108 MEDICAL ENTOMOLOGY 



They are very hard and gritty, and frequently spoil sections of this 

 part of the gut. 



The Malplghian tubes differ somewhat in appearance in the different 

 genera, but all conform to a general type, of which Tabanus may be 



taken as an example. There are four of these struc- 

 Malpighian Tubes . . 



tures, opening at the junction between the mid-gut and 



the hind-gut. They are blind tubes of great length, at least twice 

 as long as the body of the fly when dissected out. They lie in the abdom- 

 inal cavity, and are coiled and twisted around the other organs in 

 every direction, some appearing to pass into the posterior end of the 

 thorax. The terminations of one pair are found near the anterior end of 

 the mid-gut, and those of the other closely intertwined with tracheae 

 and embedded in the fat body at the sides of the rectum. The tubes are 

 of practically uniform diameter throughout, but are narrowed a little at 

 the distal end. The wall is composed of large flattened cells, which are 

 wrapped round the lumen and fitted to one another so as to form a com- 

 plete tube ; as a result of this arrangement only one nucleus at most 

 is cut in cross-section. (Plate XXII, figs. 1 and 2.) The tubes are well 

 supplied with tracheae, and are so bound down by means of them that 

 it is difficult to dissect them out entire. In stained preparations the 

 nuclei are very conspicuous on account of their size and of the uniformity 

 with which they take up the ordinary stains, no discrete particles 

 of chromatin being distinguishable. In addition to the nuclei of the 

 cells of the tubules there are a few other smaller ones, which belong 

 to the ultimate divisions of the tracheae distal to the terminations 

 of the spiral thread. These are, of course, distributed all over the tissues 

 of the insect, but are only distinguishable in situations such as this, 

 where they are not mixed up with other cells. 



The Malpighian tubules are probably excretory organs, which collect 

 the waste products from the blood and fat body and pass them into 

 the lumen of the intestine, from which they are passed out with the 

 residue from the food. Their situation in many insects forms a useful 

 landmark, but it must not be supposed that the manner in which 

 they separate the mid-gut from the hind-gut is more than what one might 

 perhaps term an accident. In many insects they open into the intestine 

 much further down, as for instance in the bugs, where the opening 

 is just above the anus. 



The alimentary canal of Tabanus will serve as a type for the rest 

 of the blood-sucking Orthorrapha, with the exception of the Culicinae, 

 which show many peculiarities, and will be dealt with separately. With 



