110 LETTERS ON ENTOMOLOGY. 
kind have a long instrument for the purpose of 
piercing a hole in which to deposit their eggs 
in safety, which is called an ovipositor. Many 
of the ichneumon kind which deposit their eggs 
in living animals have this long tail, which is a 
most admirable instrument. The grasshoppers, 
in particular, have a large and strong ovipositor, 
but it is partly concealed by the body. In h- 
custa viridissima, the green English locust, which 
is frequently found in grass, it is very con- 
spicuous. The saw-flies have for the same pur- 
pose a most admirable and effective saw, which 
is contained in their bodies ; but if I were to 
enumerate all the various and beautiful instances 
of exquisite contrivance observable even in this 
tribe, it would go far beyond the limits of a 
letter : I shall therefore mention what occurs to 
me, in a desultory manner. 
It must, I think, have often surprised you to 
see flies walking upon glass, and the cieling and 
walls. It was formerly supposed to be per- 
formed by means of the hooks with which their 
feet were furnished, and to the smokiness and 
ruggedness of the glass ; but as they walk equally 
well on it when just cleaned, and on the most 
polished mirror, we must find another reason. 
It is ascertained that they are furnished with 
