It is generally acknowledged by Entomologists that the hairy larvæ, such as the Arctians and their allies, very commonly escape parasitic attack, a circumstance attributable to the fact, that in order to permit the deposit of ova, these caterpillars must be discovered by the parasites in favorable postures, or else worried by them into such, that the spines separating, give the only opportunity for the insertion of the ovipositor. This was clearly demonstrated in an attack made by an Ichneumon upon a larva of Apatela Americana Harris, which came under my observation in the early part of last autumn. This genus belongs to the family of the Noctuidæ, the larvæ of which, like the Arctians, are clothed with dense spinular hairs. The caterpillar had secured itself by its abdominal legs to the midrib of a maple leaf, having the fore part of its body elevated, similar to the attitude common to the larvæ of the Sphingidæ.