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 Muscte vibrantes, because when 

 searching for the destined nidus of their eggs their an- 

 tennae 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 



a Reaum. vi. 30G. b Fn. Succ. 1626. ' Linn. Trans, iii. 26. 



