1900.] 63 



the pupa aud up to the mouth ot" the sac. This has, however, an 

 arraDgetnent that permits the ? moth easily to traverse its axis, 

 though its elasticity closes the I'outc up again when she leaves it. 



I do not know whether the parasitised specimen is a ^^ or $ , but 

 the silk spun by the larva agrees with the normal spinning of neither 

 (^ nor 5 . In the specimen, as T now see it. the pu|)al case o£ this 

 Dipteron (some Tachinid) is in much the position occn])ied by the 

 head of the ? pupa of opacella, lyi"g ill a mesh of loose silk, but 

 this silk does not continue forward to the month of the sac, but im- 

 mediately in front of" the Dipterous pupa, at a distance of 7 — 8 ram. 

 from the mouth of the sac, forms a smooth transverse diaphragm. 

 Through this diaphragm the Dipteron has emerged through a trans- 

 verse slit, made not by the Dipteron rupturing the silk, but obviously 

 by a weak line having been spun in just the right position by the 

 opacella larva. The Dipteron did not expand its wings, and with this 

 added difficulty to that involving the naming of many Tachinids, one 

 has to be satisfied by recognising that it is undoubtedly a Tachinid. 



How does instinct in a case like this get changed in precisely the 

 manner to suit the parasite ? This seems to be especially a difficult 

 question here, since A. opacella is affected, so far as my experience 

 goes, comparatively rarely by Dipterous parasites. 



We must suppose that as a species (say opacella) alters its habits, 

 so the parasite alters not only its own habits but the effects it pro- 

 duces on its hosts, and natural selection will favour those parasites 

 that produce effects useful to themselves. A Tachinid, for example, 

 that made an opacella spin an unopenable cocoon could not thrive, but 

 it would thrive in exact proportion to the suitability of the change 

 opacella made in her cocoon to the requirements of the parasite. How 

 the effect is produced is a very obscure matter, but it must be by 

 some effect on the nervous system of the host, either mechanically, or 

 by the production of some agent acting chemically. 



vSome Psychids are freely attacked by Tachinids, and opacella may 

 be so, though I have not met with it. If it really is not we must con- 

 clude that this Tachinid has its haunt on some other species, or on 

 Psychids generally, here to produce this desired effect on its host, and 

 that the procedure, whatever it is, is effective on opaeeUa, though 

 elaborated not in that species, but on some of its allies. Probably the 

 Tachinids have grown up along with the Psychids and perfected and 

 varied their procedure throughout the whole time the Psychids have 

 been evolving. It is tolerably certain that a Tachinid strange to 

 Psychids was not let loose on them a few years ago, and has already 

 developed such curious interactions. 



Botula, Rfigale : Jami<irii, r.)iil. 



