i 9 4 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



kind of life-history as the form last described, and are parasitic on 

 bees of the same sort. The female beetle does not, however, lay 

 her eggs near a suitable nest, but simply deposits them in the 

 ground. The six-legged larvae climb up various plants, and com- 

 monly lie in wait on or near their flowers, attaching themselves 

 at random to any hairy insects that come near enough. The 

 chances of a given larva reaching a suitable destination are ex- 

 ceedingly small, but as a set-off against this each female beetle 

 lays some 10,000 eggs, which allows for considerable wastage. 



MEMBRANE-WINGED INSECTS (HYMEI?OPTERA) AS PARASITES. 

 Among the most interesting members of this order in the present 

 connection are the Ichneumon-Flies, and other forms of similar 

 habits, in which the female is provided with a sharp ovipositor, 



Fig. 1139. The Yellow-Legged Ichneumon-Fly (Microgaster glomeratus}. a, Adult; &, larva; c, dead caterpillar 

 of Cabbage-Butterfly, surrounded by cocoons of the Ichneumon. Size of a and b indicated by the short lines. 



by means of which she deposits her eggs within the bodies of 

 the larvae, pupae, or even eggs of other insects. In some cases 

 deposition takes place not in, but on or sufficiently near, suitable 

 victims. The early stages of Butterflies and Moths are particu- 

 larly liable to such attacks, and in this way the ravages of many 

 of our familiar agricultural and garden pests are kept within 

 bounds. Some of these parasites are in turn similarly attacked 

 by insects not distantly related to them, a case of the biter bit. 

 The common Cabbage- Butterfly (Pieris brassiccz) is subject to 

 the attentions of a number of these forms. By one (Polynema 

 gracilis) its eggs are pierced, two others (Microgaster glomeratus, 

 fig. 1 1 39, and Pimpla instigator] lay their eggs in its caterpillars, 

 and still another two (Pteromalus puparum and P. ponticz) attack 

 its chrysalides. 



Certain larvae and pupae, that live where one would expect 

 them to be quite secure from these parasitic insects, are never- 

 theless sought out by them, and exposed to the murderous 

 assaults of their brood. One kind of Ichneumon- Fly (Agriotypus 

 armatus] boldly plunges into water, and lays her eggs in the 



