io8 



ENTOMOLOGY 



along the hypopharynx. Though this poison is said to facilitate the 

 process of blood-sucking by preventing the coagulation of the blood, its 

 primary use was perhaps to prevent the coagulation of proteids in the 

 juices of plants. 



Malpighian Tubes. The kidney, or Malpighian, tubes, present in 

 nearly all insects, are long, slender, blind tubes opening into the intestine 

 immediately behind the stomach as a rule (Figs 

 147, 148) , but always into the intestine. The num- 

 ber of kidney tubes is very different in different in- 

 sects; Collembola have none, while Odonata have 

 fifty or more and Locus tidae as many as one hun- 

 dred and fifty; commonly, however, there are four 

 or six, in Coleoptera, Lepidoptera and many other 



FIG. 158. Portion PIG. 159. Cross-section of Malpighian tube of silkworm 



of Malpighian tube of Bombyx mori. b, basement membrane; c, crystals; i, intima 



caterpillar, Samia ce- I, lumen; n, nucleus; p, peritoneal layer. Greatly magnified. 

 cropia, surface view. 



orders. Not more than six and frequently only four occur in the 

 embryo (Wheeler) , though these few embryonic tubes may subsequently 

 branch into many. 



The Malpighian tubes (Fig. 158) are evaginations of the proctodaeum 

 and are consequently ectodermal. A cross-section of a tube shows a 

 ring of from one to six or more large polygonal cells (Fig. 159), which 

 often project into the lumen of the tube; the nuclei are usually large and 

 may be branched, as in Lepidoptera. A chitinous intima, traversed by 

 pore canals, lines the tube, and a delicate basement membrane is present, 

 surrounded by a peritoneal layer of connective tissue. Furthermore, 

 the urinary tubes are richly supplied with tracheae. In function, the 

 Malpighian tubes are analogous to the vertebrate kidneys and con- 

 tain a great variety of substances, chief among which are uric acid and its 

 derivatives (such as urate of sodium and of ammonium) , calcium oxalate 

 and calcium carbonate. 



