THE HyMENOPTERA OF MINNESOTA 171 
tho living in the water, do not escape them, and some species attack 
spiders. 
Generally, all have long slender bodies, ovipositors sometimes very 
long and protected by a sheath. Eggs are laid either upon the outside 
or within the caterpillar or other larva. When hatched, the Ichneu- 
mon grub devours the fatty portions of its victim, the latter gradually 
Fig. 50. The parasite, Lysiphlebus, sp. 
attacking grain plant louse. From life. 
growing weaker and dying about the time it would normally pupate. 
The parasite lives largely by the absorption of the lymph and blood of 
its host. 
The genus Aphidius, an Ichneumon preying upon plant lice, is 
extremely useful to the agriculturist. The species are all small. 
Some prey upon aphids on oats, others on different forms of wheat 
aphids. Usually a parasitized aphid is swollen and globular in 
marked contrast to other non-parasitized aphids. The method of at- 
tacking plant lice on the part of this genus is interesting; “the female 
alights upon a leaf and runs about amongst the plant lice. When 
she has selected one in which to oviposit, she stands with her head to- 
ward it, and bending her abdomen under her thorax, between her legs, 
she darts her ovipositor forward into the body of the aphid.” (Com- 
stock). In this connection see the illustrations made in our labora- 
tony, bes. 50. coleand 52. 
