Qf Flies, 57 
the hollownefs'does plainly appear, which is filled with a 
corrofive water. The fly makes ufe of this auger to pre¬ 
pare a convenient lodgment for her eggs (and choofes 
thofe leaves that are moll ladeous and juicy) under the 
ikin of the leaf, from whence the worm upon gnawing 
the veffels for its fullenance, occafions the fap to flow out 
of them and to coagulate into that knotty fubftance. 
Mr. Leeuwenhoek took a lmall devouring worm from a 
larger' that lay dead by it, and put it upon a living one 
to which it immediately faftened, whilfl: the other at the 
fame time ufed all means, by bending, ftretching, coiv 
trailing, and winding its body, to free itfelf from this 
troublefome gueft, but in vain, the final! one Hill keeping 
its hold. 
Fig. 102. exhibits a tonnekin, which was a worm but 
the foregoing evening, and had caft o£F a very thin Ikin; 
this alfo confifted of feveral rin^s and circles as when in 
the worm ftate. The feet and joints thereof were very 
vifible ; A B and A C reprefents its two antennae j and 
although they were inclofed in a thin membrane, yet all 
the joints might be clearly feen. The change of this 
worm was fo fudden, that Mr. Leeuwenhoek was never 
able to fee it. 
Not only the willows and other trees, but plants alfo 
have cafes produced on their leaves, as nettles, ground 
ivy, &c. by the injedion of the eggs of an ichneumon 
fly. Thefe cafes are generally obferved to grow near to 
forne rib of the leaf, and their production thus. The 
parent in fed with its ftiff fetaceous tail, terebrates the 
rib of the leaf when tender, and makes way for its egg, 
into the very pith or heart thereof, and probably lays 
in therewith fome proper juice of its own body to pervert 
the regular vegetation of it. From this wound arifes a 
fmall excrefcence which (when the egg is hatched into a 
maggot) 
