VOL. XXir.] PHILOSOPHICAL TEANSACTIONS. 559 



in its hinder part, which at length amounted to near two thirds of the whole 

 body. 



Fig. 6 represents the aforesaid fly, just as it appeared to the naked eye ; cd 

 shows the long, slender, and hooked part. When brought before the micro- 

 scope, it seemed a sort of hook, and covered with a great number of fine 

 hairs, as seen magnified in fig. "J , and it appeared also to be hollow. Upon 

 which imagining it might rather be the case for a hook, I endeavoured to split 

 it, and then the hook itself appeared, the point of which is only delineated, 

 jagged with teeth like a saw, as ef, fig. 8. The more I viewed this hook, the 

 more I fancied that there was another inclosed in it ; nor was I mistaken, for I 

 split the first hook, and took out of it two other distinct hooks, of tlie same 

 shape ; a small part of one of which is represented at gh, fig. 9. Each hook 

 was fortified with teeth like saws, which I observed was peculiar to them, and 

 did not belong to fig. 8. After I had taken these hooks out of fig. 8, I was 

 convinced that what I took for a hook, was only a second case or sheath for the 

 other two, as ik, fig. 10, where likewise the hollowness plainly appears. And 

 there is also a cavity to be seen in fig. 9, which may contain a sharp poisonous 

 liquor. 



From this discovery, it is easy to conceive, that such flies do not only lay 

 their eggs on the leaves of trees, but also make an opening in the skin of the 

 leaf, and convey an egg into it, from whence comes the worm, which gnawing 

 the vessels for its sustenance, occasions the sap to flov/ out of them, and coa- 

 gulate into that knotty substance. Besides one small fly produced one of those 

 small worms, whose hinder part was also hooked much like that species of flies 

 that proceed from the lice of worms on currant trees. 



I took two tonnekins out of the above-mentioned knobs in willow leaves, no 

 larger than grains of coarse sand, to describe the shape of such a small animal, 

 but three hours after I missed them, and concluded, that in that time they were 

 turned to flies, and got away. I have taken dead worms out of the said knobs, 

 without being able to find the lesser sort of worm that uses to devour the other ; 

 but I observed two longish white particles on the dead worm, which were so 

 very small, that they escaped my naked eye ; I fancied that they were eggs, for 

 I could see nothing of them that was like a worni, and the third day there 

 plainly appeared two worms, exactly of the same size and shape with those which 

 devour the larger ones. 



I took a small devouring worm from a greater that lay dead by it, and from 

 which it took its nourishment, and put it upon a living worm : immediately it 

 fastened its snout in the said living worm, which at the same time used all its 

 might, with bending, stretching, contracting, and winding its body, to free 



