58 INSECT PHYSIOLOGY 



Verson's glands 



It has been observed that when the silkworm casts its skin, its body 

 may be powdered with crystals of oxalate and uric acid derived from 

 the moulting fluid; and this led to the suggestion that, at the time of 

 moulting, the Malphighian tubes are thrown temporarily out of 

 action, and their excretory function taken over by the dermal 

 glands (Verson's glands, p. 4). But this idea has been shown to be 

 mistaken; the excretory substances in the moulting fluid really come 

 from the Malpighian tubes, escape through the anus beneath the old 

 cuticle, and so spread over the surface of the body. (It is just possible 

 that the calcareous warts of Stratiomyid larvae (p. 0) may also be 

 formed in this way. On the other hand, the lime which appears in the 

 ecdysial fluid of certain Diptera at the time of pupation is said to be 

 dissolved from the Malpighian tubes, reabsorbed into the blood, and 

 excreted through the newly-formed cuticle.) 



The Malpighian tubes and the urine 



When all these subsidiary processes have been considered, the Mal- 

 pighian tubes still remain unquestionably the chief excretory organs. 

 The Malpighian tubes are relatively simple tubular glands which 

 open at the junction between the mid-gut and the hind-gut. They are 

 exceedingly variable in form: sometimes being numerous (e.g. over 

 100) and short, sometimes few in number (e.g. 2) and long; some- 

 times simple and sometimes branched; occasionally anastomosing 

 to form closed loops ; while sometimes more than one type of tube 

 is present. 



Their histological structure is no less variable. Usually their epi- 

 thelial cells bear a striated border, but this may be wanting; some- 

 times this border is of the type known as a 'honey-comb border', or 

 'Wabensaum', in which the rod-like filaments or 'microvilli' are so 

 closely packed, being separated by no more than 150-200A, that 

 they appear with the light microscope as though fused together; 

 sometimes the microvilli are more widely separated and the mobile 

 filaments form a 'brush border' or 'Burstensaum'. The cytoplasm 

 of the Malpighian tubes is exceedingly rich in mitochondria and 

 these commonly extend far into the microvilli, so that it some- 



