218 DISEASES OF INSECTS. 
or short according to the situation and circumstances of 
the larva which receives them : if this lives in the open 
air, and the access to it is easy, it is usually short and 
retracted within the body ; but if it lies concealed in deep 
holes or cavities, or shuns all approach, it is often very 
long. Thus in Pimpla Manifestator, which commits its 
eggs to the grub of a wild bee inhabiting the bottom of 
deep holes bored in posts and rails, the ovipositor is 
nearly an inch and half in length, and in some extra- 
European species three inches. How the egg is pro- 
pelled so as to pass in safety from the oviduct, along this 
extended and very slender instrument to the grub for 
which it is destined, has not been certainly ascertained : 
but from an observation of Reaumur's a it should seem 
that it is aided in its passage by some fluid ejected at the 
same time with it, or is so lubricated as to slide easily 
without being displaced. The flies we are speaking of, by 
some authors are called Muscat vibrantes, because when 
searching for the destined nidus of their eggs their an- 
tennas vibrate incessantly, and it is by the use of these 
wonderful organs that they discover it wherever it lurks. 
Bergman observed that Fcenus Jaculator searches for the 
latent grub of certain bees and other Hymenoptera with 
its antennae b : and from Mr. Marsham we learn that 
Pimpla Manifestator, before it inserts its ovipositor in the 
nest of the grub of Chelostoma maxillosa, explores it first 
with one antenna and then with the other, plunging 
them all the while intensely quivering up to the very root c . 
With respect to their size, Ichneumons vary greatly ; 
some being so extremely minute as to be invisible to the 
naked eye, unless moving upon glass ; while others, as to 
' Reaum. vi. 306. h Fn. Stiec. 1026. r Linn, Trans, iii. 26. 
