96 NATURAL HISTORY OF WASPS. 
with all that muscles in action require for their con- 
stant renewal. The air-vessels by which they are 
traversed are much more numerous than the, capil- 
lary blood vessels which ramify among our muscles ; 
each fibre is crossed or encircled by little tufts of 
trachese, springing at right angles from long straight 
tubes which run parallel to the direction of the 
muscle. 
Wasps, on account of the small size and the hard- 
ness of the mtegument of the thorax, are ill-adapted 
to display the exact mode of arrangement of the 
muscular system of this part: this must be studied 
on larger insects encased in more yielding walls. So 
I will not do more here than indicate the. general 
disposition of the muscles of the thorax. These may 
conveniently be divided into two classes, mm accord- 
ance with the distinctions between voluntary and in- 
voluntary muscles which have just been pointed out. 
First, closely nestled together in the corners between 
the ridges which project on the inside, and crowded 
into the wall-spaces along the lower and anterior 
surface of the thorax, are many little muscles which 
move the head and legs. These are of a whiter colour 
than the mass which occupies the centre of the 
thorax, and their microscopic structure shows them 
to be voluntary muscles. Other muscles of the same 
order run down into the limbs to perform the few 
simple movements of which these parts are capable. 
Besides these little slips, there are two, or rather two 
pair, of large muscles which cross the cavity of the 
thorax in opposite directions, nearly at right angles 
to each other. If we break the dried thorax of a 
wasp these large masses fall out, the empty spaces 
