318 
DIPrERA. 
Lucy, What colour are these little 
dirty creatures ? 
Mother. They always appear of the 
same hue as the mud in which they are 
found ; for it strongly adheres to their 
clammy skin ; but if washed they are 
found to be white, and so transparent, 
that their air vessels can be distinctly 
traced. Of these air vessels there are 
two, which, in the middle of the body, are 
of considerable size, but lessen in thick- 
ness towards the extremities, and they 
run along its whole length, like two pa- 
rallel stripes. When the tail is stretched 
out, these air vessels become so slender as 
to be nearly imperceptible; but when it 
is contracted, they enlarge, and as the 
tail shortens, they bend up in folds. The 
intestine, which contains the aliment, or 
food, is easily distinguished from the air 
vessels, by the dark colour of its con- 
tents. 
Lucy. But, mamma, how do they 
lengthen or shorten their tails ? 
