoderm that the other structures — crop, gizzard, 

 etc. — become developed. ^ 



It is curious to observe how highly differentiated the 

 intestine of the imago is, in an insect which rarely feeds, while \ 

 the larva, which does nothing but feed, must content itself \ 

 with so simple a structure. .>_J 



Opening into the base of the mouth is a median salivary 

 duct, which soon divides into two parts. Each of these smaller 



