OF INSECTS. 
139 
certain neuropterous groups, such as the Hemerobii, 
and, in many instances, particularly among the Or- 
thoptera and Hymenoptera, they amount to a hun- 
dred and fifty. Their length generally bears some 
proportion to their number, a deficiency the one 
way being compensated by an increase in the other. 
The longest surpass the dimensions of the body five 
or six times. Although commonly of the same dia- 
meter throughout their whole extent, they are occa- 
sionally attenuated at one or both extremities. Nu- 
merous constrictions at regular intervals, in a few 
instances, give them the appearance of being granular 
or warty. One of the most notable deviations from 
their generally simple construction is witnessed in 
the cockchafer, in which they appear fringed with a 
double row of thickset projecting processes of equal 
length, some of which are furcate ; (PI. II. fig. 3, e 3 e.) 
their composition, also, i3 greatly more simple than 
that of the alimentary canal, the coat consisting of a 
single membrane of great delicacy. It is likewise 
transparent, permitting the view of the contained fluids, 
which are most frequently brown or saffron yellow ; 
the prevalence of the latter induced Swammerdam to 
call the whole organs saffron-vessels. 
The last system of vessels which we shall notice 
at present in connection with the alimentary tube, is 
that formed by the urinary vessels. In their general 
aspect they almost seem to repeat, at the anal ex- 
tremity of the body, the salivary vessels appended to 
the head. But their presence seems to be far less 
general, or at least they are more difficult to detect. 
