AND ANATOMY OF CERTAIN HYMBNOPTERA PARASITICA. 407 



slightest notice, and with a rapid, hard, quivering stab pierces the Aphid, 

 deep into its body. 



If a number of half-grown Aphids be fixed in Gilson and stained in para- 

 carmine, on examination some will be found to contain parasitic larvae as 

 drawn in PI. 26. fig. 15 at P. In all the parasitized individuals dis- 

 covered by me, the larva is surrounded by an embryonic membrane, SA, 

 and evidently does not feed till at a much later stage. In the lower Hgure 

 in PI. 26. fig. 15 is drawn in optical section the upper part of the parasitic 

 cyst, the embryonic membrane (SA) is seen to consist of a single layer of 

 hypertro[)hied cells. 



In the case of certain Ghalcids (13) this layer has been called the pseudo- 

 serosa. The pseudo-serosa of Ghalcids, M'here present, is derived by a 

 delamination of the surface-cells of the embryonic morula, and it is more 

 than likely that this membrane in the Braconid Aphid parasites is derived 

 in a similar manner. 



As the parasitized Aphids grow, they can frequently be detected by the 

 fact that they become a little different in colour from their unparasitized 

 fellows. In the case of A. pojni, which is brown, the parasitized forms 

 become a whitish brown and the skin looks tight and shiny. Sooner or later 

 these individuals leave off feeding and die, the parasite evidently having 

 attacked their vitals. They then become the typical straw or darker brown 

 shade. These dead parasitized Aphids, if opened up just after they have 

 gone the straw-colour which characterizes them, are found to contain the 

 Bracon larva. It is just at this stage, or some time afterwards, that the 

 Chalcid and Cynipid hyperparasites begin operations. 



The Hyperparasitic Ghalcids and Cynipids seek out these dead parasitized 

 Aphids, bore a hole in the dry skin and deposit an egg at the side of, but 

 so far as I know never inside, the Braconid larva or pupa. The egg hatches 

 out and the minute larva at once fixes itself on to the Bracon pupa or larva. 

 If one examines enough of these dry Aphids, one is sure to find that in some 

 cases the hyperparasite, even when quite young, may have a Hyper-hyper- 

 parasitic larva sticking on to its body. There is then a chain of three larvge, 

 or of one pupa (Bracon), a hyperparasitic larva (Chalcid or Cynipid), and a 

 hyper-hyperparasitic larva (Chalcid ?). Both hyperparasitic and hyper- 

 hyperparasitic larva cling on quite firmly to the skin of their victim. 



By collecting a number of the straw-coloured dead Apple-Aphids and 

 keeping them in closed phials till the contained parasites, etc. hatch out, it 

 will be found that a remarkably large number of small Hymenopterous 

 insects depend for their existence on the colonies of plant-lice. It is need- 

 less to say that some of these insects are of great economic importance. 

 The hyperparasites are not beneficial, as they are engaged in destroying 

 the parasites, which are destroying the noxious plant-lice. 



