Rhyssa persuasoria : — Its Oviposition and Larval Habits. 



By Miss Evelyn Cheesman, F.E.S. — Read April lAt/t, 1921. 



The larvfe of this Hymenopteron are parasitic on the larv;e of 

 Sirex (jiijas : the female liln/ssa is furnished with an extremely long 

 ovipositor for reaching the burrows of the latter. 



During the summer of 1918, Lord Sligo happened to observe a 

 female of /i/(//ss<t pcrsiiasoyia in the act of ovipositing in a post of 

 larch wood. He carefully observed and described the mode of 

 procedure, and from his notes a sketch was made by Mr. Staniland 

 of the Imperial College of Science, which differs markedly from the 

 figure by Kiley in "Insect Life," vol. i. (178), of an allied species 

 Thalessa Uinator. 



As noted by Lord Sligo the insect first walked about on the 

 trunk for some time ; touching the bark with its antennae, then 

 halting as if waiting for some sound below. (Other observers have 

 recorded this behaviour, and it is probable that the larvte of Sirex 

 ijiijas, moving or feeding below are audible to the Itlnji^sa, who is 

 thus able to locate the burrows before inserting her ovipositor. It 

 is extremely necessary that the spot should be chosen with great 

 care, for should the position be miscalculated, or the wood be too 

 hard, the ovipositor may become fixed, and if not broken the 

 insect must starve.) 



Having decided upon the right place for commencing operations, 

 and taken her position htad downwards parallel with the post, the 

 insect raised her abdomen to its utmost capacity, raising herself at 

 the same time on the long posterior pairs of legs, until the ovipositor 

 was brought under the body, and held in position by the legs, which 

 apparently guided it. The ovipositor was now resting against the 

 wood, but at the wrong angle for boring. Next there appeared to 

 be convulsive movements of the abdomen, and the ovipositor was 

 very gradually shortened until it was brought to the required length 

 to take a vertical position at right angles to the surface of the post 

 — then boring commenced. 



As fully described by Eiley in the paper on Thalesna Innator 

 quoted above, the shortening of the ovipositor is compassed by the 

 action of some powerful muscles at its base, by which it is drawn 

 backward into a membranous sac situated between the 6th and 7th 

 segments, within which it is coiled like a watch spring. 



Subsequently the larch logs, in which Rhi/ssa had been observed 

 ovipositing, were sent to the Insect House of the Zoological 

 Gardens, Regent's Park, to be kept under observation. Five adults 

 of Sirex (jii/as, four males and one female, emerged during the 



