ANATOMY AND PHYSIOLOGY. 167 
cause trouble farther down the canal, ever getting 
into the stomach at all. With the close fitting valve 
at its entrance, and the free power of emptymg the 
crop by aid of the air vesicles, their stomach is pretty 
safe from having anything passed into it which cannot 
pass out of it. 
- Some of the tangled threads which we tear through 
are air tubes, distinguished, even to ther minutest 
ramifications, by their white colour and the presence 
of a spiral fibre wound round them. In the yellow 
threads we recognize another elementary tissue. 
These are gland tubes; they are opaque and finely 
granular, and, from their analogous position, have been 
thought by many to represent the biliary vessels of the 
higher animals. Referring again to Van der Hoeven’s 
admirable summary* of the latest—as far as I know— 
discoveries in insect anatomy and physiology, we find 
reasons for hesitating to accept this opinion without 
qualification. The economy of insects is so very diffe- 
rent from that of the Vertebrata, that we are not justi- 
fied in inferring, from mere analogy of position, as to the 
functions-of any organ. The discovery of uric acid in 
the secretion of these yellow gland-tubes has led some 
to infer that they represent the kidneys rather than, or 
as well as, the liver. But, in truth, the present state 
of our knowledge scarcely justifies any definite con- 
clusion on this matter. 
* Op. cit., p. 256. 
+ The excretions of insects have been made the subject of special 
examination by Dr. John Davy. Wasps have come in for their share 
of attention, and uric acid and urate of ammonia are set, down as 
constituents of the excretions of the perfect insect and the larva re- 
spectively. ‘Trans. Entomolog. Society.’ Vol. III, N.S. 1854-56, 
p. 18, I have often failed to find uric acid in these excretions, 
