An especially striking case of incongruence is offered by the family _Notodontidæ_, under which Boisduval, depending only on imaginal characters, united genera of which the larvæ differed to a very great extent. In O. Wilde’s work on caterpillars this family is on this account quite correctly characterized as follows:--“Larvæ of various forms, naked or with thin hairs, sixteen or fourteen legs.”[185] In fact in the whole order Lepidoptera there can scarcely be found associated together such diverse larvæ as are here placed in one imago-family; on one side the short cylindrical caterpillars of the genus _Cnethocampa_, Steph. (_C. Processionea_, _Pithyocampa_, &c.), which are covered with fine, brittle, hooked hairs, and are very similar to the larvæ of _Gastropacha_ with which they were formerly united; and on the other side there are the naked, humped, and flat-headed larvæ of the genus _Harpyia_, Ochs., with their two long forked appendages replacing the hindmost pair of legs, and the grotesquely formed caterpillars of the genera _Stauropus_, Germ., _Hybocampa_, Linn., and _Notodonta_, Ochs. The morphological congruence between larvæ and imagines declares itself most sharply in genera, where it is the rule almost without exception. In this case we can indeed be sure that a genus or sub-genus founded on the imagines only will, in accordance with correct principles, present a corresponding difference in the larvæ. Had the latter been known first we should have been led to construct the same genera as those which are now established on the structure of the imagines, and these, through other circumstances, would have stood in the same degree of morphological relationship as the genera founded on the imagines. There is therefore a congruence in a double sense; in the first place the differences between the larvæ and imagines of any two genera are equally great, and, in the next place, the common characters possessed by these two stages combined cause them to form precisely the same groups defined with equal sharpness; the genera coincide completely. So also the butterflies of the sub-family _Nymphalinæ_ can well be separated into genera by the characters of the larvæ, and these, as far as I am able to judge, would agree with the genera founded on the imagines.