THE URINARY TUBES 



351 



secreting cell. After Schindler. 



In many insects (Pentatoma, Cimex, Velia, Gerris, Haltica, Donacia, and 

 often in caterpillars), the vessels open into a sort of urinary bladder connecting 

 with the intestine on one side. 



In the larvie of some insects the blind ends of the tubes are often externally 

 bound to the rectum, in the silkworms being attached by fine threads to the 



intestine, while in some flies (Tipula and Cten- 

 ophora), two vessels may unite to form a loop. 

 In all larval Cecidomyise, the two tubes are united 

 to form a loop which curves backward, opening 

 near the vent, the proctodaeum being very short. 

 (Giard.) 



F. 345. -.4, section of urinary While usually the urinary vessels form simple 

 tube of Periplaneta ; B, part of tube tubes, iii many species of Lepidoptera and Dip- 



tera the y are b ranched > thus resembling those 

 of spiders and scorpions. Moreover, in many 

 Lepidoptera and Diptera (Fig. 308), the tubes 



are not simple, but are lobulated, and in some Hemiptera (Pentatoma, Noto- 

 necta, and Tettigonia) are twisted or lacelike. In rare cases there are two kinds 

 of urinary tubes ; in Melolontha vulgaris, two of them are partly lobulated and 

 yellow, while the other two are simple and white. Their color in beetles varies, 

 some being whitish or yellowish ; in Geotrupes, Dyticidse, Hydrophilidse, etc., 

 reddish brown ; in Gryllotalpa as well as Locusta viridissima, there are two. 

 different kinds of vessels, differing in contents and in 

 color (white or yellow), as well as histologically. 

 (Schindler.) 



The exterior of the tubes is richly provided with 

 tracheae, which often form a web around them, and 

 the fine branches often seem to attach them to the 

 intestine. In Acheta they are enveloped by a very deli- 

 cate, loose network of muscular fibres. (Schindler.) 

 The urinary tubes consist, according to Schindler, 

 of at least three cellular layers (Fig. 345) : 



1. An external, connective, nucleated membrane, 

 the peritoneal membrane. 



2. A very delicate homogeneous basal membrane, 

 the tunica propria. 



3. A single layer of large polygonal excretory cells. 



4. Lining the internal canal a chitinous layer pene- 

 trated by pore-canals, the intima often wanting. 



The secretory cells are usually of the same size, 

 but in many cases are relatively small ; sometimes 

 four to six or more form the periphery of the canal, 

 sometimes three or only two. In some insects the 

 cells are so very large that a single cell forms the 

 entire periphery. The nuclei in the Lepidoptera 

 (Papilio, Pontia, Cossus) are large and irregularly 

 branched. 



The excretions of the Malpighian vessels, derived from the blood and from 

 the fat-body, are more or less fluid and granular, sometimes pulpy. From the 

 cells they pass into the canal, thence into the intestine, and thence out of the 

 body. How, says Kolbe, the secretion passes into the intestine, whether by the 

 contraction of the fine fibrillfe of the peritoneal membrane, or by the external 

 pressure of the other organs, or by the pressure of the secretory matter behind, 



FIG. 346. Portion of a 

 urinary tube of CnUiphorn 

 vomitoria : tr, trachea ; 1, 

 lumen ; k, nucleus. After 

 Gegenbaur. 



