Walker : On Ichxeumonid^. 



77 



existing in a more or less modified form among the other families, 

 consists of three chief parts, viz. : — The terebra or borer, and the two 

 sheaths. The latter are not actually necessary during oviposition, but, 

 being semicircular and concave, unite, and include between them the 

 terebra itself, thus protecting that delicate apparatus from injury. 

 They are covered along their whole length with bristles or hairs, all 

 pointing downwards towards the tip. In some species these hairs are 

 very short, a fact that is just reversed in others, and in one species 

 that I have examined they are at least twice as long as the sheath 

 itself is thick. 



The terebra is composed of three pieces, two of them being 

 thin and filamentous, and notched or toothed at their extremities, the 

 remaining one being thickened towards the extremity and brought 

 to a fine point at the extreme end. 



If we were to make a transverse section of the terebra, we should 

 then have the appearance of a circle divided into three segments by a 

 corresponding number of radii, two of the segments being equal to one 

 another, and together less than the remaining segment, which latter 

 represents the unserrated filament in section. The central portion of 

 the circle is occupied by an aperture through which the ova are 

 expelled. 



At first I was puzzled to imagine why the terebra should consist of 

 three pieces, but I have accounted for the fact thus : Supposing that 

 the true ovipositor were a perfect cylinder, perforated for the expulsion 

 of the egg, the end would be blunt, and unsuited for piercing purposes. 

 If, again, the extremity of the cylinder were brought to a fine point, 

 the ova would be unable to find an exit. Now both these difiiculties 

 are surmounted by the ovipositor being trifid, each of the three 

 portions being finely pointed at its extremity. When an ichneumon 

 is about to oviposit, the two spiculse are firmly united to the superior 

 filament, forming a finely pointed borer, which is plunged into the 

 flesh of some unfortunate larva, slightly withdrawn, and the three 

 parts so separated as to form a cylindrical passage for the egg, which 

 is left in the wound. 



I must not omit to remark that there are two kinds of ovipositors 

 found among the Ichneumonidce. Not that they are structurally 

 different, but they merely vary in length. Those of some species are 

 said to be retracted when they do not project outwards beyond the 

 tip of the abdomen, and exserted, when prolonged beyond the terminal 

 segment, as in Pimpla instigator, which I have described. In some 

 species this instrument attains an enormous length, and I have placed 



