INSECTS
Fig. 107. A dead potato aphis that has contained a parasite, which when adult escaped through the door cut in the back of the aphis
on almost any plant, you will most likely note here and there a much swollen aphid of a brownish color. Closer examination reveals that such individuals are dead, and many of them have a large round hole in the back, perhaps with a lid standing up from one end like a trap door (Fig. 107). These aphids have not died natural deaths; each has been made the involuntary host of another insect that converted its body into a temporary
Fig. 108. Aphidius, a common small wasplike parasite of aphids
home. The guest that so ravishes its protector is the grub of a small wasplike insect (Fig. 108) with a long, sharp ovipositor by means of which it thrusts an egg into the body of a living aphid
Fig. 109. A female Aphidius inserting an egg into the body of a living aphis, where the egg hatches; the larva grows to maturity by feeding in the tissues of the aphis. (From Webster)
(Fig. 109). Here the egg hatches and the young grub feeds on the juices of the aphis until it is itself full-grown, by which time the aphid is exhausted and dead. Then the grub slits open the lower wall of the hollow corpse and spins a web between the lips of the opening and against the surface of the leaf below, which attaches the aphid shell to the support. Thus secured, the grub proceeds to give
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