370 PHYSIOLOGY OF THE INVERTEBRATA. 



have the power of emitting this secretion in the form of 

 threads, one end of which floats freely in the air until it 

 meets with some object to which it adheres. By this means 

 spiders often form natural bridges, by means of which they 

 can pass over brooks and rivers, in some cases twenty and 

 even fifty feet wide. 



Another purpose to which this secretion is applied by all 

 spiders is the formation of silken cocoons for the reception 

 of the ova, yrhich a few species (i.e., wandering spiders) carry 

 about with them. 



Fig. 52, a AND b. — Crystals of Uric Acid and Murexide. 

 a = the uric acid crystals, b = murexide crystals. 



Concerning the excretory apparatus in the Araneina, Mr. 

 A. Johnstone, P.G.S., and the author * have examined the 

 Malpighian tubules of Tegenaria domestica (Pig. 5 1 , A and B). 

 The intestines of this species form a tube-like body, which 

 dilates into a short rectum, and into this rectum the Mal- 

 pighian tubules open. 



An aqueous extract of a large number of these tubules 

 yielded uric acid (Fig. 52). The secretion is neutral to test 

 papers. 



* Pruc. Roy. Soc. Edinb., vol. 15, p. iii. 



