EXCRETION 59 



times appears as though the striated border were composed of 

 mitochondria. 



Often the tubes have a muscular coat and are capable of active 

 movements; sometimes they show only slow twisting movements 

 due to changes in the secretory pressure within them. Many histologi- 

 cal changes have been described in the active cells of the Malpighian 

 tubes : the discharge of vesicles, the eruption of vacuoles, and so 

 forth. It is not improbable that more than one cytological mechanism 

 of excretion may exist ; but certainly many of the recorded obser- 

 vations are artefacts. Studies with the electron microscope have re- 

 vealed the uptake of fluids from the lumen by a process of pinocytosis 

 between the bases of the microvilli ; and also the discharge of minute 

 vesicles, apparently from the vesicular endoplasmic reticulum, at the 

 tips of the microvilli. 



The urine which the Malpighian tubes secrete is equally varied. It 

 may be a clear fluid or a thick pasty suspension: the colourless fluid 

 passed by blood-sucking insects soon after a meal is an example of 

 the former sort, the 'meconium' of newly emerged Lepidoptera of 

 the latter. Its constituents depend naturally upon the nature of the 

 food, but it always consists primarily of water with the usual inor- 

 ganic salts, chlorides and phosphates of sodium, potassium, calcium, 

 and magnesium, in solution. Yellowish or greenish pigments ('ento- 

 mo-urochrome') are often present; but it is likely that these differ 

 chemically in different species: riboflavine is plentiful in the cells of 

 the Malpighian tubes of some insects, kynurenine in others. 



Nitrogen is excreted chiefly as uric acid ; but urea has been report- 

 ed in substantial amounts in the urine of the clothes moth, and meat- 

 eating maggots eliminate much of their nitrogen as ammonia. Some 

 aquatic insects, such as the larvae of Sialis, of Odonata and Tri- 

 choptera also eliminate their nitrogen in the form of ammonia; and 

 so indeed do Aphids, which are continually absorbing large quanti- 

 ties of water from the plant and discharging copious amounts of 

 'honey dew'. The amino-acid leucine has been recorded by many 

 earlier authors, but the evidence rests only on crystal structure and 

 requires chemical confirmation. Guanine, the main nitrogenous con- 

 stituent in the urine of many Arthropods, seems not to occur in 

 insects; but many other nitrogenous compounds such as xanthine 



