44 2 Studies in the Theory of Descent. 



possess the characteristics of many of the cater- 

 pillars of this group. 9 



A second illustration is furnished by the family 

 Ophiusida, 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 larvae, 

 like the imagines, show a striking resemblance to 

 those of the Geometrina in the absence of the 

 anterior abdominal legs. For this reason Hiibner 

 in his work on caterpillars has termed the species 

 of this family " Semi-Geometra" 



All these cases show a complete congruence in 

 the two kinds of form-relationship ; but exceptions 

 are not wanting. Thus, the family Bombycut* 

 would certainly never have been formed if the 

 larval structure only had been taken into con- 

 sideration, since, whilst the genera Gastropacha, 

 Clisiocampa, Lasiocampa y Odonestis, and their 

 allies, are thickly covered with short silky hairs 

 disposed in a very characteristic manner, the cater- 

 pillars of the genus Bombyx, to which the common 

 silkworm, B. Mori, belongs, are quite naked and 

 similar to many Sphinx-caterpillars (Cheer ocampa). 

 Are the imagines of the genera united under this 



[For Mr. A. G. Butler's observations on the genus Acro- 

 nycta, see " Trans. Ent. Soc." 1879, p. 313 ; and note 3 P- l6 9> 

 of the present volume. R.M.] 



