THE ICHNEUMON FLY. (Pimpla persuasoria.)
The mouth of this insect has jaws, but no sucking tongue. The antennæ contain more than thirty joints; and the abdomen is joined to the body by a slender pedicle. The ovipositor is enclosed in a cylindrical sheath, composed of two valves.
One distinguishing and striking characteristic of all the species of this kind of fly is the almost continual agitation of their antennæ. The name of Ichneumon has been applied to them from the service they do us by destroying caterpillars, plant-lice, and other insects; as the Ichneumon or Mangouste destroys the crocodile in the East. The tip of the abdomen of the females is armed with an ovipositor, visible in some species, though not in others; and this instrument, though so fine, is able to penetrate through mortar and plaster. The female fly uses it to deposit her eggs in the body of other insects when in the egg, caterpillar, or pupa state; so that the young as soon as they are hatched may feed upon the caterpillar, penetrating to its very entrails. These larvæ, however, contrive to suck out the nutritious juices of their prey without attacking its vitals; for the caterpillar continues to live for a long time, so as to afford them food till they have attained their full size. It is not uncommon to see caterpillars fixed upon trees, as if they were sitting upon their eggs; when it is afterwards discovered that the larvæ, which were within their bodies, have spun their threads, with which, as with cords, the caterpillars are fastened down, and so perish miserably.
“A friend of mine,” says Dr. Derham, “put about forty large caterpillars, collected from cabbages, on some bran and a few leaves in a box, and covered it with gauze to prevent their escape. After a few days we saw, from the backs of more than three-fourths of them, about eight or ten little caterpillars of one of the Ichneumon flies come out and spin each a small cocoon of silk; and in a few days the large caterpillars died.”
The Ichneumons performed great service in the years 1731 and 1732, by multiplying in the same proportion as the caterpillars, and their larvæ destroyed more of these destructive creatures than could any efforts of human industry.