» 150 NATURAL HISTORY OF WASPS. 
fluid, of a greenish colour, with a number of little 
round or oval cells floating in it, which are often 
rough on the surface, and are, on the whole, much 
smaller than human blood-globules. Perhaps these 
are not really blood-cells, only particles of fat.* The 
blood only brings and removes material; the supply 
of air is quite independent of the circulation. It is 
altogether much less important than the blood of the 
higher animals. It is small in quantity in proportion 
to the size of the animal; and its direct influence on 
the manifestations of life is very slight, much less, 
indeed, than the influence of air in the trachee. An 
insect will live and walk about for hours after the 
circulation has been interrupted by the removal of the 
abdominal viscera, but immersion in water is speedily 
fatal by obstructing the respiration. 
The only portion of the nervous system which is 
likely to arrest attention on a cursory examination of 
the internal structures of a wasp is the chain of ganglia 
which lie just beneath the ventral scales of the abdo- 
men. These need no dissection, but appear, on merely 
turning back the scales, as a row of little white knots 
on a double thread. Their importance, however, is 
not to be measured by their size, for these knotted 
threads are a portion of the central nervous system. 
From the knots or ganglia the little filaments issue 
which give motor power to the muscles ; and the fila- 
ments which bring back sensations from the neigh- — 
bouring parts tend to these ganglia as their several 
centres. The double thread connects them all into 
one common system. It extends the whole length of 
+ Van der Hoeven. ‘ Handbook of Zoology,’ Transl. Vol. I, p. 259. 
