So also the families _Liparidæ_, _Euprepiidæ_, and _Lithosiidæ_ appear sharply defined in both forms; and similar families occur likewise under the _Noctuina_, although in this group the erection of families presents great difficulties owing to the near relationship of the genera, and is always to some extent arbitrary. It is important, however, that it is precisely the transitional families which present intermediate forms both as larvæ and as imagines. Such an instance is offered by the _Acronyctidæ_, a family belonging to the group _Noctuina_. The imagines here show in certain points an approximation to the group _Bombycina_; and their larvæ, which are thickly covered with hairs, likewise possess the characteristics of many of the caterpillars of this group.[184] A second illustration is furnished by the family _Ophiusidæ_, which is still placed by all systematists under the _Noctuina_, its affinity to the _Geometrina_, however, being represented by its being located at the end of the _Noctuina_. The broad wings and narrow bodies of these moths remind us in fact of the appearance of the “geometers;” and the larvæ, like the imagines, show a striking resemblance to those of the _Geometrina_ in the absence of the anterior abdominal legs. For this reason Hübner in his work on caterpillars has termed the species of this family “_Semi-Geometræ_.” All these cases show a complete congruence in the two kinds of form-relationship; but exceptions are not wanting. Thus, the family _Bombycidæ_ would certainly never have been formed if the larval structure only had been taken into consideration, since, whilst the genera _Gastropacha_, _Clisiocampa_, _Lasiocampa_, _Odonestis_, and their allies, are thickly covered with short silky hairs disposed in a very characteristic manner, the caterpillars of the genus _Bombyx_, to which the common silkworm, _B. Mori_, belongs, are quite naked and similar to many Sphinx-caterpillars (_Chærocampa_). Are the imagines of the genera united under this family, at any rate morphologically, as unequally related as their larvæ? Whether it is correct to combine them into one family is a question that does not belong here; we are now only concerned with the fact that the two stages are related in form in very different degrees.