Mesostenus.^ 



BRITISH ICHNEUMONS. 



261 



nervure may be so obsolete as to appear wanting ; its abdonitn also varies 

 much in width, the normal form being elongate-ovate with the third 

 segment the broadest and parallel-sided, whence the abdomen tapers to its 

 base, but rarely segments two to four are of equal breadth throughout ; 

 the petiolar spiracles appear much more prominent in smaller examples, 

 and the post-peliole is either parallel-sided or gradually exfjlanate towards 

 its apex. 



This species occurs throughout central and northern I'Auope, and has 

 been there bred from Zyi^aena Ephialks, Z. trifolii (Brisch.), and Z. carni- 

 olica (Laboul.). In Britain it is by no means uncommon ; Hrockley in 

 June (Turner) ; Maldon in Essex (Fitch) ; Fairlight, Sussex (Hast. List); 

 Brundall near Norwich (Bridgman) ; South Devon (Bignell) ; Botusfleming 

 (in coll. Marsh.) ; common some years on the Cotswold Hills (Watkins, 

 Ent. Rec. 1902, p. 349). Bred at Oxford from Zyi:;aena trifolii early in 

 May (Hamm) ; and from Eucheliajacobaeae (Buckler's Larvae, ix. Appendix). 

 South and Chapman have bred it at Reigate, and Lyle in the New Forest, 

 from Zygaena Jilipendulae, and Capron took it at Shere in the same 

 county. 



Head of Larva. 



Zygaena cocoon. 



The life history of this species is comparatively well known and I have 

 been enabled, thanks to the kindness of Mr. R. M. Prideaux, to add a 

 little to what was already noted by Bignell and Moncreaff. In April the 

 female lays a solitary egg on the back of a caterpillar o{ Zygaena filipetidulae, 

 when the larva emerges in a few days and eats its way through the skin to 

 begin feeding upon the fatty portions of the caterpillar, which is full fed by 

 the end of June. The latter then weaves its cocoon as though in perfect 

 health, and usually retains sufficient strength to assume the pupa state. 

 The parasite has meanwhile been advancing towards maturity at its host's 

 expense, and completes its demolition just before, or soon after, it pupates. 

 The former then proceeds to construct a cocoon of its own within its 

 host's ; this is twelve millimetres in length and six in breadth, a little con- 

 tracted at both extremities, which are oblifiuely truncate, and do not reach 

 the apices of the Zygaena cocoon. It is sub-ovate and generally longi- 

 tudinally wrinkled, on account of the contraction of its surface. Upon the 

 outer integument being removed, the parasite's cocoon is revealed ; this is 

 of exactly the same colour as the host's, and within is shining, (juite 

 smooth and entirely empty, excepting for the larva ; it is interwoven upon 

 the host's cocoon and is much stouter and more chitinous. The Zygaena 

 caterpillar's skin occupies the extreme apex of its cocoon (i) and its 

 chrysalis, of which only the crumpled, compacted skin and chitinous parts 

 remain, is pushed closely on to it. Very rarely the caterpillar's skin is at 

 one end of the Zygaena cocoon and the pupa skin at the other. Some- 



